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Plant rhizodeposition - An important source for carbon turnover in soils
Abstract
The soil organic matter plays a key role in ecological soil functions, and has to be considered as an important CO2 sink on a global scale. Apart from crop residues (shoots and roots), left over on the field after harvest, carbon and nitrogen compounds are also released by plant roots into the soil during vegetation, and undergo several transformation processes. Up to now the knowledge about amount, composition, and turnover of these root-borne compounds is still very limited. So far it could be demonstrated with different plant species, that up to 20 % of photosynthetically fixed C are released into the soil during vegetation period. These C amounts are ecological relevant. Depending on assimilate sink strength during ontogenesis, the C release varies with plant age. A large percentage of these root-borne substances were rapidly respired by microorganisms (64—86 %). About 2—5 % of net C assimilation was kept in soil. The root exudates of maize were mainly water-soluble (79 %), and in this fraction about 64 % carbohydrates, 22 % amino acids/amides and 14 % organic acids could be identified. Plant species and in some cases also plant cultivars varied strongly in their root exudation pattern. Under non-sterile conditions the exuded compounds were rapidly stabilized in water-insoluble forms and bound preferably to the soil clay fraction. The binding of root exudates to soil particles also improved soil structure by increasing aggregate stability. Future research should focus on quantification and characterization of root-borne C compounds during the whole plant ontogenesis. Apart from pot experiments with 14CO2 labeling, it is necessary to conduct model field experiments with 13CO2 labeling in order to be able to distinguish between CO2 originating from the soil C pool and rhizosphere respiration, originating from plant assimilates. Such a separation is necessary to assess if soils are sources or sinks of CO2. The incorporation of root-borne C (14C, 13C) into soil organic matter of different stability is also of particular interest.Pflanzliche Rhizodeposition — eine wichtige Quelle für den Kohlenstoffumsatz in BödenDie organische Bodensubstanz (OBS) nimmt eine Schlüsselrolle bei den ökologischen Bodenfunktionen ein und stellt global betrachtet eine wichtige CO2-Senke dar. Neben den auf dem Feld verbleibenden Ernte- und Wurzelrückstnden werden C- und N-Verbindungen auch whrend des Pflanzenwachstums in den Boden abgegeben und unterliegen dort vielfltigen Umsetzungsprozessen. Der Kenntnisstand über die Menge, Zusammensetzung und den Umsatz dieser wurzelbürtigen Verbindungen im Boden ist noch sehr begrenzt. Daher beschftigt sich die vorliegende Publikation mit der Rhizodeposition als wichtiger Quelle des C-Umsatzes in Böden, insbesondere mit den mobilen Wurzelabscheidungen. Bisher konnte anhand verschiedener Pflanzenarten gezeigt werden, dass bis zu 20 % des photosynthetisch fixierten C whrend der Vegetation durch die Wurzeln freigesetzt werden. Es handelt sich dabei um ökologisch relevante C-Mengen. In Abhngigkeit von der Strke des Assimilatsinks whrend der Ontogenese variiert die C-Freisetzung mit dem Pflanzenalter. Ein hoher Anteil dieser wurzelbürtigen Verbindungen (64—86 %) wurde schnell durch Mikroorganismen veratmet. 2—5 % der Netto-C-Fixierung blieben im Boden zurück. Dieser Bodenrückstand war bei Wurzelabscheidungen von Mais hauptschlich wasserlöslich (79 %), und in dieser Fraktion wurden 64 % Kohlenhydrate, 22 % Aminosuren/Amide und 14 % organische Suren identifiziert. Pflanzenarten und teilweise auch -sorten variierten stark in der Zusammensetzung ihrer Wurzelexsudate. Unter insterilen Bedingungen wurden die exsudierten Verbindungen schnell in nichtwasserlöslicher Form stabilisiert und vor allem an die Tonfraktion des Bodens gebunden. Die Bindung an Bodenpartikel verbesserte die Bodenstruktur durch erhöhte Aggregatstabilitt. Zukünftige Forschungsarbeiten sollten sich auf die Quantifizierung und Charakterisierung wurzelbürtiger C-Verbindungen whrend der gesamten pflanzlichen Ontogenese konzentrieren. Abgesehen von Gefßversuchen mit 14CO2-Applikation ist es erforderlich, Feldmodellversuche mit 13CO2-Applikation durchzuführen, um zwischen der CO2-Emission aus dem Boden-C-Pool und derjenigen aus der Rhizosphrenatmung (Ursprung sind Pflanzenassimilate) unterscheiden zu können. Eine solche Trennung ist zur Beurteilung, ob Böden eine Quelle oder Senke für CO2 darstellen, zwingend erforderlich. Von besonderem Interesse ist auch der Einbau des wurzelbürtigen C (14C, 13C) in OBS-Fraktionen unterschiedlicher Stabilitt.
... In this experiment, we did see some effects of treatments on C L in the topsoil (0-30 cm) and the subsoil (30-60 cm). Several factors related to soil management may affect C L ; for example, fertilization has been shown to reduce C L due to the stimulation of microbial activity in the soil (Hütsch, Augustin, and Merbach 2002). In our case, the role of fertilization in driving the content of labile C in the soil remains unclear as we did not observe any effects of mineral fertilizer or manure addition in either grass sward or plowed tillage treatments. ...
... We did not follow the amount of C and N deposited into the soil in various treatments, doing so may have enabled us to understand short-term effects on labile compounds better. Hütsch, Augustin, and Merbach (2002) state that rhizodeposition is a major source of labile carbon in the soil, while intensive root activity might lower stocks of labile N. We did see this effect; the relationship between C L and N pot under grass swards differed from the two plowed tillage treatments (Figure 4). However, the instability of these pools makes them subject to considerable Figure 4. Relationships between labile organic carbon (C L ) and potentially mineralizable nitrogen (N pot ) in 0-30 cm topsoil layer in vineyard soils exposed to five different soil management regimes between 2 and 14 years. ...
Historical depletion of agricultural soils must be remedied to maintain their long-term food production function, including soils in intensive vineyards traditionally kept without plant cover to limit resource competition. This paper reports on the impact of five contrasting soil management regimes on indicators of soil quality such as soil organic carbon, nitrogen and their indices. We exposed sandy loam Rendzic Leptosol under a vineyard in Nitra-Dražovce (Slovakia, central Europe) to the following five treatments for 14 years: no-till sward, no-till sward+NPK100, no-till sward+NPK125, plowed tillage and (5) plowed tillage+manure. We found that grass swards continuously increased the total soil organic carbon in the topsoil, but plowed tillage resulted in no change. The availability of potentially mineralizable nitrogen was also increased by grass cover; but was not modified by manure but by mineral fertilizer addition. We tested the usefulness of carbon and management indices as indicators of changing soil C and N status and found them no better than tracking total and labile forms of both elements. In conclusion, the recovery of degraded vineyard soils under no-till grass sward cover is detectable within 14 years and is not affected by fertilization or manure addition.
... Concerning the biotic drivers of soil C and N cycling, the phenology of S. herbacea significantly affected N-NH 4 + , with a negative relation, whereas it influenced DOC with a positive relation. This relationship could be related to the plant uptake and development: the plant uptake increases with increasing phenophases until senescence, reducing N-NH 4 + in soils, whereas the DOC content increases as a result of the release of root exudates into soils (Hütsch, Augustin, and Merbach 2002). In fact, up to 20 percent of the photosynthetically fixed C is released by roots into the soil during the vegetation period as C-rich rhizodepositions, which play several functions in both plant nutrition and soil ecology (Hütsch, Augustin, and Merbach 2002). ...
... This relationship could be related to the plant uptake and development: the plant uptake increases with increasing phenophases until senescence, reducing N-NH 4 + in soils, whereas the DOC content increases as a result of the release of root exudates into soils (Hütsch, Augustin, and Merbach 2002). In fact, up to 20 percent of the photosynthetically fixed C is released by roots into the soil during the vegetation period as C-rich rhizodepositions, which play several functions in both plant nutrition and soil ecology (Hütsch, Augustin, and Merbach 2002). Regarding N-NH 4 + , the i norganic nitrogen input from melting snowpack represents a large part of the annual atmospheric nitrogen input in snowbed plant communities (e.g., Bowman 1992;. ...
The snowbed habitats represent a relevant component of the alpine tundra biome, developing in areas characterized by a long-lasting snow cover. Such areas are particularly sensitive to climate changes, because small variations in air temperature, rain, and snowfall may considerably affect the pedoclimate and plant phenology, which control the soil C and N cycling. Therefore, it is fundamental to identify the most sensitive abiotic and biotic variables affecting soil nutrient cycling. This work was performed at seven permanent snowbed sites belonging to Salicetum herbaceae vegetation community in the northwestern Italian Alps, at elevations between 2,686 and 2,840 m.a.s.l. During a four-year study, we investigated climate, pedoclimate, floristic composition, phenology, and soil C and N dynamics. We found that lower soil water content and earlier melt-out day decreased soil N-NH 4 + , N-NO 3 − , dissolved organic carbon (DOC), dissolved organic nitrogen (DON), total dissolved nitrogen (TDN), microbial nitrogen (Nmicr), microbial carbon (Cmicr), and C:Nmicr ratio. The progression of the phenological stages of Salix herbacea reduced soil N-NH 4 + and increased DOC. Our results showed that the snow melt-out day, soil temperature, soil water content, and plant phenological stages were the most important factors affecting soil biogeochem-ical cycles, and they should be taken into account when assessing the effects of climate change in alpine tundra ecosystems, in the framework of long-term ecological research. ARTICLE HISTORY
... The assessment of rhizodeposition in terms of the amounts of compounds excreted by roots and their chemical identification, the release sites, their fate, and their impact still remains a relevant challenge faced by scientists [88,89]. Recently, Oburger et al. [90] developed a new rapid method for determining total carbon concentrations in root exudates of grass species by using spectrophotometry. ...
... To address these hypotheses, we sampled decade-old bioenergy field experiments from two locations with similar climates, identical management, and contrasting soil textures to conduct a year-long laboratory incubation experiment. We assessed the production, chemical composition, and persistence of microbial residues using 13 C-labeled glucose additions as a proxy for easily assimilable sugars that dominate root exudates (Hütsch et al., 2002). As our incubations did not contain plant or root material, all cycling of labeled C can be attributed to microbial dynamics. ...
... The large concentration gradient of LMW organic compounds between the cytosols of epidermal root cells and the rhizosphere promotes outward diffusion of organic compounds. Plants can release about 5─10% of net fixed C as exudates and sugars making up the largest component of the exudate pool with a significant proportion as glucose (~ 40 − 50%) (Hütsch et al. 2002;Jones et al. 2004). Low molecular weight organic anions are usually present in the form of di-and tri-carboxylate anions such as malate, citrate, and oxalate. ...
Background and aims
The amount and type of root exudates can influence P availability in the rhizosphere directly by desorption or dissolution of soil minerals, or indirectly by decomposition of soil organic matter (SOM). This study aimed to determine the mechanisms by which specific root exudates influence the distribution and availability of P in soils with low P availability.
Methods
Water, glucose, alanine, and oxalate were delivered through a simulated root into soils for 15 days. Zymography and planar optodes were used to image potential phosphatase activity, and O2 and pH distribution, respectively. Soils were analyzed for resin extractable inorganic P (Pi), dissolved organic C (DOC), water soluble Fe, and Al, and microbial community structure. Characterization of SOM and P were conducted using ultra-high resolution mass spectrometry and ³¹P solution nuclear magnetic resonance (NMR), respectively.
Results
The addition of oxalate resulted in the greatest resin extractable Pi, DOC, and water-soluble Fe, and Al compared to the other exudates suggesting destabilization of mineral associated organic matter (MAOM) and release of organic P (Po). Both ³¹P solution NMR and ultra-high resolution mass spectrometry analysis provided evidence of mineralization of Po released from the destabilization of MAOM.
Conclusion
The study demonstrates the important role microbial and plant-derived metal chelating ligands play in destabilizing MAOM, releasing SOM and importantly Po, that when mineralized may contribute to increasing Pi availability in soils with low P availability.
... In mature broadleaf mixed forests, plants allocate less carbon to transport roots and direct a relatively higher proportion of carbon to absorptive roots in response to higher nutrient availability [48,49]. Six et al. [50] reported that the decomposition of dead roots affected aggregate stability; however, the amount of root exudates and the stimulation of soil microorganisms are related to the plant species [51]. This also explains the significant differences in soil aggregate stability observed among different forest stand types. ...
In soils, high aggregate stability often represents higher quality and anti-erosion ability; however, few studies have systematically analyzed how different forest stands affect soil aggregate stability. We selected five typical mixed forest stands on Jinyun Mountain in Chongqing, China, as research sites to evaluate soil aggregate stability. Within these sites, we analyzed the factors influencing soil aggregate stability in different stands by measuring soil characteristics and root traits. Soil aggregation stability, plant root traits, and soil properties varied among the mixed forest stands. The broadleaf tree mixed forest improved soil aggregate stability by 57%–103% over that of the Pinus massoniana mixed forest. The soil organic carbon, cation exchange capacity, Fe-Al oxides, and fine root proportion were positively correlated with soil aggregate stability. The specific root length and very fine root proportion were negatively correlated with soil aggregate stability, whereas the fine root proportion was positively correlated with this property. Specifically, we found that arbuscular mycorrhizal fungi did not affect soil aggregate stability in acid rain areas. Structural equation modeling indicated that soil aggregate stability was closely related to soil physicochemical properties and plant root characteristics. Predictive factors accounted for 69% of the variation in mean weight diameter, and plant root traits influenced soil aggregate stability by affecting soil organic matter, texture, and Fe-Al oxides. This study elucidated the impact of soil physicochemical properties and plant root characteristics on soil aggregate stability in different forest stand types, which has crucial implications for optimizing the management of various forest types.
... The rhizosphere, an area of soil in the vicinity of 1-2 mm from plant roots, has rich microbial diversity, making it closely related to biogeochemical processes [3,4]. Root exudates are composed of carbohydrates, amino acids, fatty acids, enzymes, and other organic compounds that affect soil's physicochemical properties, creating differences in the microbial composition and activity in the vicinity of the roots [5][6][7]. The rhizosphere's function is an important subject of the study of plant-soil interactions in different ecosystems [8]. ...
The rhizosphere microenvironment is crucial to plant–soil physiological processes. The differences among microbial communities in the rhizosphere and non-rhizosphere peatland topsoil (0–15 cm) and subsoil (15–30 cm) in five plant communities dominated by Carex schmidtii, Chamaedaphne calyculata, Ledum palustre, Betula fruticosa, and Vaccinium uliginosum, as well as non-rhizosphere soil in discontinuous and continuous permafrost regions, were studied. We found that the bacteria and nifH gene abundances in the C. calyculata rhizosphere soil in the discontinuous permafrost region were higher than those in continuous permafrost region, while the nirK and nifH gene abundances in the non-rhizosphere soil of the discontinuous permafrost region were lower than those in the continuous permafrost region. The ratio of bacteria to fungi decreased and that of nirK to nirS increased significantly from the discontinuous to the continuous permafrost region, indicating that permafrost degradation can change soil microbial community composition. Fungal abundance was higher in the rhizosphere than the non-rhizosphere soils, suggesting that plant roots provide a more suitable environment for fungi. Moreover, the abundances of the topsoil bacteria; the fungi; and the nirK, nirS, and nifH genes were higher than those in the subsoil because of the organic matter from plant litter as a source of nutrients. The microbial abundance in the subsoil was also more affected by nutrient availability. To sum up, the microbial abundance varied among the different types of rhizosphere and non-rhizosphere soils, and the carbon and nitrogen cycling processes mediated by soil microorganisms may be greatly altered due to permafrost degradation under climate warming.
... Plants release root exudates [44,45] to influence soil bacterial communities and attract certain bacteria to the rhizosphere [46,47]. However, the changes in root exudate composition induced by bacteria involved in plant growth remain poorly understood. ...
The use of biological inputs is an interesting approach to optimize crop production and reduce the use of chemical inputs. Understanding the chemical communication between bacteria and plants is critical to optimizing this approach. Recently, we have shown that Sphingomonas (S.) sediminicola can improve both nitrogen supply and yield in pea. Here, we used biochemical methods and untargeted metabolomics to investigate the chemical dialog between S. sediminicola and pea. We also evaluated the metabolic capacities of S. sediminicola by metabolic profiling. Our results showed that peas release a wide range of hexoses, organic acids, and amino acids during their development, which can generally recruit and select fast-growing organisms. In the presence of S. sediminicola, a more specific pattern of these molecules took place, gradually adapting to the metabolic capabilities of the bacterium, especially for pentoses and flavonoids. In turn, S. sediminicola is able to produce several compounds involved in cell differentiation, biofilm formation, and quorum sensing to shape its environment, as well as several molecules that stimulate pea growth and plant defense mechanisms.
