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Ten years of free‐air CO2 enrichment altered the mobilization of N from soil in Lolium perenne L. swards
Abstract
Effects of free-air carbon dioxide enrichment (FACE, 60 Pa pCO2) on plant growth as compared with ambient pCO2 (36 Pa) were studied in swards of Lolium perenne L. (perennial ryegrass) at two levels of N fertilization (14 and 56 g m−2 a−1) from 1993 to 2002. The objectives were to determine how plant growth responded to the availability of C and N in the long term and how the supply of N to the plant from the two sources of N in the soil, soil organic matter (SOM) and mineral fertilizer, varied over time. In three field experiments, 15N-labelled fertilizer was used to distinguish the sources of available N.
In 1993, harvestable biomass under elevated pCO2 was 7% higher than under ambient pCO2. This relative pCO2 response increased to 32% in 2002 at high N, but remained low at low N. Between 1993 and 2002, the proportions and amounts of N in harvestable biomass derived from SOM (excluding remobilized fertilizer) were, at high N, increasingly higher at elevated pCO2 than at ambient pCO2. Two factorial experiments confirmed that at high N, but not at low N, a higher proportion of N in harvestable biomass was derived from soil (including remobilized fertilizer) following 7 and 9 years of elevated pCO2, when compared with ambient pCO2.
It is suggested that N availability in the soil initially limited the pCO2 response of harvestable biomass. At high N, the limitation of plant growth decreased over time as a result of the stimulated mobilization of N from soil, especially from SOM. Consequently, harvestable biomass increasingly responded to elevated pCO2. The underlying mechanisms which contributed to the increased mobilization of N from SOM under elevated pCO2 are discussed. This study demonstrated that there are feedback mechanisms in the soil which are only revealed during long-term field experiments. Such investigations are thus, a prerequisite for understanding the responses of ecosystems to elevated pCO2 and N supply.
... Although numerous experiments have been conducted over the past two decades to investigate the role of N in constraining CO 2induced stimulation of photosynthesis and primary production, there is still no general explanation for the disparity of responses observed among different ecosystems (Bazzaz, 1990;Saxe et al., 1998;Nowak et al., 2004;K€ orner, 2006;Reich et al., 2006b;Norby & Zak, 2011). In some studies, low N availability was found to be the primary constraint responsible for the transient, small or nonexistent CO 2 fertilization effect (Schneider et al., 2004;Norby et al., 2010;Sigurdsson et al., 2013). In other studies, plant production was stimulated by eCO 2 despite apparent N-limitation (McCarthy et al., 2010;Hungate et al., 2013;Talhelm et al., 2014). ...
... AM systems showed a wider range of responses, presumably driven by their variable capacity to acquire N, either through N-fertilization or association with N 2 -fixers. For example, AMgrassland Lolium perenne at SwissFACE showed a positive CO 2induced aboveground biomass enhancement under high N, but not in low N plots (Schneider et al., 2004), consistent with the lower cost of N acq associated with N-fertilization (Fig. 3). Medicago sativa in this same experiment, however, showed a positive effect on ANPP and N acq even under low N, consistent with its N 2 -fixing capacity (L€ uscher et al., 2000) (data not included in Fig. 3 because no indication of C bg was found). ...
... Other AM ecosystems, however, do not follow this pattern. In the SwissFACE experiment, neither the AM grass Lolium perenne nor the N 2 -fixer Trifolium repens showed an increase in soil C storage after 10 yr of eCO 2 ( van Kessel et al., 2006), despite a positive effect on photosynthesis (Ainsworth et al., 2003) and a lack of N-mineralization and ANPP response under low N availability (Schneider et al., 2004). eCO 2 did not increase soil C content at GiFACE either (Lenhart et al., 2016), but the presence of legumes may have contributed to an increase in the allocation of C transfer to N 2 -fixation, rather than soil C stabilization, which would explain the strong increase in abundance of legume species from c. 1% at the beginning of the experiment to 10% in later years, together with an increasingly positive overall effect on plant biomass (Andresen et al., 2017). ...
Contents
Summary
507
I.
Introduction
507
II.
The return on investment approach
508
III.
CO 2 response spectrum
510
IV.