... Thus, here, the C input by litter and SOM decomposition may be negligible (data not shown). The soil C sequestration capacity is mainly associated with C storage as organic matter, which might enter the soil via litter or root exudates (Hütsch et al., 2002). This C sequestration capacity, which can be measured in long-term studies, may increase in eCO 2 systems via soil microorganisms (Drigo et al., 2008;Koyama et al., 2018). ...
Rising atmospheric carbon dioxide [CO2] is a main climate change driver, and soil respiration is the most relevant contributor to ecosystem respiration. However, the soil microbiome and respiration responses of semiarid agroecosystems under elevated [CO2] (eCO2) conditions must be better understood. In particular, peanut agroecosystems host rhizobia and arbuscular mycorrhizal fungi (AMF) associations. Here, we sought to address the following questions: a) Does eCO2 conditions (650 μmol CO2 m− 2 s− 1, +250 μmol CO2 m− 2 s− 1, aCO2, control) alter soil chemical properties, soil microbial community size and composition, soil respiration, and β-Glucosidase activity? b) Are these responses influenced by transient water deficit periods? We conducted this study in a typical West Texas semiarid region (no well-watered treatment and drought was not an experimental factor) during two consecutive growing seasons (May-Oct). We induced the atmospheric CO2 enrichment using field-installed Canopy Evapotranspiration and Assimilation (CETA) systems. Our results showed no consistent significant changes in soil moisture, C: N ratio, total soil microbial EL-FAME abundance, or β-Glucosidase ac- tivity. However, we found that eCO2 increased soil temperature (+1 ◦C), AMF abundance (EL-FAME marker, +49%), and soil respiration (+82%). Our findings suggest that in future semiarid climates, peanut agro- ecosystems may experience: 1) increased soil metabolic activity as a result of increased autotrophic respiration; 2) increased AMF, which could further facilitate plant nutrient and water uptake; and 3) minimal change in the total size of the microbial community and C cycling enzyme activity during the growing season. In this manu- script, we demonstrated that soil respiration and temperature could be indicators of ecosystem productivity and climate feedback. Furthermore, soil organic carbon and AMF were good indicators of poor nutrient soil ecosystem transitional health across well-watered and water-deficit cycles. This study will increase our under- standing of how these changes will affect soil ecology and climate feedback and will provide new insight into the peanut agroecosystem carbon source/sink functioning and productivity in future climates.
... More importantly, nutrient cycling, disease suppression, and control of neighbors and relatives by acids, antibiocins, and other allelochemicals or antibiotics required by plants occur immediately adjacent to roots due to root exudates and metabolic products of symbiotic and pathogenic communities of microorganisms. When they are not capable of producing enough of these substances and/or organic acids break very quickly, LAB become less dominant in that soil zone (Bais et al., 2006;Chen and Aviad, 2015;Grayston et al., 1998;Hütsch et al., 2002;Ingham and Rollins, 2008;Walker et al., 2003). It is reported that LAB in a given soil depend very much on available plant species and the environmental conditions they create in that soil (temperature, moisture, exudates, etc) (Grayston et al., 1998). ...
Lactic acid bacteria (LAB) are ubiquitous, Gram-positive, probiotic, and facultative aerophilic microorganisms. They are commonly found in wide range of environments including food-rich environments, decaying plants, milk products, the human gut, vaginal flora, and on the skin of various living organisms. These multifaceted bacteria have multiple roles including promoting food safety; promoting plant growth; improving soil, animal, and human health. They are also an integral part of sustainable farming strategies with a low risk of resistance to chemical pesticides, making them an environmentally friendly and effective way of managing pests and diseases and improving plant production. While the traditional role of LAB in food processing and human health sectors has been widely studied and documented, there is increasing attention to their additional roles that empirical evidences and research have validated such as serving as biofertilizers, biocontrol, and biostimulant agents in plant production through the production of bacteriocins, organic acids, and other compounds. However, there is still a gap in unlocking the relationship between LAB, soil, and plant hosts. Through a review of the literature and metadata, this study aims to discuss the less-explored relevance of LAB in soil-plant systems and spotlight the prospects for increased acceptance as sustainable and safe soil and plant health enhancers. The first part concentrates on analysing the existing metadata, while the discussion part essentially focus on literature review.
... Therefore, labile carbon from plants can be employed to improve plant growth via enhanced SOM mineralization in nutrient-deficient agricultural soil (Billings et al. 2010). Plant rhizodeposition is an important source of labile carbon (Hütsch et al. 2002). C release varies with the plant age and photo assimilate sink strength. ...
A major obstacle to the creation of efficient biobased disease management practices continues to be the poor integration of traditional agricultural practices and cutting-edge technical approaches. The present review will expand the understanding of organic amendments and metabolites-mediated microbial community metabolism and their mechanistic aspects in disease-suppressive soil (DSS). Organic amendments have been shown to promote the biocontrol potential of resident soil microbiota. Organic amendments positively affect the labile carbon, cation exchange content (CEC) and microbial enzymatic activity. DSS is considered a rich source of beneficial soil microbial community that produces a plethora of antibacterial metabolites. Multiple gene clusters associated with known metabolites offer mechanistic insights associated with disease-suppressive phenotypes. Organic amended soil has higher abundance of chemotaxis genes. Several strains of Bacillus and Pseudomonas produce key metabolites, phenazines, 2,4-diacetylphloroglucinol, pyoluteorin, pyrrolnitrin, cyclic lipopeptides and volatile organic compounds in DSS. High-resolution metagenomics combined with bioinformatics tools would be instrumental in the identification of biomarkers associated with suppressive soils. The integration of traditional and genomic approaches can be employed to infer the untapped potential of resident soil microbiomes.
... Several other reasons might explain the differences in C transfer into the soil of the different genotypes. Hütsch et al. [45] showed that cereal root exudates, which are 80% water-soluble (64% carbohydrates, 22% amino acids and 14% organic acids), are rapidly (1-2 days) stabilized into water-insoluble forms and bound preferentially to clay particles. Warembourg and Estelrich [46] also indicated that only 13% to 21% of total root exudates are found in the soil matrix as water-insoluble or stable organic matter. ...
The transfer of atmospheric carbon (C) in soils is a possible strategy for climate change mitigation and for restoring land productivity. While some studies have compared the ability of existing crops to allocate C into the soil, the genetic variations between crop genotypes have received less attention. The objective of this study was to compare the allocation to the soil of atmospheric C by genetically diverse wheat genotypes under different scenarios of soil water availability. The experiments were set up under open-field and greenhouse conditions with 100 wheat genotypes sourced from the International Maize and Wheat Improvement Centre and grown at 25% (drought stressed) and 75% (non-stressed) field capacity, using an alpha lattice design with 10 incomplete blocks and 10 genotypes per block. The genotypes were analyzed for grain yield (GY), plant shoot and root biomass (SB and RB, respectively) and C content, and stocks in plant parts. Additionally, 13C pulse labeling was performed during the crop growth period of 10 selected genotypes for assessing soil C inputs. The average GY varied from 75 to 4696 g m−2 and total plant biomass (PB) from 1967 to 13,528 g m−2. The plant C stocks ranged from 592 to 1109 g C m−2 (i.e., an 87% difference) under drought condition and between 1324 and 2881 g C m−2 (i.e., 117%) under well-watered conditions. Atmospheric C transfer to the soil only occurred under well-drained conditions and increased with the increase in the root to shoot ratio for C stocks (r = 0.71). Interestingly, the highest transfer to the soil was found for LM-26 and LM-47 (13C/12C of 7.6 and 6.5 per mille, respectively) as compared to LM-70 and BW-162 (0.75; 0.85). More is to be done to estimate the differences in C fluxes to the soil over entire growing seasons and to assess the long-term stabilization of the newly allocated C. Future research studies also need to identify genomic regions associated with GY and soil C transfer to enable the breeding of “carbon-superior” cultivars.
... The majority of the plant-microbe interactions occur in rhizosphere: a narrow zone of soil surrounding the roots of living plants. The plant roots deposit carbon-containing organic compounds, including sloughed-off root cells and tissues, water-soluble and volatile compounds, and mucilages, collectively known as rhizodeposits (Hütsch et al. 2002). The rhizodeposits promote microbial life in the rhizosphere (Tian et al. 2020), which results in altered and, often, enhanced microbial communities compared to the non-rhizosphere (bulk) ...
Irrigated and rain-fed rice fields are unique agroecosystems and anthropogenic wetlands whose main feature is seasonal flooding. Flooded soils are characterized by spatiotemporal shifts and oscillation of the oxygen status and redox potential, sustaining varieties of microbial metabolisms, where bacteria and methanogenic archaea play principal roles and thus have been the major research targets. In this review, we focus on the diversity and ecology of protists—often overlooked biological entities—in wetland rice field soils. Protists with different ecological functions, i.e., phagotrophs, phototrophs, saprotrophs, and parasites, inhabit a rice field soil with a community- and individual-level adaptation to the wide range of oxygen tensions and redox potential. Other agricultural managements like fertilization and char application also influence the protist community. They link to the material cycling in rice soil and affect the activities and community composition of the microorganisms involved in the biogeochemical cycles. Rice roots are the hot spot for protists, which control the rhizospheric bacterial community and could increase the plant productivity through enhancing nutrient release and altering bacterial activities. This review highlights the essential roles of protists in a wetland rice field soil and needs for further research to fill the gaps in knowledge regarding the diversity and functions of the protists in this unique agroecosystem.
... In the case of pMFCs, rhizodeposits are the main feed for electrifying bacteria. Of all the photosynthetic carbon fixed by different plant species, almost 20-40% is released into soil in various forms collectively known as rhizodeposits [79,80]. Rhizodeposits mainly include dead cells, secretions, sloughed-off root cap and border cells, root exudates, lysates, mucilage, and gases. ...
The review considers a new direction of bioelectrochemical systems – plant-microbial fuel cells (pMFC to generate environmentally friendly bioelectricity and wastewater treatment. Electroactive bacteria (EAB), exoelectrogens, serves as a crucial catalytic machinery in pMFC, which works by from reducing waste streams organic/inorganic substrates. The pMFC relies on the rhizospheric zone to power the exoelectrogens at the anodic compartment with the excretion of rhizodeposits with concomitant bioelectricity production. The pMFC's can be capable of the continuous output of bioelectricity without depending on the biomass (biofuels) by utilizing the waste streams of plant carbon sources, i.e., rhizodeposition, for their functioning. The review provides a detailed description of pMFC features such as the basis, the function of plants and the rhizodeposits released by them, electrical characteristics, internal resistances, substrate kinetics and redox reactions of the root environment, mechanisms of electron transport, a description of the microbial community capable of electrogenesis; presents the most common pMFC designs – tubular and flat-plate, their characteristics and comparison. At the end of the review, the issues of pMFC technology application for wireless energy-neutral sensing, next-generation farming and agricultural applications are highlighted. Overall, the present review is a correctly timed energy-concept based nut-shell by showcasing the possible integration of pMFCs with bioelectricity production, wastewater treatment, CO2 sequestration, and industrial commodities production capabilities. PMFCs have promising potential for inclusion in many artificial ecosystems like agricultural fields, agroecosystems, constructed wetlands, and natural ecosystems.
... Microbial decomposition of organic matter is the most fundamental process to release nutrients into soil solution (Schimel and Bennett 2004), but the activity of the soil microbial community is often limited by the availability of labile carbon (Hütsch et al. 2002). Labile carbon is dominantly provided by roots as organic root exudates like sugars, organic acids and amino acids. ...
Aims
We investigated the role of plants and their plant-derived carbon in shaping the microbial community that decomposes substrates and traced the return of nutrients from decomposition back to plant shoots in order to understand the importance of plants for ecosystem element cycling.
Methods
We performed a greenhouse experiment having plant communities with and without arbuscular mycorrhizal fungi (AMF) and ingrowth cores that held different ¹⁵ N labeled substrates. We determined the microbial community structure using molecular sequencing and the net assimilation of plant carbon into soil microorganisms using a ¹³ CO 2 pulse and ¹³ C measurements of microbial biomarkers. We determined the return of nitrogen back to the shoots using the ¹⁵ N signal, which was provided from the decomposition of the substrate added to the ingrowth cores.
Results
We observed that the microbial community composition in the ingrowth cores and their net ¹³ C assimilation depended on the presence of AMF and the added substrate. Both plant communities had similar ¹⁵ N uptake into their shoots, but the net N uptake cost was significantly lower in presence of AMF. In the presence of AMF also lower net N uptake cost was observed for the decomposition of plant-derived and microorganism-derived substrates compared to inorganic nitrogen suggesting that AMF actively controls the decomposer comunity and their carbon demand.
Conclusion
Our results identify for the first time a functional overlap of soil microorganisms as identical substrate is decomposed by different microorganisms suggesting functional redundancy of microbial communities. In consequence a better understanding of ecosystem element cycling can only be achieved when the whole plant-microorganism-organic matter-soil continuum is investigated.
... Bakhshandeh, et al. [36], for example, demonstrated that differences between root/shoot ratios of wheat genotypes are an indicator for differences in below-ground carbon disposition. Furthermore, below-ground carbon inputs are not only driven by root and stubble biomass, but also by rhizodeposition [37], which can account for more than 50% of plant-derived soil carbon inputs of cereals [38]. The amount, chemical composition and associated microbial turnover of the rhizodeposition varies between cultivars just as root biomass does [39,40]. ...
Carbon sequestration has been proposed as a way to mitigate the impact of CO2 on the
climate. At the COP21, the ‘4 per 1000 Soils for Food Security and Climate’ initiative was launched with the goal to increase global soil organic carbon (SOC) stocks by 4‰ per year. The Thyrow long-term field experiment DIV.2 was chosen to determine the feasibility of this 4 per 1000 goal under the dry and sandy conditions in Eastern Germany. The effects of different fertilizing regimes on SOC contents and winter rye yields were
investigated. Winter rye is a representative crop for the region and grown as a monoculture in the experiment. The 4 per 1000 goal was achieved in all treatments including the unfertilized control, although ploughing takes place and straw is removed every year. The highest carbon sequestration of up to 0.5 t ha−1 a−1 was provided by a combination of mineral and manure fertilization. In three out of four years, no yield difference was observed between mineral-only fertilization (120 kg ha−1 N) and a combination of mineral and organic N (97.4 kg ha−1 plant available N) fertilization. Yields increased over the years in the treatment with pure organic N and decreased in all other treatments.
... Second, vegetation appears to play a major role in soil organic matter decomposition (Faz Cano et al., 2002). HT likely reduced litterderived C and rhizodeposit C inputs (mostly O-alkyl C) by suppressing litterfall and fine root biomass (Hütsch et al., 2002;Li et al., 2020). Furthermore, HT could reduce the labile organic matter decomposition (e.g., O-alkyl C), which was likely attributed to the nutrient limitation under heavy thinning. ...
Forest thinning is a common forest management practice and has complex effects on soil organic carbon (SOC). However, the mechanisms underlying SOC and its components (physical fractions and chemical compositions) in response to forest thinning remain poorly understood. Four thinning treatments including control (CK with no thinning), light thinning (20% of basal area removed, LT), moderate thinning (40% of basal area removed, MT), and heavy thinning (60% of basal area removed, HT) were applied in a pine plantation. After nine years of thinning, SOC physical fractions and chemical compositions, soil heterotrophic respiration, plant biomass, soil properties, soil microbial biomass, and soil enzyme activity were measured. The results showed that HT significantly reduced SOC by an average of 39.15% relative to CK, which was likely due to the decreases in litter production, soil nutrients, and fine root biomass. Compared with CK, HT significantly reduced particulate organic carbon (POC), which was correlated with the decreases in soil nutrients and microbial biomass nitrogen. Moreover, MT and HT significantly reduced the percentage of alkyl-C and alkyl-C: O alkyl-C, whereas only HT increased the percentage of labile functional group of O alkyl-C. The changes in SOC chemical compositions were related to plant biomass and the decomposition of soil labile organic matter. Collectively, combining SOC fractions, plant biomass, and microbial decomposition could provide a comprehensive understanding of SOC dynamics in response to thinning in managed forests. We recommend that less than 60% thinning intensity would be reasonable in view of increasing forest productivity and maintaining soil organic carbon.
... Although soil glucosidase activity is considered a good indicator of management-induced changes in SH (Bandick & Dick, 1999;Stott, Andrews, Liebig, Wienhold, & Karlen, 2010), it may not be sensitive enough for differentiating the effects of management variations on microbial activity in soils and rhizospheres of AF practices. The substrate for glucosidase, cellobiose, is a product of cellulose decomposition, and ~80% of some plant root exudates are water-soluble carbohydrates, organic acids, and amino acids (Hütsch, Augustin, & Merbach, 2002). Because of the wide C/N ratio of these root exudates, rapid metabolism by the proliferating rhizosphere microbial community results in temporary N immobilization before synthesis of extracellular enzymes can mineralize N from organic materials and the dead microbial biomass (Paterson, 2003;Schenck zu Schweinsberg-Mickan, Jörgensen, & Müller, 2012). ...
Agroforestry (AF) is an intensive land management practice where trees and shrubs are intentionally integrated into crop and livestock management practices to optimize benefits arising from biophysical interactions among the components. The five main AF practices are: riparian buffers, alley cropping, windbreaks, silvopasture, and forest farming. Due to the interactions among the components, AF provides numerous ecosystem services including soil health (SH) benefits. This chapter highlights the benefits of AF practices on SH including soil carbon (SC), physical properties, biological properties, chemical properties, decontamination, biodiversity and ecosystem services (ES) provided by improved SH of AF.
Agroforestry was approved by both the afforestation and reforestation programs and under the Clean Development Mechanisms of the Kyoto Protocol for carbon sequestration (CS). Riparian buffers reduce sediment losses, filter nutrients, retain those within the watershed, and thereby improve SH and land productivity. Windbreaks, riparian buffers, alley cropping, and silvopasture improve SC, and soil physical, chemical, and biological properties. Multi-species AF practices promote greater soil biodiversity and influence nutrient cycling, CS, decontamination, soil reactions and land productivity as compared to less diverse management practices. Adoption of agroforestry improves SH and thereby enhances supporting, provisional, regulating, and cultural ES. Although research findings are limited on SH benefits of AF for the temperate regions, existing limited literature suggests that integration of AF significantly enhances SH. In the future, data from standardized experimental designs and management plans could be extensively evaluated to identify SH indicators for developing a ranking system to use as guidelines for the establishment of AF for improved SH and ES.
Intensification of crop sequence (ICS) has been proposed as a key field practice to preserve soil health and achieve more sustainable agricultural systems. However, the effects of ICS are site‐specific and vary according to soil characteristics, climatic conditions, the duration of the crop sequencing and the types of crop involved. Soil aggregate stability (AS) and soil organic carbon (SOC) stock are useful indicators of soil health as they are closely linked to diverse soil services and functions and are sensitive to management practices. We performed a meta‐analysis of 33 studies to analyse the impact of ICS on SOC stock and AS in field experiments in the central‐eastern region of Argentina. Our results showed that ICS increased SOC stock and AS, with an overall mean change of 7% and 22%, respectively. Fine‐textured soils showed the greatest SOC stock increase (12%), comparable to the increase observed in coarse‐textured soils (11%); in medium‐textured soils this increase was less than half (5%). Coarse‐textured soils had the greatest increase in AS (32%), followed by medium‐textured and fine‐textured soils, which also showed notable improvements (25% and 19%, respectively). Greater diversity of crops resulted in larger increases in both AS and SOC. ICS generated a larger increase of SOC stock in the soil surface (0–10 cm) than in the subsurface (10–20 cm), whereas the opposite was found for AS. Long‐term studies (≥9 years) had the greatest effect on AS and SOC stock. Regression analysis revealed that the initial carbon stock influenced SOC stock results following ICS, increases being greater when initial carbon stock contents were smaller. Introducing gramineous species into the crop sequence was associated with a greater improvement of AS and SOC stock. Finally, the mean rate of carbon sequestration from ICS in all the studies amounted to 0.28 Mg ha ⁻¹ yr ⁻¹ . Overall, ICS is a useful strategy for improving SOC storage and AS in this region, though results may vary according to soil characteristics and management practices.
Maize and cowpea are among the staple foods most consumed by most of the African population, and are of significant importance in food security, crop diversification, biodiversity preservation, and livelihoods. In order to satisfy the growing demand for agricultural products, fertilizers and pesticides have been extensively used to increase yields and protect plants against pathogens. However, the excessive use of these chemicals has harmful consequences on the environment and also on public health. These include soil acidification, loss of biodiversity, groundwater pollution, reduced soil fertility, contamination of crops by heavy metals, etc . Therefore, essential to find alternatives to promote sustainable agriculture and ensure the food and well-being of the people. Among these alternatives, agricultural techniques that offer sustainable, environmentally friendly solutions that reduce or eliminate the excessive use of agricultural inputs are increasingly attracting the attention of researchers. One such alternative is the use of beneficial soil microorganisms such as plant growth-promoting rhizobacteria (PGPR). PGPR provides a variety of ecological services and can play an essential role as crop yield enhancers and biological control agents. They can promote root development in plants, increasing their capacity to absorb water and nutrients from the soil, increase stress tolerance, reduce disease and promote root development. Previous research has highlighted the benefits of using PGPRs to increase agricultural productivity. A thorough understanding of the mechanisms of action of PGPRs and their exploitation as biofertilizers would present a promising prospect for increasing agricultural production, particularly in maize and cowpea, and for ensuring sustainable and prosperous agriculture, while contributing to food security and reducing the impact of chemical fertilizers and pesticides on the environment. Looking ahead, PGPR research should continue to deepen our understanding of these microorganisms and their impact on crops, with a view to constantly improving sustainable agricultural practices. On the other hand, farmers and agricultural industry players need to be made aware of the benefits of PGPRs and encouraged to adopt them to promote sustainable agricultural practices.
Aims
N rhizodeposition plays a vital role in regulating rhizosphere soil N dynamics. However, its effects on the rhizosphere soil N cycle in clonal plants and the underlying mechanism remain unclear.
Methods
¹⁵N solutions with different concentrations were injected into young or connected mature ramets of Moso bamboo (Phyllostachys edulis), respectively, in the field, to explore the response of rhizosphere N transformation to N rhizodeposition.
Results
¹⁵N injection increased the ¹⁵N abundance in the rhizosphere soil of both young and mature ramets, with higher ¹⁵N abundance in 20–40-cm layer than that in 0–20-cm layer. Functional genes involved in nitrification and denitrification in the rhizosphere were upregulated when young ramets received low- and medium-N solutions. However, the genes showed diverse responses to high-N treatment in young ramets, with increased abundances of ammonia-oxidizing bacteria and decreased levels of ammonia-oxidizing archaea, comammox, and denitrifiers. In contrast, the functional genes involved in denitrification in the 20–40-cm layer of rhizosphere soil were upregulated by high-N treatment in mature ramets. Additionally, partial-least-squares path modeling revealed ¹⁵N abundance in the soil indirectly affected N mineralization, N mineralization directly affected nitrification, and nitrification directly affected denitrification in a positive manner.
Conclusion
The variations of N rhizodeposition and transformation process in both connected ramets is highly consistent in responding to different N treatments, but the nitrifiers and denitrifiers in the deeper rhizosphere soil were more sensitive to N rhizodeposits than those in the upper rhizosphere soil.
The priming effect (PE) is the short-term increase or decrease in the rate of soil organic matter mineralization in
response to a stimulus, such as the addition of carbon (C) and/or nitrogen (N) to the soil. Literature has generally
framed the PE in terms of CO2 evolved from soil organic carbon mineralization, but fewer publications have
focused on how the PE affects the soil N cycle and nitrous oxide (N2O) production from soil organic N miner�alization (SOM-N), despite the potency of N2O as a greenhouse gas and ability to destroy stratospheric ozone.
This review summarizes our current understanding of how the PE can alter the rates of SOM-N mineralization
and subsequently amplify, diminish, or maintain N2O production in and release from soils, henceforth referred to
as N2O priming. Additionally, the concept of process priming, the differential augmentation of N2O-producing
processes (e.g. priming of nitrification) is introduced. Diverse results across studies suggest that the mechanisms
of N2O priming cannot be fully explained by a single hypothesis, and it is currently unclear how significant the
contribution of N2O priming to net N2O emissions is, but a preliminary estimate suggests that N2O emissions
resulting from priming mechanisms can range from -39 – 76% following C and N amendments compared to a
control. To disentangle the complexity of N2O priming, an expansion of current research efforts is required. The
promotion of open data sharing and publication of full datasets will facilitate the development and validation of
models that can accurately simulate the complexity of soil N dynamics and account for the feedback effects of
climate change on N2O priming, which is a key research gap. This is particularly the case in under-studied areas
such as permafrost-affected soils of arctic, subarctic, and alpine regions, and vulnerable tropical regions, where
climate warming may amplify N2O priming.
Carya cathayensis is an important economic nut tree that is endemic to eastern China. As such, outbreaks of root rot disease in C. cathayensis result in reduced yields and serious economic losses. Moreover, while soil bacterial communities play a crucial role in plant health and are associated with plant disease outbreaks, their diversity and composition in C. cathayensis are not clearly understood. In this study, Proteobacteria, Acidobacteria, and Actinobacteria were found to be the most dominant bacterial communities (accounting for approximately 80.32% of the total) in the root tissue, rhizosphere soil, and bulk soil of healthy C. cathayensis specimens. Further analysis revealed the abundance of genera belonging to Proteobacteria, namely, Acidibacter, Bradyrhizobium, Paraburkholderia, Sphaerotilus, and Steroidobacter, was higher in the root tissues of healthy C. cathayensis specimens than in those of diseased and dead trees. In addition, the abundance of four genera belonging to Actinobacteria, namely, Actinoallomurus, Actinomadura, Actinocrinis, and Gaiella, was significantly higher in the root tissues of healthy C. cathayensis specimens than in those of diseased and dead trees. Altogether, these results suggest that disruption in the balance of these bacterial communities may be associated with the development of root rot in C. cathayensis, and further, our study provides theoretical guidance for the isolation and control of pathogens and diseases related to this important tree species.
Biochar is often used as an amendment to enhance soil fertility by directly increasing soil pH and nutrient availability. However, biochar may also improve soil fertility indirectly by altering the succession of bacterial communities that, in turn, may alter nutrient supply and availability. To determine how biochar affects soil bacterial richness and diversity, as well as how bacterial communities respond to biochar across space and time, we studied the rhizosphere and bulk soils of potted barley plants for 2 years. Adding biochar significantly increased bacterial community richness (Chao 1 richness index) by the end of the second year in the rhizosphere (P = 0.037), but in bulk soils, we observed an increase in richness in Year 1 that dissipated by Year 2. In contrast to richness, adding biochar only had a significant effect on bacterial community diversity (Shannon diversity index) in Year 1 seedling stage (P < 0.001), but the effect dissipated thereafter. We also found that adding biochar increased the relative abundances of Actinobacteria and Proteobacteria but decreased the relative abundances of Acidobacteria and Chloroflexi, suggesting these communities were sensitive to biochar inputs. The biochar-sensitive genera belonging to Actinobacteria and Proteobacteria made up 45%–58% of sensitive taxa in both rhizosphere and bulk soils. Of the Proteobacteria sensitive to adding biochar, Nitrosospira and Sphingomonas were most abundant in the rhizosphere relative to bulk soils. However, despite the initial increase of biochar sensitive responders in the rhizosphere, their numbers decreased after 2 years and had 179 fewer genera than bulk soils. Our findings suggest the effect of adding biochar was relatively short-lived and that the influence of the plant phenology was a stronger driver of bacterial community change than biochar inputs 2 years after its application. Altogether, the succession of soil bacterial community structure reflected changes in the soil environment induced by the combined effect of biochar, rhizospheric inputs, and plant phenology, suggesting that changes in microbial community composition observed after amending soils with biochar, may also contribute to changes in soil fertility.
Carbon (C) derived from roots, rhizodeposition of living roots and dead root decomposition, plays a critical role in soil organic C (SOC) sequestration. Recent studies suggest that root inputs exert a disproportionate influence on SOC formation, and therefore, it is necessary to test separately the effects during root growth (i.e. rhizode-position) and dead root decomposition (i.e. belowground litter) under natural conditions. A field experiment was carried out in grasslands with three typical plants: Stipa bungeana (St.B), Artemisia sacrorum (Ar.S), and Thymus mongolicus (Th.M) to differentiate the effects of root growth vs decomposition on C inputs by using in-growth soil cores and litter bag methods. For root growth experiment, the SOC content increased (2.5-3.8 g⋅kg − 1) with root biomass, especially in soil under Ar.S. C input increased mineral-associated organic C (MAOC%) from 57% to 65%. The SOC δ 13 C was consistent with roots δ 13 C, indicating the roots were the primary source of SOC. The root decomposition experiment showed that the increase in SOC (0.74-2.6 g⋅kg − 1) was highest at 90 days. Root decomposition rate was fast before 45 days, and SOC increased with the MAOC% during this period. After 45 days, particulate organic C % (POC%) was raised and was higher than in the control soil. The increase in SOC under root growth (2.5-3.8 g⋅kg − 1) after one year was greater as compared to the rate under root decomposition (-0.4-0 g⋅kg − 1). It suggested that C released during root growth was more effectively retained in the soil than that caused by dead root decomposition. The rise in MAOC during root growth and decomposition mainly explains the increase in SOC. Our results provided direct field based evidence illustrating the specific contribution of root growth and decomposition to SOC.
Different multipurpose tree species integrated into agroforestry may exhibit variable effects on soil characteristics, which not only depend on planted tree species but also on tree management practices being adopted. In the present study, the effect of different tree management practices on soil organic carbon, dehydrogenase activity, microbial biomass carbon and potentially mineralizable nitrogen was assessed in three well-established agroforestry models viz. model 1: crown pruning management [three levels: 0 (unpruned), 50 and 75%] in Albizia procera, model 2: tree density management (three levels: 200, 400 and 800 trees ha⁻¹) in Hardwickia binata, and model 3: in situ soil moisture conservation (SMC) measures [four levels: normal planting (control), stone mulch, deep basin and deep basin + deep ploughing] in Emblica officinalis-based agroforestry. The aim was to determine (1) whether tree management practices have any effect on soil biological properties, and (2) what levels of these management practices are desirable in selected agroforestry systems. We hypothesized that these practices will improve soil in terms of biological properties, with moderate levels being more beneficial. For the purpose, soil samples were collected from two sampling locations [rhizosphere (> 1.5 m from tree base) and the non-rhizosphere zone (outside the tree canopy i.e. < 5 m from tree base), and at each location, from two soil depths (0–15 and 15–30 cm). The findings revealed that unpruned trees of A.procera caused the maximum improvement in soil, followed by trees subjected to 50 and 75% crown pruning. The maximum tree density of H. binata (800 trees ha⁻¹) yielded the highest values of the studied soil biological parameters, followed by 400 and 200 trees ha⁻¹. The SMC measures adopted in E. officinalis had variable effects on studied parameters, with deep basin and deep basin + deep ploughing outweighing stone mulch and normal planting. The values of all the parameters were significantly higher in the rhizosphere as well as in the upper soil (0–15 cm). Conclusively, the study suggests that light crown pruning in A. procera, stocking of 400–800 trees ha⁻¹ of H. binata, and deep basin and/or deep basin + deep ploughing SMC measures in E. officinalis may be adopted as desirable management practices for optimum soil biological health in the semi-arid region of Central India.
Legume crop production has many benefits for agricultural systems. Through the rhizodeposition process, they release a significant amount of C and N into the soil, increasing soil organic C and reducing the use of N fertilizer. Rhizodeposition is known as a dynamic process influenced by many factors.
The aim of this study was to study the contribution of root exudation and root senescence to the rhizodeposition of atmospheric C and N during vegetative and reproductive growth in annual and perennial legumes and to understand how this is linked to the fixation capacities of C and N and root functional traits.
An original approach that combined ¹³CO2 labelling and the ¹⁵N dilution method was developed to measure the rhizodeposition of atmospheric C and N throughout plant growth by two annual grain legumes (pea and faba bean) and two perennial forage legumes (white and crimson clovers).
C rhizodeposition was found to increase proportionally with N rhizodeposition during reproductive development and the differences observed between species were related to the C and N fixation abilities. The use of root traits such as specific root length, root tissue density and root dry matter content suggests a strong contribution of root exudation to C rhizodeposition at vegetative growth and a strong contribution of root senescence to both C and N rhizodeposition during reproductive growth.