Discussion
516
Acknowledgements
518
References
518
Summary
Land ecosystems sequester on average about a quarter of anthropogenic CO 2 emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants’ ability to store additional carbon (C) under rising CO 2 , but these mechanisms are not well understood. Here, we review findings from elevated CO 2 experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO 2 may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N‐fixing microbes. We found that N‐acquisition efficiency is positively correlated with leaf‐level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N‐fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts.
... Most field studies have involved mixtures of herbaceous species in swards, either natural or introduced, but responses of monocultures or nearly pure areas of species have also been determined, e.g. for Trifolium subterraneum (Lilley et al. 2001), Phalaris aquatica (Lilley et al. 2001;Volder et al. 2007), Pascopyrum smithii (Hunt et al. 1996), Lolium perenne (Hebeisen et al. 1997;Ainsworth et al. 2003;Schneider et al. 2004), and Trifolium repens (Hebeisen et al. 1997). The responses to climate change factors of C 3 and C 4 herbaceous species, with their different photosynthetic pathways and mechanisms, have been compared in field experiments and controlled environments (Crush 1994;Greer et al. 1995;Hunt et al. 1996;Ghannoum & Conroy 1998;Lee et al. 2011;Morgan et al. 2001). ...
... Reviewed literature from 1980-1997 showed that C 4 and C 3 grasses (Poaceae) increased total biomass significantly under elevated CO 2 concentration by 33% and 44%, respectively (Wand et al. 1999). Under controlled conditions, C 3 and C 4 Panicum grasses grown in pots failed to respond to CO 2 enrichment in nitrogen-deficient soil (Ghannoum & Conroy 1998), and in other studies, relatively high growth stimulation occurred at elevated CO 2 concentration under high (Bowler & Press 1996;Schneider et al. 2004) or limiting (Wong & Osmond 1991) nitrogen levels, or was not influenced by nitrogen status (Hocking & Meyer 1991). Reich et al. (2001) found growth and other responses to elevated CO 2 concentration and soil N status for 16 grassland species and there was no interaction between CO 2 concentration and soil N status. ...
... Effects of elevated CO 2 concentration and elevated temperature on herbaceous seed germination and seedling survival are less certain. Studies investigating the interaction of elevated CO 2 and soil N (Wong & Osmond 1991;Ghannoum & Conroy 1998;Schneider et al. 2004) are inconclusive. Because of the uncertainty around the interactions of elevated CO 2 concentration and soil N status, these findings have implications for establishing and maintaining ground cover on eroded soils, which are characterised by low organic matter content, low nutrient status and low water storage capacity (Rosser & Ross 2011). ...
... T he CO 2 fertilization of plant growth by rising atmospheric CO 2 concentrations [CO 2 ] sequesters carbon in plant biomass and thus has the potential to slow the future rate of increase in [CO 2 ] and thus the pace of associated climate change 1 , but how this fertilization varies with environmental conditions remains unclear. The CO 2 fertilization effect may be limited, for example, by low supply of soil resources [2][3][4][5][6] . This hypothesis follows logically from multiple resource limitation theory 2,7 , which suggests that responses to elevated [CO 2 ] (eCO 2 ) may depend on levels of other resources ( Supplementary Fig. 1). ...
... Moreover, eCO 2 can enhance or reduce the eCO 2 fertilization effect by influencing soil resource supply [15][16][17] , and thereby altering the relative limitations imposed by different resources (that is, shifting position on the soil resource axes in Supplementary Fig. 1), or by favouring species that are more or less responsive to eCO 2 or that themselves alter soil resource supply 18 . For example, the N limitation feedback hypothesis [4][5][6]16 suggests that negative impacts of eCO 2 on N cycling can constrain responses to eCO 2 . In contrast, eCO 2 fertilization may be enhanced under conditions of low water supply given that eCO 2 reduces stomatal conductance, and thus plant water use, increasing soil moisture 8,9,[11][12][13][14][15][19][20][21][22][23] . ...
... Past studies show that interactive effects among multiple globalchange factors on ecosystem processes are common, although not ubiquitous [4][5][6]8,10,24,25 , and models of global-change impacts on ecosystems point to the importance of such potential interactions 26 . However, an understanding of the interactive effects of eCO 2 and multiple soil resources on ecosystems is limited by the low number of pertinent studies. ...
Rising atmospheric CO2 concentrations can fertilize plant growth. The resulting increased plant uptake of CO2 could, in turn, slow increases in atmospheric CO2 levels and associated climate warming. CO2 fertilization effects may be enhanced when water availability is low, because elevated CO2 also leads to improved plant water-use efficiency. However, CO2 fertilization effects may be weaker when plant growth is limited by nutrient availability. How variation in soil nutrients and water may act together to influence CO2 fertilization is unresolved. Here we report plant biomass levels from a five-year, open-air experiment in a perennial grassland under two contrasting levels of atmospheric CO2, soil nitrogen and summer rainfall, respectively. We find that the presence of a CO2 fertilization effect depends on the amount of available nitrogen and water. Specifically, elevated CO2 levels led to an increase in plant biomass of more than 33% when summer rainfall, nitrogen supply, or both were at the higher levels (ambient for rainfall and elevated for soil nitrogen). But elevated CO2 concentrations did not increase plant biomass when both rainfall and nitrogen were at their lower level. We conclude that given widespread, simultaneous limitation by water and nutrients, large stimulation of biomass by rising atmospheric CO2 concentrations may not be ubiquitous.
... The uncertainty regarding the impact of rising atmospheric CO 2 on the magnitude of the terrestrial C sink is presently the largest unknown for terrestrial ecosystem feedbacks on climate change (IPCC, 2013). A key part of this uncertainty is related to how and to what extent nitrogen (N) availability constrains the elevated CO 2 (eCO 2 )-induced stimulation of terrestrial net primary productionas often seen in field experiments (Schneider et al., 2004;Reich et al., 2006;Norby et al., 2010;Reich & Hobbie, 2013) and whether or not this constraint will become stronger over time (Hungate et al., 2003;Luo et al., 2004). ...
... While increased N sequestration in long-lived plant biomass and soil organic matter and slower decomposition of litter with lower N concentration may contribute to PNL in eCO 2 (Rastetter et al., 1992;Luo et al., 2004), there are thus also other processes by which eCO 2 may sustain increased plant N acquisition over long time periods (#P1-3 in Table 1). There are examples of longterm free-air CO 2 enrichment (FACE) experiments with declining (Norby et al., 2010), approximately constant (Schneider et al., 2004;McCarthy et al., 2010;Reich & Hobbie, 2013;Li et al., 2014;Talhelm et al., 2014) or increasing (Schneider et al., 2004;Reich & Hobbie, 2013) temporal response trends of eCO 2 -induced productivity enhancement. For forests, it has earlier been indicated that the initial productivity enhancement in eCO 2 declines rapidly during the first few years, but data available at that time only allowed for analysis of temporal response trends of up to five years (K€ orner, 2006). ...
... While increased N sequestration in long-lived plant biomass and soil organic matter and slower decomposition of litter with lower N concentration may contribute to PNL in eCO 2 (Rastetter et al., 1992;Luo et al., 2004), there are thus also other processes by which eCO 2 may sustain increased plant N acquisition over long time periods (#P1-3 in Table 1). There are examples of longterm free-air CO 2 enrichment (FACE) experiments with declining (Norby et al., 2010), approximately constant (Schneider et al., 2004;McCarthy et al., 2010;Reich & Hobbie, 2013;Li et al., 2014;Talhelm et al., 2014) or increasing (Schneider et al., 2004;Reich & Hobbie, 2013) temporal response trends of eCO 2 -induced productivity enhancement. For forests, it has earlier been indicated that the initial productivity enhancement in eCO 2 declines rapidly during the first few years, but data available at that time only allowed for analysis of temporal response trends of up to five years (K€ orner, 2006). ...
... The uncertainty regarding the impact of rising atmospheric CO 2 on the magnitude of the terrestrial C sink is presently the largest unknown for terrestrial ecosystem feedbacks on climate change (IPCC, 2013). A key part of this uncertainty is related to how and to what extent nitrogen (N) availability constrains the elevated CO 2 (eCO 2 )-induced stimulation of terrestrial net primary productionas often seen in field experiments (Schneider et al., 2004;Reich et al., 2006;Norby et al., 2010;Reich & Hobbie, 2013) and whether or not this constraint will become stronger over time (Hungate et al., 2003;Luo et al., 2004). ...