Synthesis. Both C and N rhizodeposition appeared to be controlled by traits indicative of resource acquisition and root development.
Background
In areas prone to water erosion, crop selection strategies should be based on assessment of their effects on soil structural properties.
Aims
The present study compared the effects of the cultivation of forage maize ( Zea mays L.) and forage oat ( Avena sativa L.) and their cultivars on soil aggregation relative to potato ( Solanum tuberosum L.) or wheat ( Triticum aestivum L.) at a hydrothermally limited site on the Loess Plateau, China.
Methods
The water‐stable aggregate (WSA) distribution in soil was measured under three cultivars in each of maize, oat, wheat, and potato (a total of 12 cultivars from four crops) in their flowering stage of three cropping seasons, when root biomass was largest.
Results
In each year, the water‐stable macroaggregates (>0.25 mm) content and mean weight diameter (MWD) of WSAs in the top 20 cm of soil did not differ between tested cultivars of every crop but increased under maize and oat, compared with those under wheat or potato. The increased soil aggregation under maize and oat, compared with wheat or potato, was consistent with the pattern of change in root biomass but was not consistent with the changes in root length density, root surface area, or root mean diameter across the crops. The water‐stable macroaggregates content and MWD of soil was positively correlated with root biomass across cultivars and crop species within each cropping season.
Conclusions
We suggest that increased root biomass under maize and oat relative to potato or wheat resulted in increased soil aggregation in maize and oat cultivated soils. It is demonstrated that, in areas prone to soil water erosion, planting high‐biomass‐yielding crops such as maize and oat is more beneficial for increasing soil aggregation and stability, compared with low‐biomass‐yielding crops such as wheat or potato.
Nematolojik açıdan tuzak bitki uygulamaları, topraktaki nematod populasyonunu baskılamak amacıyla uygulanabilecek bitki temelli stratejilerden biridir. Tuzaklama stratejilerinde, nematod ve konukçusu arasındaki parazitik ilişki kritik bir öneme sahiptir. Bir alanda tuzak bitkilerin kullanım şekli, tuzak bitki olarak kullanılan bitkinin özelliği ve bu bitkinin imha edilme zamanına bağlı olarak değişmekle beraber; genellikle ana ürün ile aynı zamanda tekli sıralar, çoklu sıralar veya düzensiz dağılım şeklinde ya da ana üründen önce yetiştirilme şeklinde olabilir. Her ne kadar, nematodlar için tuzak bitki olarak bilinen bitki sayısı sınırlı olsa da, nematodun konukçusu ile olan beslenme davranışı da dikkate alındığında, hassas bitkilerin bile tuzaklama amacıyla kullanılabilecek potansiyelde olması, bu uygulamanın bir mücadele stratejisi olarak kullanılabilme potansiyelini artırmaktadır. Özellikle, tuzak bitkilerin hassas bitkiler arasında kısa süreli yetiştirilmesiyle, üretim yapılan alandaki nematod populasyonunun etkili bir şekilde azaldığı ve kendinden sonra yetiştirilen bitkide, belirgin verim artışı sağlandığı bilinmektedir. Ayrıca, kimyasal kullanımının da azalmasına katkı sağlayan bu yaklaşımlar, hem ekonomik olarak daha karlı bir üretimin yapılmasını, hem de çevre ve insan sağlığı için güvenli ve sürdürülebilir bir üretimin gerçekleştirilebilmesini sağlamaktadır. Bu nedenle, kök-ur nematodları ile mücadelede tuzak bitkilerin kullanımının ele alındığı çalışmada, öncelikle kök-ur nematodunun biyolojisi hakkında özet bilgi verilerek, nematod biyolojisi ve tuzaklama stratejileri ile bağlantısı açıklanmaya çalışılmıştır. Tuzaklamanın 2 uygulama şekli olan; tuzak özelliğine sahip bitkilerin kullanıldığı uygulamalar ile hassas bitkilerin tuzak olarak kullanıldığı uygulamalar ve tuzaklama uygulamasını destekleyici stratejiler, günümüze değin yapılan örneklerle derlenerek özetlenmiştir.
Background
Soil aggregation and organic carbon (OC) content are important indicators of soil quality that can be improved with plant residue amendments. The extent of the effects of plant residue amendments on soil aggregation and OC content across different plant residue and soil types is not fully understood.
Aim
In this meta‐analysis, we evaluated the effects of plant residue amendments on soil aggregation and OC content for different plant residues (fresh, charred) and soil types varying in clay content, initial OC content, and pH.
Methods
Our meta‐analysis included 50 published studies (total of 299 paired observations). We estimated the response ratios of mean weight diameter (MWD) and separate aggregate size classes, total soil OC (TSC), and aggregate‐associated OC. We also considered the effect of experimental factors (study duration, residue type, residue amount, initial soil OC, clay content, and pH).
Results
The benefit of plant residue amendment on soil aggregation was larger in soils with initially low OC content and neutral pH. Initial soil OC content and pH were more important than soil clay content for OC storage in soil aggregates. Both fresh and charred plant residue amendments were effective in forming aggregates, whereas charred residues were more effective in increasing TSC. We found only a weak positive relationship between the response ratio of TSC and MWD indicating that other factors besides soil aggregation contributed to the increase in soil C storage.
Conclusions
While plant residue amendments can enhance soil aggregation and TSC, these effects are likely governed by the type of plant residue and soil properties such as the initial soil pH and OC content.
Cadmium is an essential element for plant growth and development. Its accumulation in soil is more hazardous to human and animal health than to plants and microorganisms. A pot greenhouse experiment was conducted to determine the usability of Sinapis alba L. and Avena sativa L. for the phytoremediation of soil contaminated with cadmium and to verify cellulose viability in the remediation of soil under cadmium pressure in doses from 4 to 16 mg Cd2+ kg−1 soil d.m. (dry matter) The effect of cadmium on soil microbiome was investigated with the culture method and the variable region sequencing method. Sinapis alba L. and Avena sativa L. were found viable in the phytoremediation of soil contaminated with Cd2+. Avena sativa L. was more potent to accumulate Cd2+ in roots than Sinapis alba L. Although the fertilization of Cd2+- contaminated soil with cellulose stimulated the proliferation of microorganisms, it failed to mitigate the adverse effects of Cd2+ on bacterial diversity. Bacteria from the Sphingomonas, Sphingobium, Achromobacter, and Pseudomonas genera represented the core microbiome of the soils sown with two plant species, contaminated with Cd2+ and fertilized with cellulose. Stimulation of the growth and development of these bacteria may boost the efficacy of phytoremediation of cadmium-contaminated soils with Sinapis alba L. and Avena sativa L.
Thermogravimetry (TG) is a simple method that enables rapid analysis of soil properties such as content of total organic C, nitrogen, clay and C fractions with different stability. However, the possible link between TG data and microbiological soil properties has not been systematically tested yet, which limits TG application for soil and soil organic matter assessment. This work aimed to search and to validate relationships of thermal mass losses (TML) to total C and N contents, microbial biomass C and N, basal and substrate-induced respiration, extractable organic carbon content, anaerobic ammonification, urease activity, short-term nitrification activity, specific growth rate and time to reach the maximum respiration rate for two sample sets of arable and grassland soils. Analyses of the training soil set revealed significant correlations of TML with basic soil properties such as carbon and nitrogen content with distinguishing linear regression parameters and temperatures of correlating mass losses for arable and grassland soils. In a second stage, the equations of significant correlations were used for validation with an independent second sample set. This confirmed applicability of developed equations for prediction of microbiological properties mainly for arable soils. For grassland soils was the applicability lower, which was explained as the influence of rhizosphere processes. Nevertheless, the application of TG can facilitate the understanding of changes in soil caused by microorganism’s activity, and the different regression equations between TG and soil parameters reflect changes in proportions between soil components caused by land-use management.
Nonphotosynthetic plant metabolic processes are powered by respiratory energy, a limited resource that metabolic engineers—like plants themselves—must manage prudently.
Microbial communities play critical roles in fixing carbon from the atmosphere and fixing it in the soils. However, the large-scale variations and drivers of these microbial communities remain poorly understood. Here, we conducted a large-scale survey across China and found that soil autotrophic organisms are critical for explaining CO2 fluxes from the atmosphere to soils. In particular, we showed that large-scale variations in CO2 fixation rates are highly correlated to those in autotrophic bacteria and phototrophic protists. Paddy soils, supporting a larger proportion of obligate bacterial and protist autotrophs, display four-fold of CO2 fixation rates over upland and forest soils. Precipitation and pH, together with key ecological clusters of autotrophic microbes, also played important roles in controlling CO2 fixation. Our work provides a novel quantification on the contribution of terrestrial autotrophic microbes to soil CO2 fixation processes at a large scale, with implications for global carbon regulation under climate change.
To thrive in nutrient-poor waters, coral reefs must retain and recycle materials efficiently. This review centers microbial processes in facilitating the persistence and stability of coral reefs, specifically the role of these processes in transforming and recycling the dissolved organic matter (DOM) that acts as an invisible central currency in reef production, nutrient cycling, and organismal interactions. The defining characteristics of coral reefs, including high productivity, balanced metabolism, high biodiversity, nutrient recycling efficiency, and structural complexity, are inextricably linked to microbial processing of DOM. The composition of microbes and DOM in reefs is summarized, and the spatial and temporal dynamics of biogeochemical processes carried out by microorganisms in diverse reef habitats are explored in a variety of key reef processes, including decomposition, accretion, trophic transfer, and macronutrient recycling. Finally, we examine how widespread habitat degradation of reefs is altering these important microbe–DOM interactions, creating feedbacks that reduce reef resilience to global change.
Expected final online publication date for the Annual Review of Marine Science, Volume 15 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
There is a lack of research on soil microplastics in arid oases considering the rapid economic development of northwestern China. Here, we studied the occurrence and sources of microplastics in soil, as well as the relationships between microplastics and adsorbed heavy metals in the Ebinur Lake Basin, a typical arid oasis in China. Results showed that (1) the average microplastic content in all soil samples was 36.15 (±3.27) mg/kg. The contents of microplastics at different sampling sites ranged from 3.89 (±1.64) to 89.25 (±2.98) mg/kg. Overall, the proportions of various microplastic shapes decreased in the following order: film (54.25%)>fiber (18.56%)>particle (15.07%)>fragment (8.66%)>foam (3.46%); (2) among all microplastic particles, white particles accounted for the largest proportion (52.93%), followed by green (24.15%), black (12.17%), transparent (7.16%), and yellow particles (3.59%). The proportions of microplastic particle size ranges across all soil samples decreased in the following order: 1000–2000 µm (40.88%)>500–1000 µm (26.75%)>2000–5000 µm (12.30%)>100–500 µm (12.92%)>0–100 µm (7.15%). FTIR (Fourier transform infrared) analyses showed that polyethylene terephthalate (PET), polypropylene (PP), polycarbonate (PC), polyethylene (PE), and polystyrene (PS) occurred in the studied soil; (3) random forest predictions showed that industrial and agricultural production activities and the discharge of domestic plastic waste were related to soil microplastic pollution, in which agricultural plastic film was the most important factor in soil pollution in the study area; and (4) seven heavy metals extracted from microplastics in the soil samples showed significant positive correlations with soil pH, EC, total salt, N, P, and K contents (P<0.01), indicating that these soil factors could significantly affect the contents of heavy metals carried by soil microplastics. This research demonstrated that the contents of soil microplastics are lower than other areas of the world, and they mainly come from industrial and agricultural activities of the Ebinur Lake Basin.
Forest fires can alter the biological properties of soils. There is increasing evidence that fires cause a shift in soil microbial communities, which play a central role in forest carbon and nutrient cycling. In this study, we evaluate the effect of soil heating on soil microbial functions. We hypothesised that fire reduces the catabolic functional diversity of soil, and that post‐fire plant growth enhances its recovery. To test this, we experimentally heated a forest soil at 200°C (T200) or 450°C (T450). Heated and unheated soils were then incubated in tubs with or without live grass ( Lolium perenne L.). We determined the functional profiles by measuring the substrate‐induced respiration (SIR) using the Microresp™ technique and analysed nutrient availability at the end of the incubation. At both temperatures, soil heating altered the respiration responses to substrate additions and the catabolic functional diversity of soils. Functional diversity was initially reduced in T200 soils but recovered at the end of the incubation. In contrast, T450 soils initially maintained the catabolic functional diversity, but decreased at the end of the incubation. Heating‐induced nutrient availability stimulated the growth of grass, which in turn increased the response to several substrates and increased the functional diversity to values similar to the unheated controls. Our results suggest that fire‐driven alteration of soil microbial communities has consequences at a functional level, and that the recovery of plant communities enhances the recovery of soil microbial functions.
Highlights
Soil experimental heating altered microbial functions and reduced soil functional diversity.
Soil heating also increased nutrient availability, enhancing plant growth.
Growth of plants promoted the recovery of soil functional diversity.
Post‐fire recovery of functional diversity may be related to the recovery of photosynthetic tissues.
Background
Rhizodeposits regulate rhizosphere interactions, processes, nutrient and energy flow, and plant-microbe communication and thus play a vital role in maintaining soil and plant health. However, it remains unclear whether and how alteration in belowground carbon allocation and chemodiversity of rhizodeposits influences microbiome functioning in the rhizosphere ecosystems. To address this research gap, we investigated the relationship of rhizosphere carbon allocation and chemodiversity with microbiome biodiversity and functioning during peanut ( Arachis hypogaea ) continuous mono-cropping. After continuously labeling plants with ¹³ CO 2 , we studied the chemodiversity and composition of rhizodeposits, along with the composition and diversity of active rhizosphere microbiome using metabolomic, amplicon, and shotgun metagenomic sequencing approaches based on DNA stable-isotope probing (DNA-SIP).
Results
Our results indicated that enrichment and depletion of rhizodeposits and active microbial taxa varied across plant growth stages and cropping durations. Specifically, a gradual decrease in the rhizosphere carbon allocation, chemodiversity, biodiversity and abundance of plant-beneficial taxa (such as Gemmatimonas , Streptomyces , Ramlibacter , and Lysobacter ), and functional gene pathways (such as quorum sensing and biosynthesis of antibiotics) was observed with years of mono-cropping. We detected significant and strong correlations between rhizodeposits and rhizosphere microbiome biodiversity and functioning, though these were regulated by different ecological processes. For instance, rhizodeposits and active bacterial communities were mainly governed by deterministic and stochastic processes, respectively. Overall, the reduction in carbon deposition and chemodiversity during peanut continuous mono-cropping tended to suppress microbial biodiversity and its functions in the rhizosphere ecosystem.
Conclusions
Our results, for the first time, provide the evidence underlying the mechanism of rhizosphere microbiome malfunctioning in mono-cropped systems. Our study opens new avenues to deeply disentangle the complex plant-microbe interactions from the perspective of rhizodeposits chemodiversity and composition and will serve to guide future microbiome research for improving the functioning and services of soil ecosystems.
Soil, as an important constituent, is not only useful for producing food but also for maintaining environmental sustainability. It contains microorganisms with useful traits, which are necessary for enhancement of agricultural productivity. Rhizosphere, one of the complex ecosystems on Earth, is a hot spot for these microorganisms, which are beneficial for plant growth and development. Rhizospheric microorganisms play an important role in improving efficient use of nutrients, such as nitrogen, and crop sustainability. In the functioning of rhizospheric microorganisms, plant genotype plays a crucial role. Current advances in the understanding of plant natural pathways have resulted in the discovery of many of the enzymes and corresponding genes. These genes are required for the biosynthesis and transport of a variety of rhizosphere signaling molecules, supporting metabolic engineering to control the rhizosphere. Moreover, genetic variations have been revealed in several crops such as cereals, particularly maize, wheat, and rice. The rhizosphere has been targeted for efficient use of nutrients, most notably, of nitrogen, phosphorus, and potassium. Although utmost progress is being made for potassium. Numerous studies reported that drought tolerance in wheat was improved by rhizospheric bacteria. Further, preparing isolates of rhizospheric bacteria from harsh environments is a promising as well as a novel way to improve plant water‐use efficiency. Important bacterial species like Rhizobium , Pseudomonas , Azospirillum , and Bacillus found to have positive impacts on crops by enhancing both above and belowground biomass and hence play positive roles in achieving sustainable agriculture. These new advancements importantly contribute toward solving food‐security issues under changing climatic conditions. Harnessing rhizosphere microbiomes for drought‐resilient crop production can improve plant growth, and they offer the potential to increase crop resilience to future drought. Therefore plant–microbe interactions are indispensable for combating food crisis, which is one of the greatest global challenge.