... While increased N sequestration in long-lived plant biomass and soil organic matter and slower decomposition of litter with lower N concentration may contribute to PNL in eCO 2 (Rastetter et al., 1992;Luo et al., 2004), there are thus also other processes by which eCO 2 may sustain increased plant N acquisition over long time periods (#P1-3 in Table 1). There are examples of longterm free-air CO 2 enrichment (FACE) experiments with declining (Norby et al., 2010), approximately constant (Schneider et al., 2004;McCarthy et al., 2010;Reich & Hobbie, 2013;Li et al., 2014;Talhelm et al., 2014) or increasing (Schneider et al., 2004;Reich & Hobbie, 2013) temporal response trends of eCO 2 -induced productivity enhancement. For forests, it has earlier been indicated that the initial productivity enhancement in eCO 2 declines rapidly during the first few years, but data available at that time only allowed for analysis of temporal response trends of up to five years (K€ orner, 2006). ...
... While increased N sequestration in long-lived plant biomass and soil organic matter and slower decomposition of litter with lower N concentration may contribute to PNL in eCO 2 (Rastetter et al., 1992;Luo et al., 2004), there are thus also other processes by which eCO 2 may sustain increased plant N acquisition over long time periods (#P1-3 in Table 1). There are examples of longterm free-air CO 2 enrichment (FACE) experiments with declining (Norby et al., 2010), approximately constant (Schneider et al., 2004;McCarthy et al., 2010;Reich & Hobbie, 2013;Li et al., 2014;Talhelm et al., 2014) or increasing (Schneider et al., 2004;Reich & Hobbie, 2013) temporal response trends of eCO 2 -induced productivity enhancement. For forests, it has earlier been indicated that the initial productivity enhancement in eCO 2 declines rapidly during the first few years, but data available at that time only allowed for analysis of temporal response trends of up to five years (K€ orner, 2006). ...
A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in Free-Air CO2 Enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) In all three ecosystem types, this relationship was positive, linear, and strong (r(2) = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. Since the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2 , and this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial time scale covered by FACE studies, N limitation of eCO2 -induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
... Finalement, la méthode du marquage continu permet d'éviter les désavantages des deux méthodes précédentes. Il consiste à exposer en continu les plantes à de l'air marqué au 13 C dans une serre afin d'assurer une stabilité du CO2 marqué et une homogénéité de marquage au cours du temps dans le matériel plante (Schneider et al., 2004;Shahzad et al., 2012). La plupart des études, pour des raisons financières, utilisent du 13 CO2 provenant de la combustion d'énergie fossile comme source de marquage (δ 13 Cfossil ≈ -38 ‰; δ 13 Cair ≈ -8 ‰) Pausch et al., 2013). ...
... Une méthode basée sur la mesure différentielle de CO2 entre l'entrée et la sortie d'une serre existe (Schneider et al., 2004) -L'incertitude de l'estimation du RPE dépend de l'intensité de marquage choisie. ...