ABSTRACT
This investigation was conducted to determine the ability of plant growthpromoting
rhizobacteria (PGPR) to elicit growth of rice plants (var. CO51) in
soil incubated with Blue-R textile dye. Two commercial strains of PGPR,
nitrogen-fixing Azospirillum lipoferum Az204 and phosphorus-solubilizing
Bacillus megaterium var phosphaticum PSB1, were used to prepare a carrier
and liquid-based formulations. Seed biotization of carrier-based PGPR consortium
followed by a foliar spray substantially resulted in a higher plant
biomass, germination percent, and vigor index as compared to noninoculated
plants in Blue-R-amended soil. The photosynthetic efficiency is
also higher, which is evident by the total chlorophyll content in PGPR-treated
seedlings. Similarly, the carrier-based PGPR inoculated plants revealed higher
nutrient uptake as well as N, P, and K content in shoots as compared to the
non-inoculated plants. PGPR inoculation through seeds showed better survival
and colonization of A. lipoferum Az204 and B. megaterium PSB1 in the
rhizosphere. A. liopferum Az204 remarked good colonization in inner tissues
of roots, stems, and leaves. It was concluded that PGPR application through
seeds enhances crop fitness, modulates rhizosphere services, and alleviates
oxidative stress caused by Blue-R in rice plants. Therefore, rhizosphere engineering
by PGPR through seeds followed by subsequent foliar sprays can be
used as a sustainable strategy for textile wastewater-irrigated soils.
Cutting trees removes all parts of their photosynthetic area, which affects rhizosphere assembly. However, information regarding the underground alteration process after tree cutting is insufficient. This study aimed to observe the fate of both root exudation and the rhizosphere microbial community following tree cutting. The study included 540 Calliandra calothyrsus Meissn. The experimental layout was a completely randomized block design with 3 blocks (cutting age) × 2 (cutting and not cutting) × 180 trees. Composite soil samples were collected from trees at 0-20 cm depth and stumps at 0, 2, 4, 8, and 12 weeks after cutting to observe the soil sugar content, pH, and functional group population. This study demonstrated that cutting reduced the flux of sugars below ground by 80% and caused rapid acidification (pH less than 5.0) of the soil. Total soil sugar depletion is presumed to be a mechanism by which C. calothyrsus survives and regrows after cutting. Sugar depletion affects significant shifts in the size and structure of the rhizosphere microbial community. Increasing soil acidity is another survival strategy to limit close competitor populations in the rhizosphere. This study confirms that C. calothyrsus is a proper species for developing in the coppice-harvesting-system (CHS) energy estate.
Rhizosheaths are aggregated, sheath-like soils that physically adhere to root surface, and they form on herbaceous plant roots worldwide especially in semiarid grasslands. Representing a strong root−soil−microbe interaction, the rhizosheaths are expected to have distinct soil organic carbon (SOC) signatures from rhizosphere soils of non-rhizosheath forming plants. However, such signatures remain unclear, which hinders our understanding of root effects on SOC cycling in grasslands. We compared SOC characteristics between rhizosheath and non-rhizosheath soils of eight herbaceous plant species, collected from a semiarid grassland of North China, using solid-state ¹³C nuclear magnetic resonance spectroscopy and biomarker analyses. We further examined the temporal dynamics of SOC characteristics of rhizosheath soils from early, middle, and late plant growth stages. Compared to non-rhizosheath SOC, rhizosheath SOC had more root inputs of both labile substrates (carbohydrates and free alkanoic acids) and relatively recalcitrant suberin- and lignin-derived compounds. Moreover, the labile inputs provided more substrates for microbial degradation of cutin-derived compounds. These indicators of labile substrate availability increased significantly from the early to late growth stages. Overall, our findings clarify the molecular characteristics of rhizosheath SOC and its temporal dynamics, both of which suggest a critical role of rhizosheath in shaping the rhizosphere microenvironment and regulating SOC cycling.
Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to address the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emissions of the greenhouse gas nitrous oxide (N2O) from soils. A better understanding of how to mitigate N2O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N2O emissions due to crop residue application to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation of residues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N2O emissions. Other practices such as incorporation timing and interactions with fertilisers are less conclusive. Several of the evaluated N2O mitigation measures implied negative side-effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N2O emissions without strong negative side-effects, which require further research. These are: a) treatment of crop residues before field application, e.g., conversion of residues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use of residues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management.
Though primary sources of carbon (C) to soil are plant inputs (e.g., rhizodeposits), the role of microorganisms as mediators of soil organic carbon (SOC) retention is increasingly recognized. Yet, insufficient knowledge of sub-soil processes complicates attempts to describe microbial-driven C cycling at depth as most studies of microbial-mineral-C interactions focus on surface horizons. We leveraged a well-studied paleo-marine terrace (90 ka) located near Santa Cruz, CA, to characterize the short-term (days to weeks) and intermediate-term (months to years) fate of two low molecular weight organic carbon (LMWOC) compounds at three depths in the soil profile (∼25 cm, A horizon; ∼75 cm A/B transition; and ∼125 cm, B horizon). We employed isotopically-labeled glucose (GLU) and oxalic acid (OXA) to represent two common classes of rhizodeposits: carbohydrates and organic acids. Using a combination of laboratory (9 d) and field (490 d) incubations we traced the fate of GLU- and OXA-C through dissolved-, metal-associated-, microbially-respired CO2 and bulk SOC pools. Our results suggest new SOC retention (i.e., defined as ¹³C label identified in solid or aqueous fractions) over intermediate time frames (490 d) is correlated with patterns in short-term (9 d) cycling dynamics, which in turn is related to the theoretical efficiency by which microorganisms process each substrate. For all horizons (A, A/B, and B) GLU-C was converted to CO2 more quickly than OXA-C with modeled decomposition rates ∼2–4 times faster for GLU depending on microbial density (higher in A than B horizon). The faster decomposition rates of GLU-C increased fractional recovery (0.399 ± 0.026 to 0.504 ± 0.030 for GLU-C) compared to OXA-C (0.035 ± 0.003 to 0.127 ± 0.010) among all horizons in our field experiment (490 d). Though the overall proportion of GLU-C recovered in solid fractions did not vary significantly with horizon, based on ¹³C recovered in aqueous fractions the apparent mechanism for retention did. After the 9 d laboratory incubation, fractional recovery for GLU-C among C pools associated with microbial biomass was almost 20x higher than OXA-C (0.192 versus 0.010, respectively across all horizons). More than a year later, 43–46% of GLU-C retained in the field incubation was extractable with a neutral salt (representing a pool of soil C residing within or available to microbial biomass) among A and A/B horizons, while only 6% of retained GLU-C was similarly extractable in the B horizon. Thus, it appears among depths with higher microbial density (A, A/B horizons), anabolic recycling is the most likely process contributing to the persistence of glucose C, whereas abiotic sinks contributed more to intermediate-term stability for GLU-C in the B horizon. By contrast, most OXA-C was lost, presumably as CO2, over the short-term from the A and A/B horizons (fractional recovery: 0.136 ± 0.011 and 0.091 ± 0.002, respectively). However, though substantially lower than GLU-C recovered at the conclusion of our field experiment, the fraction of oxalic acid C retained in the B horizon over both short- (0.72 ± 0.037) and intermediate-time (0.127 ± 0.010) frames was several-fold higher than for overlying horizons. The specific process(es) (e.g., more efficient microbial utilization, metal-organic complexation, direct adsorption to the mineral matrix, etc.) contributing to higher retention for OXA-C at depth are discussed but remain unresolved.
Purpose
Simultaneously interacting rhizosphere processes determine emergent plant behaviour, including growth, transpiration, nutrient uptake, soil carbon storage and transformation by microorganisms. However, these processes occur on multiple scales, challenging modelling of rhizosphere and plant behaviour. Current advances in modelling and experimental methods open the path to unravel the importance and interconnectedness of those processes across scales.
Methods
We present a series of case studies of state-of-the art simulations addressing this multi-scale, multi-process problem from a modelling point of view, as well as from the point of view of integrating newly available rhizosphere data and images.
Results
Each case study includes a model that links scales and experimental data to explain and predict spatial and temporal distribution of rhizosphere components. We exemplify the state-of-the-art modelling tools in this field: image-based modelling, pore-scale modelling, continuum scale modelling, and functional-structural plant modelling. We show how to link the pore scale to the continuum scale by homogenisation or by deriving effective physical parameters like viscosity from nano-scale chemical properties. Furthermore, we demonstrate ways of modelling the links between rhizodeposition and plant nutrient uptake or soil microbial activity.
Conclusion
Modelling allows to integrate new experimental data across different rhizosphere processes and scales and to explore more variables than is possible with experiments. Described models are tools to test hypotheses and consequently improve our mechanistic understanding of how rhizosphere processes impact plant-scale behaviour. Linking multiple scales and processes including the dynamics of root growth is the logical next step for future research.
One-step immunoassay detects a target analyte simply by mixing a sample with a reagent solution without any washing steps. Herein, we present a one-step immunoassay that uses a peptide mimicking a target analyte (mimotope). The key idea of this strategy is that the mimotopes are screened from an autodisplayed FV-antibody library using monoclonal antibodies against target analytes. The monoclonal antibodies are bound to fluorescence-labeled mimotopes, which are quantitatively released into the solution when the target analytes are bound to the monoclonal antibodies. Thus, the target analyte is detected without any washing steps. For the mimotope screening, an FV-antibody library was exhibited on the outer membrane of E. coli with a diversity of >10⁶ clones/library using autodisplay technology. The targeted clones were screened from the autodisplayed FV-antibody library using magnetic beads with immobilized monoclonal antibodies against food allergens. The analysis of binding properties of a control strain with mutant FV -antibodies composed of only CDR1 and CDR2 demonstrated that the CDR3 regions of the screened FV-antibodies showed binding affinity to food allergens. The CDR3 regions were synthesized into peptides as mimotopes for the corresponding food allergens (mackerel, peanuts, and pig fat). One-step immunoassays for food allergens were demonstrated using mimotopes against mackerel, peanut, and pig fat without any washing steps in solution without immobilization of antibodies to a solid support.
Unterschiede in den Sprossgehalten an Cu, Zn und Cd bei 11 Spinatsorten veranlassten die nähere Untersuchung der zwei Sorten Tabu und Monnopa auf Kenngrößen des Cu-, Zn- und Cd-Aneig-nungsvermögens in Abhängigkeit von der P-Ernährung. Die Anzucht der Pflanzen hierzu erfolgte in einer P-armen Parabraunerde (pH: 6,3; Gesamtgehalte Cu: 89, Zn: 297, Cd: 2,4 mg kg—1) mit zwei P-Niveaus in Gefäßen unter Freilandbedingungen. Zur Ermittlung der Nettoaufnahmeraten, Wurzellängen/Sprossmasse-Verhältnisse und der Cu-, Zn- sowie Cd-Konzentrationen in den Boden-lösungen erfolgte die Ernte 26 bzw. 40 Tage nach der Saat. In einem separaten Experiment wurde die Wurzelexsudation organischer Säuren beider Sorten nach Anzucht in Quarzsand mit variiertem P-Angebot geprüft. Beide Sorten reagierten auf die P-Düngung mit Verdoppelung der Sprossmasse. Beide Sorten zeigten bei +P (0,68—0,77% P in der TM) ähnliche SM-Gehalte der Sprosse, ähnliche Wurzellängen/Sprossmasse-Verhältnisse (WSV) und Elementaufnahmeraten der Wurzeln sowie gleiche Elementkonzentrationen in den Bodenlösungen. Bei P-Mangel zeigte Sorte Tabu (0,52% P in der TM) im Vergleich zur Sorte Monnopa (0,48% P in der TM) erhöhte Cu- und signifikant erhöhte Zn- und Cd-Gehalte in den Sprossen, sowie bei kleineren WSV aber auch erhöhte Nettoaufnahmeraten der Wurzeln. Letztere korrespondierten mit den unter der Sorte Tabu gemessenen höheren Elementkonzentrationen in der Bodenlösung des wurzelnahen Bodens sowie der höheren Exsudationsrate dieser Sorte an Oxalat, Citrat und Malat (3,9; 1,0; 0,7 nmol cm—1 h—1). Die entsprechenden Werte für Monnopa betrugen: 1,7; 0,3; 0,4 nmol cm—1 h—1. Die höhere Cu-, Zn- und Cd-Löslichkeit, die durch die exsudierten organischen Säuren verursacht wurden, scheint für die höhere Nettoaufnahmerate der Sorte Tabu verantwortlich zu sein.
Field and pot experiments showed that the P demand of wheat is highest in early stages of growth (up to 1.67 μg P per cm² root surface and day). The needed orthophosphate ions H2PO4− and HPO4²-move from soil to the root by diffusion. This process is controlled by the concentration gradient of the diffusible phosphate and the effective diffusion coefficient according to Pick's first law. Root excretions (rhizodeposition) are able to affect both characteristics. The water soluble portion of rhizodeposition contains more than 50% of up to 8 different sugars, 10–40% carboxylic acids and 10–15 amino acids and amides. The composition varies in dependence on the age of the root parts and on nutrition (Zea mays L., Brassica napus L., Pisum sativum L.). Diffusion experiments using small soil blocks showed that 50–75% of the root exudates were decomposed by respiration within 3 days. The rest was largely chemically converted. Originally present sugars disappeared. Due to the biosynthesis of different organic acids from the individual sugars the mobilisation of Ca3(PO4)2 by Pantoea agglomerans increased when the sugar mixture was derived from the rhizodeposition of P deficient plants with more pentoses instead of glucose and fructose (mainly effect of anions). In the rhizosphere therefore a mixture of rhizodeposition and its conversion products exists which affects the binding of phosphorus in soil and the P transport to the root. This should be considered both for the development of new soil extractants and for modelling the P supply to plants.
Root exudation of carbon (C) plays a major role in processes occurring in the plant rhizosphere. Environmental factors affecting root exudation have been identified but their effects are rarely quantified. The purpose of this work was to evaluate the impact of both the microflora and the chemical composition of the growth medium on root exudation, taking into account soluble exudates and mucilage fraction. Maize plants (Zea mays L.) were grown for 12 days in hydroponic conditions and then transferred in three root bathing solutions (demineralized water, KCl or nutrient solution) during 24 hours. In each case, presence of microflora was tested with a comparison between plants inoculated with maize rhizospheric strain and axenic plants. Exudation was measured in terms of C and biomass production. A strong interaction was noticed between microflora and chemical composition of the root bathing solution. In fact, the presence of rhizospheric microflora induces a stimulation of soluble exudates only in KCl and Nutrient solutions. In demineralized water, a different response was observed with a higher C release for axenic plants, probably due to the osmotic shock induced to the roots. Concerning mucilage fractions, small quantities were recovered on all treatments. This work demonstrates that the chemical composition of the root bathing solution and presence of microorganisms significantly modify the amount of soluble exudates. Attention must therefore be paid to the cultural conditions when exudation is studied because of the sensitivity of this process to root environment.
Soil respiration constitutes a major component of the global carbon cycle and is likely to be altered by climate change. However, there is an incomplete understanding of the extent to which various processes contribute to total soil respiration, especially the contributions of root and rhizosphere respiration. Here, using a stable carbon isotope tracer, the authors separate the relative contributions of root and soil heterotrophic respiration to total soil respiration in situ. The Free-Air Carbon dioxide Enrichment (FACE) facility in the Duke University Forest (NC) fumigates plots of an undisturbed loblolly pine (Pinus taeda L.) forest with COâ that is strongly depleted in ¹³C. This labeled COâ is found in the soil pore space through live root and mycorrhizal respiration and soil heterotroph respiration of labile root exudates. By measuring the depletion of ¹³COâ in the soil system, the authors found that the rhizosphere contribution to soil COâ reflected the distribution of fine roots in the soil and that late in the growing season roots contributed 55% of total soil respiration at the surface. This estimate may represent an upper limit on the contribution of roots to soil respiration because high atmospheric COâ often increases in root density and/or root activity in the soil.