La présence de plantes accélère la décomposition de la matière organique du sol (MOS) au travers de l’apport de composés riches en énergie (rhizodépôts et litières) stimulant les microorganismes ; un phénomène appelé « rhizosphere priming effect » (RPE). Une augmentation de la photosynthèse, activité pourvoyeuse d'énergie rhizodéposée, pourrait augmenter le RPE et l’offre du sol en nutriments. Récemment, le modèle SYMPHONY couplant activités photosynthétiques et microbiennes du sol suggère un ajustement de l'offre du sol en nutriments (delta minéralisation-immobilisation) à la demande des plantes. Cependant, le rôle clé de la photosynthèse sur cet ajustement offre-demande reste à étudier expérimentalement.L’objectif général de la thèse est d'étudier le rôle des interactions des activités photosynthétiques et microbiennes du sol dans les régulations des flux de carbone (C) et d'azote (N) des écosystèmes. Trois écosystèmes types ont été étudiés : la prairie, la monoculture de blé et un nouveau système de culture (NSC) associant blé et plantes pérennes de la prairie. Nous émettons l’hypothèse que les plantes pérennes, via une activité photosynthétique pourvoyant les microorganismes en énergie tout au long de l'année, sont essentielles à l'ajustement offre-demande en N. De nombreux défis techniques ont été relevés afin de construire une plateforme expérimentale de 40 mésocosmes sous éclairage et température naturels. Cette plateforme permet de coupler marquage 13C des plantes, mesures continues des échanges de CO2, du RPE, de la production végétale, du stockage de C du sol, le taux de minéralisation-immobilisation d'N et du lessivage d'N. Ce dispositif nous a permis de déterminer la contribution du RPE dans les flux de C des écosystèmes comprenant la production nette de l’écosystème (NEP), la production primaire brute (GPP) et la respiration de l’écosystème (RE) exprimées en g C m-2 24h-1. Nous avons montré une relation positive linéaire entre (1) RPE et GPP et (2) RPE et biomasse aérienne (AGB) (g C m-2). A partir de ces relations, le RPE peut être prédit en utilisant les équations suivantes : (...). Nous montrons un ajustement offre-demande de l’N au cours des saisons : une forte activité photosynthétique (printemps) est liée à un RPE et un delta minéralisation-immobilisation d’N élevés alors qu’une faible activité photosynthétique (automne) est liée à un RPE et un delta minéralisation-immobilisation d’N faibles. Cet ajustement était observé dans la prairie et dans le NSC mais pas en monoculture de blé. Logiquement, la lixiviation d’N était importante en monoculture de blé alors qu’elle était quasi nulle en prairie et dans le NSC. Après deux années de maintien des trois écosystèmes types, la production aérienne totale du NSC était équivalente à la prairie, tous deux étant supérieurs d’environ un facteur deux à la monoculture de blé. Ces résultats confirment l’importance des plantes pérennes dans la synchronisation offre-demande de l’N. L’ensemble de ces investigations souligne l’importance des activités des plantes et des processus rhizosphériques dans la régulation des cycles CN des écosystèmes. Ces régulations pourront être étudiées in situ et à l'échelle globale grâce aux proxys de ces processus rhizosphériques (RPE, ajustement offre-demande) déterminés dans la thèse. Des activités photosynthétiques et rhizosphériques tout au long de l'année sont essentielles à l'ajustement offre-demande en nutriments conduisant à une forte production primaire, à la fermeture des cycles des nutriments et au stockage de MOS. Ces découvertes offrent l'opportunité de construire de nouveaux systèmes de culture, à l'image de l’association blé-plantes pérennes étudiée, à hautes performances agro-environnementales.
... Plant pin hits were correlated to pin hits of plots where biomass was harvested as reference (Kr€ oel-Dulay et al., 2015). At other sites, typically grasslands, biomass was harvested above defoliation height as part of the management type and hereby directly determined the agriculturally relevant forage production (Schneider et al., 2004). We have aligned these measures as equally valid estimates of annual biomass production in this analysis. ...
... Thus, biomass response during the first years of a CO 2 experiment have to be treated with caution as compared to biomass increase under a slowly developing change in climatic conditions (Rastetter et al., 1997). However, by deploying the climate treatments for many years in long-term experiments (beyond this initial phase), realistic acclimation responses to future climatic conditions are likely (Mueller et al., 2016;Schneider et al., 2004). Still, it is surprising that manipulations of eCO 2 commonly alter the direction of the biomass response over time. ...
full text : http://nora.nerc.ac.uk/514680/
Field experiments that expose terrestrial ecosystems to climate change factors by
manipulations are expensive to maintain, and typically only last a few years. Plant biomass
is commonly used to assess responses to climate treatments and to predict climate
change impacts. However, response to the treatments might be considerably
different between the early years and a decade later. The aim of this data analysis
was to develop and apply a method for evaluating changes in plant biomass
responses through time, in order to provide a firm basis for discussing how the ‘shortterm’
response might differ from the ‘long-term’ response. Across 22 sites situated in
the northern hemisphere, which covered three continents, and multiple ecosystems
(grasslands, shrublands, moorlands, forests, and deserts), we evaluated biomass
datasets from long-term experiments with exposure to elevated CO2 (eCO2), warming,
or drought. We developed methods for assessing biomass response patterns to the
manipulations using polynomial and linear (piecewise) model analysis and linked
the responses to site-specific variables such as temperature and rainfall. Polynomial
patterns across sites indicated changes in response direction over time under eCO2,
warming, and drought. In addition, five distinct pattern types were confirmed within
sites: ‘no response’, ‘delayed response’, ‘directional response’, ‘dampening response’,
and ‘altered response’ patterns. We found that biomass response direction was as likely
to change over time as it was to be consistent, and therefore suggest that climate
manipulation experiments should be carried out over timescales covering both shortand
long-term responses, in order to realistically assess future impacts of climate
change.