Model calculations were made in order to quantify the effect of carboxylate excretion on phosphate (P) uptake by a single root. The uptake of chemically mobilized P increased exponentially with increasing concentration of adsorbed citrate or oxalate in soil because of the exponential relationship between adsorbed carboxylate and the solubilizing effect of carboxylate on P. The effect of local citrate excretion compared with uniform citrate excretion along the whole root was also calculated. Local exudation increased the uptake of chemically mobilized P because the higher concentration of citrate increases the solubilization of P. Additionally the effect of citrate excretion by root clusters e.g. proteoid roots was evaluated. Uptake of chemically mobilized P by root clusters was much higher than that of single roots, especially if the ratio of P buffering to citrate buffering was high. This is often the case in P fixing soils where by definition P buffering is high and citrate buffering is low because of the short time of reaction between root excreted citrate and rhizosphere soil. The reason for the superiority of cluster roots lies in the fact that most of the mobilized P is transported away from a single root to be absorbed by neighbouring roots in the clusters. This phenomenon demonstrates the strong ecological significance of cluster roots in relation to nutrient mobilization. The calculations on the effect of oxalate excretion by sugar beet roots on the uptake of mobilized P show that under P fixing conditions the influx of mobilized P will exceed that of P transported by diffusion to the root surface by a factor of 1.5-6.0.
The retention of dissolved organic matter in soils is mainly attributed to interactions with the clay fraction. Yet, it is unclear to which extent certain clay-sized soil constituents contribute to the sorption of dissolved organic matter. In order to identify the mineral constituents controlling the sorption of dissolved organic matter, we carried out experiments on bulk samples and differently pretreated clay-size separates (untreated, organic matter oxidation with H2O2, and organic matter oxidation with H2O2 + extraction of Al and Fe oxides) from subsoil horizons of four Inceptisols and one Alfisol. The untreated clay separates of the subsoils sorbed 85 to 95% of the dissolved organic matter the whole soil sorbed. The sorption of the clay fraction increased when indigenous organic matter was oxidized by H2O2. Subsequent extraction of Al and Fe oxides/hydroxides caused a sharp decrease of the sorption of dissolved organic matter. This indicated that these oxides/ hydroxides in the clay fraction were the main sorbents of dissolved organic matter of the investigated soils. Moreover, the coverage of these sorbents with organic matter reduced the amount of binding sites available for further sorption. The non-expandable layer silicates, which dominated the investigated clay fractions, exhibited a weak sorption of dissolved organic matter. Whole soils and untreated clay fractions favored the sorption of "hydrophobic" dissolved organic matter. The removal of oxides/hydroxides reduced the sorption of the lignin-derived "hydrophobic" dissolved organic matter onto the remaining layer silicates stronger than that of "hydrophilic" dissolved organic matter.
Model calculations on the uptake of inorganic nutrients by single roots and root systems do not include chemical nutrient mobilization by root exudates. In this paper, a model description of Nye (1984) was applied to the case of a single root (cylindrical case) and chemical mobilization of phosphate (P) by carboxylates. Experimental results are reported which allow the quantification of soil parameters required for the model calculations. These include the solubilizing effect of carboxylates on P. The experiments showed a strong positive correlation between carboxylate adsorption to the soil solid phase and chemical P mobilization by the carboxylate. Below a concentration of adsorbed citrate and oxalate of 10 μmol g -1 soil no mobilization occurred in the 3 soils which were investigated. Differences in pH did not affect P solubility, but did, however, affect the quantity of adsorbed carboxylate and thereby the mobilization of P. Citrate and oxalate have a greater ability to mobilize soil P as compared with oxaloacetate and malate. Model calculations which are based on these experimental results are presented in a second paper.
Limited knowledge of root distributions in agroforestry systems has resulted in assumptions that various tree species are more suited to agroforestry than others, because they are presumed to have few superficial lateral roots. This assumption was tested for Grevillea robusta when grown with maize (Zea mays) in an agroforestry system in a semi-arid region of Kenya. At a site with a shallow soil, root lengths of both species between the soil surface and bedrock were quantified by soil coring, at intervals over four cropping seasons, in plots containing sole stands and mixtures of the trees and crop; the trees were 4–6 years old and they were severely pruned before the third season. Profiles of soil water content were measured using a neutron probe. Prior to pruning of the trees, recharge of soil water below the deepest maize roots did not occur, resulting in significant (P<0.05) suppression of maize root lengths and downward root growth. Maximum root length densities for both species occurred at the top of the soil profile, reaching 1.1–1.7 cm cm −3 for G. robusta, but only 0.5 cm cm −3 for maize grown with trees. Root populations in mixed plots were dominated by G. robusta at all times, all depths and all distances from trees and maize and, thus, there was no spatial separation of the rooting zones of the trees and crop. Competition between G. robusta and maize for soil water stored near the surface was unavoidable, although pruning reduced its impact; complementary use of water by the trees and crop would only have been possible if alternative sources of water were available.
In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeo and South American 5) maize genotypes. In a growth solution containing 6 M Al3+, Tuxpeo and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al3+ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.
The short-term transfer of nitrogen (N) from legumes to grasses was investigated in two laboratory studies. One study was done in pots where the roots of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were allowed to co-exist, and a second study was performed using a micro-lysimeter system designed to maintain nutrient flow from the clover to the grass, whilst removing direct contact between the root systems. The 15N-dilution technique was used to quantify the transfer of N between species. Levels of ammonia and amino acids were measured in root exudates. The amounts of N transferred were in the same order of magnitude in both the pot and micro-lysimeter experiments. In the micro-lysimeter experiment, 0.076 mg of N were transferred per plant from clover to ryegrass during the course of the experiment. Ammonium exudation was much higher than amino acid exudation. The most abundant amino acids in both clover and ryegrass root exudates were serine and glycine. However, there was no correlation between the free amino acid profile of root extracts and exudates for both plant species: Asparagine was the major amino acid in clover roots, while glutamine, glutamate and aspartate were the major amino acids in ryegrass roots. Comparison of exudates obtained from plants grown in non-sterile or axenic conditions provides evidence of plant origin of ammonium, serine and glycine.
Controversies exist in interpreting rhizosphere C flow obtained by different 14CO2 labelling methods. However, there is a need for the standardisation of methods in order to be able to compare values obtained
for different plants, different stages of development and different habitats. Perennial bromegrass (Bromus erectus Huds) grown in soils of different fertility was exposed to a 14CO2 atmosphere for different periods of time: 1 h, 298 h and 78 days. The evolution of 14CO2 in the soil was measured during and after labelling. The 14C contents of plant and rhizosphere compartments were then estimated. The time-sequence of the rate of 14CO2 evolution after 1 h of labelling, indicated a maximum after around 20 h, followed by an exponential decrease. When expressed
as a percentage of net 14C assimilation, root-soil respiration accounted for 14% and 18% in the nutrient-poor and nutrient-rich soils, respectively.
Integration of the hourly values over several days showed that the dynamics of the evolution rate were similar for the 298-h
and 78-day experiments, thus indicating that rhizosphere C flow was dominated by newly assimilated C. This was confirmed by
the proportions of below-ground 14C, measured for roots, respiration and soil, which were not significantly affected by the labelling regime. The differences
were, however, found to be significant between the two types of soils. The conclusion was that the conditions for plant growth
during labelling were more important than the length of time of labelling, and that this explained the discrepancies in the
literature-cited values. A succession of short-term 14C labelling of plants at different development stages followed by an allocation period of about 1 week is proposed to give
a reliable estimation of the dynamics of C flow in the rhizosphere.
en Living plants change the local environment in the rhizosphere and consequently affect the rate of soil organic matter (SOM) decomposition. The rate may increase for 3‐ to 5‐folds, or decrease by 10 % to 30 % by plant cultivation. Such short‐term changes of rate (intensity) of SOM decomposition are due to the priming effect. In the presence of plants, a priming effect occurs in the direct vicinity of the living roots, and it is called rhizosphere priming effect (RPE). Plant‐mediated and environmental factors, such as, plant species, development stage, soil organic matter content, photosynthesis intensity, and N fertilization which affect RPE are reviewed and discussed in this paper. It was concluded that root growth dynamics and photosynthesis intensity are the most important plant‐mediated factors affecting RPE. Environmental factors such as amount of decomposable C in soil and Nmin content are responsible for the switch between following mechanisms of RPE: concurrence for Nmin between roots and microorganisms, microbial activation or preferential substrate utilization. Succession of mechanisms of RPE along the growing root in accordance with the rhizodeposition types is suggested. Different hypotheses for mechanisms of filling up the C amount loss by RPE are suggested. The ecosystematic relevance of priming effects by rhizodeposition relates to the connection between exudation of organic substances by roots, the increase of microbial activity in the rhizosphere through utilization of additional easily available C sources, and the subsequent intensive microbial mobilization of nutrients from the soil organic matter.
Abstract
de Einflussfaktoren auf Priming‐Effekte in der Rhizosphäre
Wurzeln lebender Pflanzen verändern die lokale Umgebung in der Rhizosphäre und beeinflussen oft die Intensität des Abbaus der organischen Bodensubstanz (OBS). Beim Anbau von Pflanzen kann die OBS‐Mineralisation um das 3‐ bis 5‐fache im Vergleich zur Schwarzbrache ansteigen oder um 10 % bis 30 % abnehmen. Solche kurzfristigen Änderungen der Intensität des OBS‐Abbaus gehören zum Phänomen des Priming‐Effektes. Da beim Pflanzenwachstum die Priming‐Effekte in unmittelbarer Nähe von lebenden Wurzeln stattfinden, werden sie Rhizosphäre‐Priming‐Effekte (RPE) genannt. Dieser Artikel gibt eine kritische Analyse und Diskussion des Erkenntnisstandes über die Faktoren, die die RPE beeinflussen. Dazu gehören Faktoren, die über die Pflanze bzw. die Umwelt wirken, wie Pflanzentyp und ‐entwicklung, Photosyntheseintensität, Gehalt an organischer Bodensubstanz und N‐Düngung. Es kann geschlussfolgert werden, dass die Dynamik des Wurzelwachstums und die Intensität der Photosynthese die wichtigsten über die Pflanze wirkenden Faktoren sind. Die Umweltfaktoren wie der Gehalt an umsetzbarem Kohlenstoff und mineralischem Stickstoff (Nmin) im Boden bedingen den Wechsel zwischen folgenden Mechanismen der RPE: (1) Konkurrenz für Nmin zwischen den Wurzeln und den Mikroorganismen, (2) Aktivierung der Rhizosphärenmikroorganismen durch die Wurzel‐exsudate und (3) bevorzugte Nutzung der Rhizodeposite als leichtverfügbares Substrat und nicht der OBS. Dieser Wechsel wird, in Übereinstimmung mit den Typen der Rhizodeposite und der Sukzession der Mikroorganismen, durch die räumliche Abfolge der RPE entlang der wachsenden Wurzel erklärt. Unterschiedliche Mechanismen der Auffüllung der C‐Verluste der OBS, die während der RPE entstehen, werden diskutiert. Die ökosystemare Bedeutung der Priming‐Effekte durch die Rhizodeposition besteht in der Stimulation der Rhizosphärenmikroorganismen durch den leichtverfügbaren Kohlenstoff zum intensiven mikrobiellen Abbau der organischen Bodensubstanz und in der Mobilisation zusätzlicher Nährstoffe speziell in Perioden des intensiven Pflanzenwachstums.
Application of an antibiotic (streptomycin) and a fungicide (benomyl) in order to separate root respiration and microbial respiration of exudates was tested using two plant species (ryegrass and spring wheat) growing on a Haplic Luvisol. Both xenobiotics were added to the soil with growing plants either separately or in combination. Plants were 14C-pulse labelled and both labelled and total CO2 emission from soil were measured. After seven days plants were harvested, dried and total carbon and C content in shoots and roots were determined.
Growing plants increased the total CO2 emission from soil by about 2.5 times in comparison to unplanted soil. The temporal pattern of 14CO2 evolution in control treatments was similar as reported in the literature with maximum emission rates (0.55 % and 0.15 % of assimilated C per hour for wheat and ryegrass, respectively) during the first day after labelling. It was affected by both xenobiotics used but in different ways. With ryegrass streptomycin decreased CO2 emission rates during the first day after labelling, while it left the second phase of tracer emission unchanged; with spring wheat the 14CO2 evolution rates were reduced on the second day. Applied xenobiotics did not change the 14C content in the shoots, but smaller tracer amounts were found in the roots and in soil-derived CO2. This was more pronounced for ryegrass than for spring wheat plants.
Short term labelling of plants with 14CO2 made it possible to separate root and microbial activity in the rhizosphere.
Measurements on wheat plants grown in the presence of root microflora and exposed to an atmosphere containing 14CO2 showed a succession of two phases of 14CO2 evolution around the roots when only one phase occurred with plants grown under axenic conditions.
A series of experiments was then undertaken in order to test the hypothesis of a time sequence for root CO2 and microbial CO2 resulting from utilization of exudates by the rhizosphere microflora.
Quantitative analysis of labelled root exudates found in the soluble phase around the roots showed large differences between plants grown under axenic or non-axenic conditions.
In both cases, labelling of the plants induced a liberation of labelled substances in the root solution with a maximum after a few hours. In the presence of microorganisms, this was followed by a second phase of liberation. The time sequence was approximately that of 14CO2 evolution.
A qualitative study of the substances liberated in both kinds of cultures has been made using gel filtration techniques. It showed that the nature and composition of the compounds liberated during the second phase were different from those measured during the first phase and in axenic cultures.
A bacterial strain producing polysaccharides of known molecular weight and isolated from the rhizosphere of a graminaceae was used to inoculate axenic cultures of wheat. The progressive evolution of the soluble substances towards one type of compound, the bacterial polysaccharide, demonstrated the existence of microbial activity on root exudates, the maximum being observed during the second phase of 14CO2 evolution. These results demonstrated a succession of root and microbial activities in the rhizosphere and the validity of respirometric methods as a means to quantify them.
Considerable progress has been achieved in quantifying the consumption of photosynthate by growing plant roots. The evolution of CO2 in the rhizosphere either from root or microbial respiration is considerable and may consume as much as 20% of the total photosynthetic production. However, an experimental resolution of these two components is difficult due to physiological interactions between the roots and rhizosphere microorganisms. This work was aimed at the determination of the actual respiration rate of nonsterile roots during a short (6 h) inhibition of microbial respiration with selected specific inhibitors. p-Hydroxymercuriphenylsulphonate turned out to be an effective inhibitor of microbial respiration. This substance reduced the CO2 development in the rhizosphere to 16 - 24%, which is considered an estimate of the actual root respiration rate. The corresponding microbial respiration of 76 - 84% indicates that the organic carbon input by growing roots and its metabolic significance in the rhizosphere is much higher than that so far assumed.
Root-derived C influences soil microbial activities that regulate N transformations and cycling in soil. The change in 13C abundance of soil microbial biomass was used to quantify contributions from maize (Zea mays L.), a C4 plant, to root zone-available C during growth in soil with a long history of C3 vegetation. Effects of root-derived available C on microbial transformations of N were also evaluated using a 15NH415NO3 fertilizer tracer. Root-released C (microbial respired C4C + soil residue C4C) accounted for 12% (210 kg C ha−1) of measured C fixed by maize at 4 wk and 5% at maturity when root-released C totaled 1135 kg C ha−1. Of the C4C remaining in soil, only 18–23% was found in microbial biomass, indicating either a rapid turnover rate of biomass or a lower availability of C4 substrates. Average daily production of root-derived available C was greatest during 4–8 wk maize growth (7 kg C ha−1 d−1) when 4–11% of the soil microbial biomass came from this C source. At maize maturity, 15% of the microbial biomass (161 kg C ha−1) came from root-derived available C, which totaled 402 kg ha−1. Of the 15N remaining in bare and cropped soils, averages of 23 and 16% (10 and 2 kg N ha−1) were found in microbial biomass, and 64 and 2% (28 and 0.2 kg N ha−1) were in inorganic 15N form, leaving 13 and 82% (6 and 10 kg N ha−1) as non-biomass organic N, respectively; this suggests that N cycling through microbial biomass was enhanced by root-derived C. Denitrification and N2O losses from planted soils were low (1–136 g N ha−1 d−1) when soil water-filled pore space (WFPS) was <50%, but increased to 0.02–3.4 kg N ha−1 d−1 when soils were wetted to 85–95% WFPS when N2 comprised 70–99% of denitrification products. The maximum denitrification rate was 1.5 times greater, and the cumulative denitrification losses 77% greater during early growth stages in planted soil as compared to bare soil when adequate NO3−N (> 2–3 mg kg−1) was present in the soil. The presence of maize plants increased denitrification losses from soil by 19 to 57% (average of 29%) during early growth stages when the release of root-derived C was greatest.
The addition of activated charcoal to a nutrient solution for the hydroponic culture of cucumber resulted in significant increases in the dry weight of the plant and fruit yield. Hydrophobic root exudates were collected at different growth stages with Amberlite XAD-4 resin and bioassayed with lettuce seedlings. The exudates at the reproductive stage were more phytotoxic than those at the vegetative stage. The exudates contained organic acids such as benzoic,p-hydroxybenzoic, 2,5-dihydroxybenzoic, 3-phenylpropionic, cinnamic,p-hydroxycinnamic, myristic, palmitic, and stearic acids, as well asp-thiocyanatophenol and 2-hydroxybenzothiazole, all of which, except 2-hydroxybenzothiazole, were toxic to the growth of lettuce.
The production and utilization of root-derived C is fundamental to the functioning of ecosystems. The objectives of this experiment were to quantify the amount of root-released C produced by two barley (Hordeum vulgare L.) cultivars, to evaluate the direct and indirect effects of shoot C, root C and root length on the root-released C and to quantify the kinetics of the decomposition of root-released C in soil for two barley cultivars. Seedlings of two barley cultivars, Abee and Samson, were cultured in nutrient solution for 20 d and then pulse-labelled with 14C for 5 d. Samson released more C than Abee during the entire experimental period. Root length had the greatest direct effect on root-released C for the two barley cultivars. Kinetic analysis of the decomposition of root-released C added to soil
showed that the labile components of the added material was 87.3% for Abee and 74.4% for Samson with half-lives of 4.3 d and 4.5 d, respectively. The half-lives of the resistant components ofthe root-released C added to soil and microbially-derived material were 37.7 d for Abee and 29.6 d for Samson, respectively. The amount of root-released C and its decomposition rate in soil varied with cultivars used.
Six areas of native grassland were labelled with 14C during a growing season. Transfers from the foliage to the roots and root respiration were measured. Plant production and turnover rates were determined by sampling the labelled material at different periods following exposure to 14CO2.Above to beneath ground plant production ratios ranged between 1.1 and 1.9 with maximal translocation to the roots occurring during the drier summer months. The distribution of the photosynthates in the roots at different depths changed with time and soil moisture content. The upper part of the soil (0–10 cm) contained 49–77% of the labelled C found beneath the soil surface. Measurement of transfers with time of the above ground labelled C from living to dead plant and litter categories gave an insight into foliage dynamics and made it possible to estimate the seasonal shoot production at 130g Cm−2 (1300kg ha−1). Root growth represented 100g Cm−2 (1000 kg ha−1).Calculations of root and soil respiration were based on the CO2 profiles in the soil. The fluxes of labelled and unlabelled CO2 at the soil surface were estimated using the diffusion equation method. Respiration by roots and closely associated soil organisms accounted for 12 per cent of the net assimilation of CO2 by the plants. This proportion was constant throughout the season and represented 19 per cent of the total CO2 evolved at the soil surface.
The present work describes an original method to follow rate of 14CO2 and total CO2 production from rhizosphere respiration after plant shoots had been pulse-labelled with 14CO2. We used a radioactivity detector equipped with a plastic cell for flow detection of beta radiation by solid scintillation
counting. The radioactivity detector was coupled with an infrared gas analyser. The flow detection of 14CO2 was compared to trapping of 14CO2 in NaOH and counting by liquid scintillation. First, we demonstrated that NaOH (1 M) trapped 95% of the CO2 of a gaseous sample. Then, we determined that the counting efficiency of the radioactivity flow cell was 41% of the activity
of gaseous samples as determined by trapping in NaOH (1 M) and by counting by static liquid scintillation. The sensitivity
of the 14CO2- flow detection was 0.08 Bq mL−1 air and the precision was 2.9% of the activity measured compared to 0.9% for NaOH trapping method. We presented two applications
which illustrate the relevance of 14CO2-flow detection to investigations using 14C to trace photoassimilates within the plant-soil system. First, we examined the
kinetics of 14CO2 production when concentrated acid is added to NaH14CO3. This method is the most commonly used to label photoassimilates with 14C. Then, we monitored 14CO2 activity in rhizosphere respiration of 5-week old maize cultivated in soil and whose shoots had been pulse-labelled with
14CO2. We conclude that alkali traps should be used for a cumulative determination of 14CO2 because they are cheap and accurate. On the other hand, we demonstrated that the flow detection of 14CO2 had a finer temporal resolution and was consequently a relevant tool to study C dynamics in the rhizosphere at a short time
scale.
The amounts of carbon released into soil from roots of wheat and barley seedlings grown under three environmental conditions
for 3 weeks with shoots in constant specific activity 14CO2 are reported. This carbon loss was measured as respired 14CO2 from both the root and the accompanying microbial population and as root derived 14C-labelled organic C compounds in the soil. With a 16 h photoperiod, growth at 15 °C constant or 18 °C day/14 °C night gave
a loss of 33–40% of the total net fixed carbon (defined as 14C retained in the plant plus 14C lost from the root). The proportion of 14C translocated to the roots that was released into the soil did not change with temperature, so carbon distribution within
the plant must have changed. With a 12 h photoperiod and a temperature regime of 18 °C/14 °C carbon loss from the roots was
decreased to 17–25% of the total fixed carbon.
Wheat and bailey plants were grown for 3 weeks in a constant environment chamber containing approximately atmospheric concentrations of carbon dioxide (0.03%) labelled with 14C. The roots of the plants were maintained under sterile or non-sterile conditions in soil contained in sealed pots which were regularly flushed with air. This enabled the quantities of 14C-labelled carbon dioxide produced in the soil by plant and microbial activity to be separately assessed. At harvest, the 14C and total carbon contents of the roots and shoots and of the water-soluble and insoluble material present in the soil were measured.
These procedures enabled both the amounts of organic materials released into soil by the roots of growing plants and the effects of micro-organisms on the process to be determined. Under sterile conditions between 5 and 10% of the photosynthetically fixed carbon may be released by roots compared with 12–18% by the roots of plants growing in unsterilized soil. These latter values are equivalent to 18–25% of the dry matter increments of the plants. The results indicate also that the increased evolution of carbon dioxide by cropped as compared to fallow soils can largely be ascribed to the immediate utilization by micro-organisms of substances released by roots.
The allocation of C between roots and shoots, and the fluxes of C to roots and soil were measured in wheat grown under two soil moisture regimes during the vegetative stages of growth. Wheat plants were grown in columns of soil in a glasshouse, and the fate of pulse-labelled 13C was followed as root and shoot C, root and microbial respiration, rhizodeposition and shoot respiration. Relatively more assimilates were allocated to the roots under limited water conditions as indicated by higher percentages of the excess 13C recovered in the roots. Changes during the vegetative growth in the excess 13C allocated to the roots occurred at the onset of tillering and booting stage, and were associated with changes in the activity of the sinks. The excess 13C below ground under limited soil water increased by 21% at leaf formation and by 43% at booting, relative to that under adequate soil water. The increase resulted mainly from an increase in the excess 13CO2 respired from roots and microorganisms, but not from the recovery of 13C in the soil. We conclude that the increase in the C below ground under limited soil water arose from more C made available to the roots by the reduction in shoot C. This was due to a smaller reduction in root growth rates than in shoot growth rates under the water-limited conditions.
Knowledge on the quantity and dynamics of rhizodeposition under ecologically realistic conditions may elucidate various aspects of soil organic matter dynamics. Data from a field experiment with 14C pulse-labelling of spring wheat at different development stages, were used to estimate rhizosphere carbon fluxes. Not only the flux of C to the roots was assessed but also the fluxes of organic and inorganic release of root-derived material. C fluxes were calculated from curves fitted to data on shoot and root biomass and to data on 14C distribution at different development stages. The 14C distribution curves were extrapolated from the first labelling date (elongation stage) down to crop emergence and from the last labelling date (dough ripening stage) up to crop harvest, using different extrapolation procedures. The results show that while the maximum shoot growth rate occurred around ear emergence, the flux of C to the roots had a maximum around tillering. Over the entire growing season, shoot growth amounted to 5730 kg Cha−1 and 2310 ± 90 kg C ha−1 was translocated belowground. Of this 920± 150 kg C ha−1 was lost in root respiration and 500 ±120 kg C ha−1 was released as young photosynthate rhizodeposits, which are defined as organic materials released from the roots within 19 days after assimilation. Root growth amounted to 940 ±40 kg C ha−1, of which, however, 370 ±40 kg C ha−1 was lost again through root decay. Root turnover during the growing season, defined as root decay divided by root growth, was therefore 37–42%. Most of the organic input to soil (56–64%) occurred through rhizodeposition, while 36–44% was comprised in root biomass at crop harvest. The model used for the calculation of the carbon fluxes is discussed.
For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the 13C isotope to determine the incorporation of maize residues into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the soil respiration. The maize-derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one 'continuous maize' and the other 'continuous rye' (reference site) from the long-term field experiment 'Ewiger Roggen' in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ13C of DOC and CO2 were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize-derived C below the ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha-1 which was equal to about 31% of the estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m-2 and one third of the DOC derived from maize C. The specific DOC production rate from the maize-derived SOC was 2.5 times higher than that from the older humus formed by C3 plants. The total CO2-C emission for 16 weeks was 18 g m-2. Fifty-eight percent of the soil respiration originated from maize C. The specific CO2 formation from maize-derived SOC was 8 times higher than that from the older SOC formed by C3 plants. The ratio of DOC production to CO2-C production was three times smaller for the young, maize-derived SOC than for the older humus formed by C3 plants.
In order to quantify rhizosphere respiration (R(rh)) during the growing season, we monitored the isotope ratio (δ13C) of soil CO2 in a corn (Zea mays L.) crop that was grown on a soil developed from C3 plant material. The R(rh) was defined as the sum of CO2 respired by plant roots and CO2 respired by microbes that feed on organic material produced by the roots. The δ13C of soil CO2 in the corn plot changed from an early season low of approximately -20‰, to mid-season values of approximately -14‰, before declining again at the end of the growing season. In the control plot (no C4 plants present) δ13C values of soil CO2 were significantly lower than values in the corn plot, averaging approximately -22‰ throughout most of the season. We observed no significant change in the δ13C value of soil CO2 in either the corn or control plots during a diurnal sampling period. The value of R(rh) was 1.71 g CO2 m-2 d-1 27 d after planting (DAP), reached 10 g CO2 m-2 d-1 at 44 DAP, remained at about that value until 76 DAP, and gradually decreased to 2.6 g CO2 m-2 d-1 at 141 DAP. The CO2 respired by the rhizosphere was equivalent to 18 to 25% of crop net photosynthesis and 24 to 35% of crop net CO2 assimilation during most of the growing season.
A mobile laboratory was developed to administer a controlled flow of 13C labelled CO2 at ambient concentrations (∼350 ppm) in the field. The stable isotope delivery (SID) system consists of an isotope-mixing unit with flow control to a series of 12 independent labelling chambers. In-line CPU controlled infrared gas analysers allow automated measurement of chamber CO2 concentrations and gas flow management. A preliminary experiment was established on an upland pasture located at the NERC Soil Biodiversity experimental site, Sourhope, UK, in August 1999. The objective of this investigation was to determine the magnitude of pulse-derived C incorporation into a typical upland plant community. To achieve this, the SID system was deployed to pulse-label vegetation with CO2 enriched with 13C (50 atom %) at ambient concentrations (∼350 ppm) on two consecutive days in August 1999. Samples of headspace CO2, shoot and root were taken on four occasions over a period of 28 days after 13C labelling. These materials were then prepared for 13C/12C ratio determination by continuous-flow/combustion/isotope ratio mass spectrometry (CF-C-IRMS). Results showed that pulse derived CO2-C was assimilated at a rate of 128 ± 32 µg g shoot-C hour−1. Dynamic samplings showed that pulse-derived 13C concentrations in the labelled plant tissues declined by 77.4 ± 6% after 48 hours. The rapid decline in 13C concentrations in plant matter was the result of C loss from the plant in the form of respired CO2 and root exudates, and dilution by subsequent unlabelled C assimilates. This novel system offers considerable potential for in situ tracer investigations. Copyright © 2000 John Wiley & Sons, Ltd.
I examined the below-ground transfer of C in spring barley (Hordeum vulgare L. cv. Alexis) grown in the field in soil cores contained in stainless steel cylinders, and compared results obtained by 14C pulse-labelling and root washing. At four growth stages, shoots of three cylinders were labelled with 14CO2 for 8–10 h, and the distribution of 14C determined 6 days after labelling (day 6). At maturity (127 days after sowing), the yield of grain, straw and macro-roots (isolated by root washing) was 273.2, 261.7 and 45 g C m−2 respectively. Throughout the growing season, 67–73% of the macro-root C and 45–78% of the 14C retained below-ground (day 6) was in the 0–15 cm of soil layer. Generally, the proportion of below-ground 14C recovered in the 0–15 cm layer decreased throughout the growing season. At early elongation, 36.7% of 14C recovered was translocated below-ground with 7.6, 20.7 and 8.3% being recovered in macro-roots, macro-root free soil and as rhizosphere respiration (root and microbial), respectively, the corresponding values at late grain filling being only 0.7, 1.1 and 3.0%. The half life of 14C deposited in the soil (macro-root free soil) was estimated to 6–42 days depending on the intensity of root growth. The total below-ground transfer of C during the growth of spring barley was estimated to 165.2 g C m−2 (1652 kg C ha−2). Rhizosphere respiration and macro-root C isolated by root washing at maturity accounted for 23.3 and 27.2%, respectively. The amount of C translocated below-ground corresponds to one-third of the above-ground C harvested at maturity.
The distribution of photosynthetically-fixed carbon between plant and soil was measured for wheat grown for 63 days in a (CO2)-C-14 atmosphere from early tillering to anthesis and at two different moisture regimes. Rhizosphere CO2-C released from soil columns (100 mm dia x 1 m), each containing a single wheat plant, ranged between 447 and 1423 mg C and was evolved with a maximum rate 1.6-7.3 times that of the unplanted soil columns. The amount of C translocated below ground was equivalent to 1765 kg C ha-1 and was not significantly affected by the moisture treatment. Between 31 and 38% of the soil C-14 residue was found incorporated into the soil microbial biomass, with differences not significantly related to the different moisture regimes. Humic acid and humin fractions, essentially free from protein and carbohydrate components, contained 38 42% of the soil C-14 residue.
Winter barley (Hordeum vulgare L. cultivar ‘Marinka’) was grown in the field in 30 stainless steel cylinders arranged in ten triplets. At four growth stages (Feekes Stage 4, 6–7, 10.3–4 and 10.5.4–11.1), the shoots of two or three triplets were exposed to 14CO2 for 8 – 10 h using a closed, temperature-regulated cabinet. Five days after labelling, the barley was cut at ground level and the cylinders retrieved for analyses. Below-ground transfer of 14C was divided into macro-roots (roots isolated by soil washing), macro-root-free soil, and rhizosphere (root and microbial) respiration. Additional labelled cylinders were left in the field and retrieved at crop maturity on 2 August or 11 October.
In presence of vegetation, the CO2-C produced by respiration activity in soils originates from plant C (rhizosphere respiration, R(rh)) and from soil C (soil respiration, R(s)). Quantitative estimates of the CO2 produced by each source are required in many studies of C dynamics in the soil plant system. In this study, we (i) used measurements of the 13C value of soil CO2 to separate total soil respiration (R1) into subcomponents R(rh) and R(s) in a maize (Zea mays L.) field under undisturbed conditions and (ii) compared these R(rh) estimates with values obtained using the root-exclusion approach. The maximum contribution of R(rh) to total respiration was 45%, observed in August. Estimates of R(rh) increased from zero 30 d after planting to 2 g CO2-C m -2 d -1 70 d after planting, remained relatively constant at that level in August, and then decreased until the end of the growing season. The total C losses as R(rh) were 17% of the crop net assimilation. Estimates of R(s) gradually declined from 3.3 g CO2-C m -2 d -1 in late June to 1.4 g CO2-C m -2 d -1 at the end of the season. Losses of soil C represented = 6% of total soil C. Variable values of δ13C of the soil CO2 in the control plot after Day 250 made the technique less reliable late in the season. However, several observations indicated that the approach has potential to provide quantitative estimates of R(rh) and R(s). First, the seasonal pattern of the R(rh) estimates coincided with that of the plant growth and physiological activity. Second, the cumulated R(rh) across the growing season agreed well with published data obtained using 14C labeling techniques. Third, in the maize plot, variation in the estimated R(s) was closely correlated with changes in soil temperature with a Q10 of 1.99 (r2 = 0.87). Finally, the estimates of R(rh) obtained using the isotopic approach agreed well with those obtained using the root exclusion technique.