... In another seminal long-term grassland study, the SwissFACE experiment investigated CO 2 × N effects in a perennial ryegrass (Lolium perenne) sward, with similar results. In this setting, Schneider et al. (2004) observed an increased harvestable biomass (+32%) under eCO 2 but only at high N supply (56 g N m −2 yr −1 vs. 14 g N m −2 yr −1 ). An earlier study in the SwissFACE plot showed no significant CO 2 × N interaction in daily integrated CO 2 uptake (A'), light-saturated CO 2 uptake (A sat ), or g s (Ainsworth et al., 2003); however, low-N plants exhibited twice the extent of photosynthetic acclimation (down-regulation) over the course of long-term eCO 2 application (see Section 3.1; Figs. 2 and 3). ...
... The effects of eCO 2 on soil N levels and cycling have been highly variable, having been shown to decrease, remain stable, or increase under eCO 2 (Gill et al., 2002;Finzi and Schlesinger, 2003;Luo et al., 2004;Schneider et al., 2004;Reich et al., 2006a,b;Dijkstra et al., 2008;Langley et al., 2009;Phillips et al., 2009;Norby et al., 2010;Zak et al., 2011;Mueller et al., 2013;Rütting and Andresen, 2015). ...
... At high fertilisation (N450), soil surface N balance of the pure grass swards was on average only slightly positive (+37 kg N ha − 1 yr − 1 ), because of the large amount of N removed from the field with the harvested biomass. This demonstrates the great capacity of the grass species Lolium perenne L. and Dactylis glomerata L. with their dense root systems (Hebeisen et al., 1997) to efficiently deplete mineral N from the soil (Daepp et al., 2000;Schneider et al., 2004). Consequently, the increase in SMN and NCSS at the high N fertiliser application of N450 was only minor, although the amount of fertiliser N not recovered in the harvested plant material was high for Grs-N450 (= 151 kg N ha − 1 yr − 1 ). ...
... The incorporation of super-nodulating legumes into mixed pasture swards should also be considered (Humphries, 2012). For example, in ryegrass pasture, adding N under eCO2 increased yield (Schneider et al., 2004), while eCO2 conditions also increased N uptake in wheat (Butterly et al., 2016). Supplementation of the pasture with additional N generated by SNF in super-nodulating legumes should improve photosynthesis and yield quantity and quality in non-legumes grown in the pasture. ...
Pasture legumes play a vital role in sustainable and productive agricultural systems by providing grazing forage and facilitating nitrogen cycling within the pasture sward. Through the process of symbiotic nitrogen fixation (SNF), legumes increase nitrogen availability by establishing a mutualistic association with rhizobia bacteria housed within root nodules, which fix atmospheric nitrogen in exchange for plant-derived carbohydrates. In response to this high carbohydrate demand from nodules, legumes display autoregulation of nodulation (AON) to restrict nodules number to the minimum required to sustain nitrogen supply under current photosynthetic levels. Mutations to the AON pathway can result in super-nodulating legumes, which typically grow smaller than wild-type plants, due to the high carbohydrate cost of producing and maintaining excessive nodule numbers. Recent research has demonstrated that these altered AON super-nodulating mutants are more responsive to elevated CO2 (eCO2) conditions, out-performing their non-mutant counterparts in biomass production and nutritional content (Zhang et al., 2023). It is thought that super-nodulating mutants are carbon-limited and can perform better at eCO2 through improved photosynthesis, facilitated by the re-investment of nitrogen assimilates into photosynthetic machinery. As eCO2 conditions are predicted to increase in future climates, harnessing the benefits of symbiotic nitrogen fixation through AON mutants has the potential to improve the CO2 fertilisation effect, without protein yield penalties. This paper critically examines our current understanding of super-nodulating legumes and identifies the necessary steps for translating this research into practical applications within pasture systems.