Living plants change the local environment in the rhizosphere and consequently affect the rate of soil organic matter (SOM) decomposition. The rate may increase for 3- to 5-folds, or decrease by 10% to 30% by plant cultivation. Such short-term changes of rate (intensity) of SOM decomposition are due to the priming effect. In the presence of plants, a priming effect occurs in the direct vicinity of the living roots, and it is called rhizosphere priming effect (RPE). Plant-mediated and environmental factors, such as, plant species, development stage, soil organic matter content, photosynthesis intensity, and N fertilization which affect RPE are reviewed and discussed in this paper. It was concluded that root growth dynamics and photosynthesis intensity are the most important plant-mediated factors affecting RPE. Environmental factors such as amount of decomposable C in soil and N min content are responsible for the switch between following mechanisms of RPE: concurrence for Nmin between roots and microorganisms, microbial activation or preferential substrate utilization. Succession of mechanisms of RPE along the growing root in accordance with the rhizodeposition types is suggested. Different hypotheses for mechanisms of filling up the C amount loss by RPE are suggested. The ecosystematic relevance of priming effects by rhizodeposition relates to the connection between exudation of organic substances by roots, the increase of microbial activity in the rhizosphere through utilization of additional easily available C sources, and the subsequent intensive microbial mobilization of nutrients from the soil organic matter.
This study addresses the issue of carbon (C) fluxes through below ground pools within the rhizosphere of Lolium perenne using the 14C pulse labeling. Lolium perenne was grown in plexiglas chambers on topsoil of a Haplic Luvisol under controled laboratory conditions. 14C-CO2 efflux from soil, as well as 14C content in shoots, roots, soil, dissolved organic C (DOC), and microbial biomass were monitored for 11 days after the pulsing. Lolium allocates about 48 % of the total assimilated 14C below the soil surface, and roots were the primary sink for this C. Maximum 14C content in the roots was observed 12 hours after the labeling and it amounts to 42 % of the assimilated C. Only half of the 14C amount was found in the roots at the end of the monitoring period. The remainder was lost through root respiration, root decomposition, and rhizodeposition. Six hours after the 14C pulse labeling soil accounted for 11 %, DOC for 1.1 %, and microbial biomass for 4.9 % of assimilated C. 14C in CO2 efflux from soil was detected as early as 30 minutes after labeling. The maximum 14C-CO2 emission rate (0.34 % of assimilated 14C h-1) from the soil occurred between four and twelve hours after labeling. From the 5th day onwards, only insignificant changes in carbon partitioning occurred. The partitioning of assimilated C was completed after 5 days after assimilation. Based on the 14C partitioning pattern, we calculated the amount of assimilated C during 47 days of growth at 256 g C m-2. Of this amount 122 g C m-2 were allocated to below ground, shoots retained 64 g C m-2, and 70 g C m-2 were lost from the shoots due to respiration. Roots were the main sink for below ground C and they accounted for 74 g C m-2, while 28 g C m-2 were respired and 19 g C m-2 were found as residual 14C in soil and microorganisms.
Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by 14 CO 2 pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total 14 CO 2 efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after 14 C pulse labelling decreased during plant development from 14 to 6.5% of the total 14 C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the 14 C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total 14 CO 2 efflux from the soil was approximately 41%. Close correlation was found between cumulative 14 CO 2 efflux from the soil and the time when maximum 14 CO 2 efflux occurred (r=0.97). The average total of CO 2 efflux from the soil with Lolium perenne was approximately 21 µg C-CO 2 d −1 g −1 . It increased slightly during plant development. The contribution of plant roots to total CO 2 efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total 14 C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m −2 at the beginning of the growth period and 50–65 g C m −2 towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost.
The effects of shading wheat plants on rhizosphere respiration and rhizosphere priming of soil organic matter decomposition were investigated by using a natural abundance 13 C tracer method and 14 C pulse labeling simultaneously. Seven days with strongly reduced photosynthesis (12/60 h day/night period) resulted in only half of the total CO 2 ef¯ux from soil compared to the treatment with a 12/12 h day/ night period. The CO 2 ef¯ux from unplanted soil amounted to only 12 and 20% of the total CO 2 ef¯ux from the soil with non-shaded and shaded plants, respectively. On average 75% of total CO 2 ef¯ux from the planted soil with prolonged night periods was root-derived. Rhizosphere respiration was tightly coupled with plant photosynthetic activity. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO 2 ef¯ux, and thereby, total CO 2 ef¯ux from soils. Clear diurnal dynamics of the total CO 2 ef¯ux intensity indicate the existence of an endogenous control mechanism of rhizosphere respiration. The light-on events after prolonged dark periods lead to strong increases of root-derived and therefore of total CO 2 ef¯ux from soil. After 14 C pulse labeling, two maxima of the root-derived 14 CO 2 ef¯ux were measured (6 and 24 h). This result demonstrated the diurnal dynamics of the rhizosphere respiration of recently-assimilated C in both the normal light conditions and shaded plants as well. The total amount of root-derived C respired in the rhizosphere was 17.3 and 20.6% of the total assimilated C for non-shaded and shaded plants, respectively. Both methods used, 13 C natural abundance and 14 C pulse labeling, gave similar estimates of root-derived CO 2 during the whole observation period: 1.80 ^ 0.27 and 1.67 ^ 0.37 mg C kg 21 h 21 (^SD), respectively. Both tracer methods show that the cultivation of wheat led to the increasing decomposition intensity of soil organic matter (priming effect). Additionally, 13 C natural abundance allows tracing of the dynamics of the priming effect depending on the light-on events. q
The loss of organic material from the roots of forage rape (Brassica napus L.,) was studied by pulse-labelling 25-d-old non-sterile sand-grown plants with 14CO2. The distribution of 14C within the plant was measured at 0, 6 and 13 d after labelling whilst 14 C accumulating in the root-zone was measured at more frequent intervals. The rates of 14C release into the rhizosphere, and loss of 14CO2 from the rhizosphere were also determined. These data were used to estimate the accumulative loss of 14C from roots and loss respiratory 14CO2 from both roots and associated micro-organisms. Approximately 17-19% of fixed 14CO2 was translocated to the roots over 2 weeks, of which 30-34% was released into the rhizosphere, and 23-24% was respired by
the roots as 14 CO2. Of the 14C released into the rhizosphere, between 35-51% was assimilated and respired by rhizosphere micro-organisms.Copyright 1993, 1999 Academic Press
Recent advances in the application of molecular genetic approaches have emphasized our potentially huge underestimate of microbial diversity in a range of natural environments(1). These approaches, however, give no direct information about the biogeochemical processes in which microorganisms are active(2). Here we describe an approach to directly link specific environmental microbial processes with the organisms involved, based on the stable-carbon-isotope labelling of individual lipid biomarkers. We demonstrate this approach in aquatic sediments and provide evidence for the identity of the bacteria involved in two important biogeochemical processes: sulphate reduction coupled to acetate oxidation in estuarine and brackish sediments(3,4), and methane oxidation in a freshwater sediment(5). Our results suggest that acetate added in a C-13-labelled form was predominantly consumed by sulphate-reducing bacteria similar to the Gram-positive Desulfotomaculum acetoxidans and not by a population of the more widely studied Gram-negative Desulfobacter spp. Furthermore, C-13-methane labelling experiments suggest that type I methanotrophic bacteria dominate methane oxidation at the freshwater site.
Seedlings of Norway spruce (Picea abies [L.] Karst.), which had been grown under sterile conditions for three months, were treated for one week in a hydroculture system with either 500 μM AlCl3 or 750 μM CaCl2 solutions at pH 4. Organic acids were determined in hot-water extracts of ground root tissue. Oxalate (3.3-6.6 μmol (g root dry weight)-1) was most abundant. Malate, citrate, formate, acetate, and lactate concentrations ranged between 1-2 μmol (g root dry weight)-1. Organic substances and phosphate found in the treatment solutions at the end of the experimental period were considered to be root exudates. Total root exudation within a 2-day period ranged from 20-40 μmol C (g root weight)-1. In root exudates, organic acids, and total carbohydrates, total amino acids, and total phenolic substances were quantified. Citrate and malate, although present in hot-water extracts of root tissue, were not detected in root exudates. Phosphate was released from Ca-treated plants. In Al treatments, there was indication of Al phosphate precipitation at the root surface. Oxalate and phenolics present in the exudates of Norway spruce seedlings are ligands that can form stable complexes with Al. However, concentrations of these substances in the treatment solutions were at micromolar levels. Their importance for the protection of the sensitive root apex under natural conditions is discussed.
Plant roots release in the rhizosphere diverse organic materials which may have different effects on soil structure. We have evaluated the effect of natural and modelled root-released materials on soil aggregates and the biodegradation of carbon from roots in the soil. The effects of root mucilage from maize and of a modelled soluble exudate were compared with those of simple compounds (glucose, polygalacturonic acid). For all treatments, soil was amended with 2 g C kg−1 soil and incubated for 30 days at 25°C. The biodegradation of mucilage was similar to that of polygalacturonic acid, and slower than the decomposition of modelled exudates and glucose. Addition of all substrates increased the stability of aggregates, but the duration of this effect depended on the chemical nature of the material. Compared with the control, the proportion of stable aggregates after 30 days of incubation was multiplied by 3.8 for root mucilage, by 4.2 for modelled soluble exudates, by 2.5 for polygalacturonic acid and by 2.0 for glucose. The different fractions of root exudates in the rhizosphere evidently affected the aggregate stability.
For a quantitative analysis of SOC dynamics it is necessary to trace the origins of the soil organic compounds and the pathways of their transformations. We used the ¹³C isotope to determine the incorporation of maize residues into the soil organic carbon (SOC), to trace the origin of the dissolved organic carbon (DOC), and to quantify the fraction of the maize C in the soil respiration. The maize-derived SOC was quantified in soil samples collected to a depth of 65 cm from two plots, one ’continuous maize’ and the other ’continuous rye’ (reference site) from the long-term field experiment ’Ewiger Roggen’ in Halle. This field trial was established in 1878 and was partly changed to a continuous maize cropping system in 1961. Production rates and δ¹³C of DOC and CO2 were determined for the Ap horizon in incubation experiments with undisturbed soil columns. After 37 years of continuous maize cropping, 15% of the total SOC in the topsoil originated from maize C. The fraction of the maize-derived C below the ploughed horizon was only 5 to 3%. The total amount of maize C stored in the profile was 9080 kg ha⁻¹ which was equal to about 31% of the estimated total C input via maize residues (roots and stubble). Total leaching of DOC during the incubation period of 16 weeks was 1.1 g m⁻² and one third of the DOC derived from maize C. The specific DOC production rate from the maize-derived SOC was 2.5 times higher than that from the older humus formed by C3 plants. The total CO2-C emission for 16 weeks was 18 g m⁻². Fifty-eight percent of the soil respiration originated from maize C. The specific CO2 formation from maize-derived SOC was 8 times higher than that from the older SOC formed by C3 plants. The ratio of DOC production to CO2-C production was three times smaller for the young, maize-derived SOC than for the older humus formed by C3 plants.
Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as: –homogeneous labelling of whole plants with ¹⁴ C
–distinction between root and microbial respiration
–separation of soil zones of known distances from the roots
–determination of microbial soil biomass.
These methods were applied to study the following aspects: –release of organic plant carbon into the soil by growing roots
–utilization of this plant carbon by the microbial biomass in the rhizosphere
–related influence on the turnover of soil organic matter, and
–spatial range of such root influence in the soil.
About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO 2 , while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C‐input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%).
Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.
Perennial rye-grass plants were pulse labelled with [14C]-CO2 over a range of temperatures (5–25°C). The fate of the label was determined within the plant and soil. The temperature at which plants were pulse labelled had a marked effect on the distribution of the label within the plant and soil system. Root-soil respiration increased from 5.7 to 24.15% when expressed as a percentage of net assimilated label. The percentage of label remaining in the plant root and in the soil was greater at 5 and 25°C, with a minimum for both these components at 15°C. At 15°C the percentage of net assimilated label that remained in the shoots was greater than at other temperatures, with this percentage decreasing at the lower and higher temperatures.
The present work describes an original method to follow rate of (CO2)-C-14 and total CO2 production from rhizosphere respiration after plant shoots had been pulse-labelled with (CO2)-C-14. We used a radioactivity detector equipped with a plastic cell for flow detection of beta radiation by solid scintillation counting. The radioactivity detector was coupled with an infrared gas analyser. The flow detection of (CO2)-C-14 was compared to trapping of (CO2)-C-14 in NaOH and counting by liquid scintillation. First, we demonstrated that NaOH (1 M) trapped 95% of the CO2 of a gaseous sample. Then, we determined that the counting efficiency of the radioactivity flow cell was 41% of the activity of gaseous samples as determined by trapping in NaOH (1 M) and by counting by static liquid scintillation. The sensitivity of the (CO2)-C-14- flow detection was 0.08 Bq mL(-1) air and the precision was 2.9% of the activity measured compared to 0.9% for NaOH trapping method. We presented two a
The composition of root-derived substances is of great importance for the understanding of processes in the rhizosphere. Therefore, methods allowing a comprehensive collection and chemical analysis of the organic root exudates are necessary. In this study, we compare different methods with regard to their suitability to collect and characterize root exudates. Because the percolation or water logging method failed to quantitatively extract root exudates, a dipping method was developed which allowed an almost complete sampling of coldwater-soluble root exudates. By 14CO2 labeling of the shoots the composition of root exudates was found to be influenced by plant species and growth stage. In comparison to pea plants maize plants had a higher share of carboxylic acids and a lower share of sugars. Younger maize plants exuded considerably higher amounts of 14C labeled organic substances per g root dry matter than older ones. During plant development the relative amount of sugars decreased at the expense of carboxylic acids. The described methods are well suited for the elucidation of the influence of growth factors on root exudation.
The root-borne C- and N-flux in the plant/soil system was studied by determining the 14C- or 15N-balances in pot trials with soil as a substrate (14CO2- or 15NH3-application to the shoots, comparison of sterile and nonsterile treatments for quantification of root-borne substances). The following results were obtained: 1. The amount of (primary) root-borne carbon compounds released into soil was (besides root respiration) 11-20% of net-CO2-assimilation or 13-32% of the 14C incorporated into the plants (= 1 t C · ha-1). 5-6% of 15N assimilated by the plants were released as root-borne N compounds (= 15 kg N · ha-1). 2. A considerable portion of the root-borne C (about 6% = 600 kg C · ha-1) was found in the rooted soil zone at the end of the experiments (rhizodeposition). 3. (Primary) root-borne C and N compounds found in immediate vicinity of the roots (about 60-80%) were mainly water soluble, whereas most of the C and N compounds found in a greater distance were water insoluble. The water soluble exudates consisted mainly of neutral (carbohydrates) and acid fractions (organic acids). The basic fraction (amino acids) made up a small portion only. 4. The root-borne C and N compounds influenced the nutrient balance of soil and plant directly and/or indirectly via microbes (depending on species, variety and nutritional status of plants). 5. Microbes stimulated the release of C- and N-compounds, but rapidly respired 65-85% of the root-borne C-compounds, thereby putting a burden on the C-budget of the "host" plant. 6. It could be shown by the example of hup+ Rhizobium meliloti strains (tested by 3H2-incorporation) and the wheat-Serratia-association, that energy efficient microbenplant systems can improve plant performance.
Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO4 solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil-grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO4. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under-estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO4 for 2 h, root exudates of soil-grown traditional, tall rice cultivars (Dular, B40 and Intan), high-yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH4 emissions. Therefore, the use of high-yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate-induced CH4 production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH4 emission from rice agriculture to the atmosphere.
A microcosm unit is described which readily allows manipulation of experimental conditions to enable the subsequent impact on root exudation release to be monitored with time. Festuca ovina and Plantago lanceolata seedlings were grown in this microcosm unit over a 34 day experimental period under conditions of high (3.75 mol m–3 N) or low (1.25 mol m–3 N) nitrate-nitrogen treatment. At the end of the experimental period the seedlings in the microcosms were labelled with [14C]-CO2 and the fate of the label within the plant and its release by the roots monitored. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period as well as during the 14C-chase period and comparison of plant C budgets using these two measurements is discussed. Nitrogen treatment as found to have a greater effect on exudate release by F. ovina than by P. lanceolata seedlings as indicated by both the total organic carbon and 14C results. The use and applications of the microcosm unit are discussed.