... Ainsworthet al. , 2003). Results from long-term FACE experiments are furthermore reported to differ from short-term responses with water and nutrients availability and demand being discussed as main abiotic drivers (Schneider et al. , 2004). Based on the existing literature and our observations we postulate, that transgenerational memory effects of elevated CO 2 on carboxylation capacity and stomatal sensitivity to drought modulated by water and nutrients availability and demand could be a main biotic mechanism causing inconsistency in observed responses between single-and multigenerational FACE experiments. ...
Increasing atmospheric CO and drought are major symptoms of anthropogenic climate change with profound effects on plant growth. Transgenerational memory (i.e. influence of the parental environment on offspring phenotype and performance) has been suggested as a relevant mechanism for plants to build-up adaptative capacity for rapid environmental changes. However, this mechanism of pre-adaptation remains poorly investigated so far. We investigated intra- and transgenerational effects of elevated CO on drought response of wheat. We used seeds from a FACE (Free Air Carbon Dioxide Enrichment) experiment with ambient and elevated CO to grow plants in climate chambers in which we varied CO , atmospheric water demand and soil moisture. We quantified photosynthetic efficiency, stomatal sensitivity and biomass production. We observed intragenerational upregulation of photosynthetic efficiency but transgenerational downregulation of photosynthetic efficiency, stomatal sensitivity and water use efficiency as response to maternally elevated CO . Plant biomass was affected by drought and experimental CO but not by maternal CO . Our study showcases the importance of transgenerational memory effects when studying climate change response of plants and could have major implications for our understanding of global dynamics of carbon sequestration. It highlights the pressing need for multi-generational experiments accounting for transgenerational memory effects of elevated CO .
... To date, most of our understanding of the e[CO 2 ] effect stems from short-term studies of only a few years. Due to the high costs associated with CO 2 enrichment studies, only a few of them offer insights into the effect of e[CO 2 ] on plants after a decade or more (Liebermann et al., 2019;McCarthy et al., 2010;Reich et al., 2018;Schneider et al., 2004;Talhelm et al., 2014). Relying on short-term CO 2 experiments to project climate-vegetation feedbacks, Earth system models (ESMs) predict that e[CO 2 ] stimulate plant productivity (and associated C stocks) and thereby counteract future increases in atmospheric [CO 2 ] (Huntingford et al., 2013;Sitch et al., 2008). ...
The degree to which elevated CO2 concentrations (e[CO2]) increase the amount of carbon (C) assimilated by vegetation plays a key role in climate change. However, due to the short‐term nature of CO2 enrichment experiments and the lack of reconciliation between different ecological scales, the effect of e[CO2] on plant biomass stocks remains a major uncertainty in future climate projections. Here, we review the effect of e[CO2] on plant biomass across multiple levels of ecological organization, scaling from physiological responses to changes in population‐, community‐, ecosystem‐, and global‐scale dynamics. We find that evidence for a sustained biomass response to e[CO2] varies across ecological scales, leading to diverging conclusions about the responses of individuals, populations, communities, and ecosystems. While the distinct focus of every scale reveals new mechanisms driving biomass accumulation under e[CO2], none of them provides a full picture of all relevant processes. For example, while physiological evidence suggests a possible long‐term basis for increased biomass accumulation under e[CO2] through sustained photosynthetic stimulation, population‐scale evidence indicates that a possible e[CO2]‐induced increase in mortality rates might potentially outweigh the effect of increases in plant growth rates on biomass levels. Evidence at the global scale may indicate that e[CO2] has contributed to increased biomass cover over recent decades, but due to the difficulty to disentangle the effect of e[CO2] from a variety of climatic and land‐use‐related drivers of plant biomass stocks, it remains unclear whether nutrient limitations or other ecological mechanisms operating at finer scales will dampen the e[CO2] effect over time. By exploring these discrepancies, we identify key research gaps in our understanding of the effect of e[CO2] on plant biomass and highlight the need to integrate knowledge across scales of ecological organization so that large‐scale modeling can represent the finer‐scale mechanisms needed to constrain our understanding of future terrestrial C storage. The degree to which elevated CO₂ concentrations (e[CO₂]) increase the amount of carbon assimilated by vegetation plays a key role in climate change. Yet, it remains highly uncertain. Here, we review the effect of e[CO₂] on plant biomass across multiple levels of ecological organization, scaling from physiological responses to population‐, community‐, ecosystem‐, and global‐scale dynamics. We find that evidence for a sustained biomass response to e[CO₂] varies across ecological scales. By exploring these discrepancies, we identify gaps in our understanding of the effect of e[CO₂] on plant biomass and highlight the need to integrate knowledge across scales of ecological organization.
... Elevated [CO 2 ] effect on C assimilation and partition aboveground is linked to N availability and acquisition belowground (Terrer et al., 2018). And low-N availability in soil is a primary constraint reducing the stimulating effect of e[CO 2 ] on biomass accumulation by reducing photosynthetic capacity and limiting leaf area expansion and thus light interception (Norby et al., 2010;Reich & Hobbie, 2013;Schneider et al., 2004;Sigurdsson et al., 2013). ...
The stimulation effect of elevated [CO2] (e[CO2]) on plant growth is modulated by nitrogen (N) availability, yet the mechanisms of this modulation under patchy N supply remain largely elusive. This study aimed to investigate the mechanisms by which patchy nutrient supply in the root‐zone influence on N uptake, partitioning and use efficiency of tobacco plants (Nicotiana tabacum L.) grown under e[CO2]. A split‐root pot experiment was conducted on tobacco plants grown at ambient (a[CO2], 400 μmol L−1) and e[CO2] (800 μmol L−1) conditions during anthesis. Plants were subjected to two fertilizer levels [0 and 113‐29‐214 (N‐P‐K) mg kg−1 soil] and three fertilization regimes (partial root‐zone fertilization, PRN, couple root‐zone fertilization, CRN and no fertilization, CK). Elevated [CO2] significantly decreased N concentrations in all tobacco organs, with the greatest reduction in leaves. Positive responses of tobacco biomass and NUE to e[CO2] were recorded, and a large amount of 15N labelled fertilizer‐N was partitioned to stems at the expense to leaves. Compared to the partially fertilized root, an equal N concentration was observed in the non‐fertilized root. In addition, compared with CRN, PRN increased the root exudates of sugar and organic acid; however, the increased root biomass by nutrient patchiness did not enhance plant total N uptake. Collectively, e[CO2] could sustain N assimilation and distribution of tobacco plants in response to natural heterogeneous nutrient available in the soil caused by patchy fertilization.
... Under elevated CO 2 , plant request for nitrogen is likely to increase and nitrogen application improves CO 2 effects on plant growth (Li et al., 2017). It appears that the nitrogen supply may have a more positive effect on the photosynthetic processes to increase the plants productivity under enriched CO 2 conditions (Schneider et al., 2004;Ainsworth and Long, 2005;Butterly et al., 2016). ...
An open-top chamber experiment was conducted in the present study to investigate the growth and biochemical responses of six sorghum genotypes under two irrigation regimes (based on 40% and 75% soil-water depletion) and two N fertilizer levels (0 and 105 kgN ha⁻¹), at two atmospheric CO2 conditions (390 ± 50 and 700 ± 50 μmol mol⁻¹⁾. The results revealed that water limited stress decreased the plant dry weight by reducing the plant leaf area, SPAD value, Fv/Fm, leaf RWC and membrane stability index (MSI), while it increased the specific activity of APX, CAT and POX enzymes, DPPH, LPC, Phe, TSC, H2O2, MDA and EL. The lowest decrease of the plant dry weight due to limited water stress was observed in the GS5 genotype, which was followed by the lowest decrease in the leaf RWC and MSI; also, the highest increase was seen in APX, Phe and TSC, while the lowest one was recorded in EL. Elevated CO2 improved DPPH and Phe under both control and water limited conditions, resulting in the decrease of APX, POX, H2O2 and EL, while a more pronounced effect was observed in the stress conditions. Furthermore, with the application of nitrogen, the reduction in DPPH, H2O2 and MDA values was greater under water limited stress rather than control conditions. It could be, therefore, concluded that the responses of sorghum genotypes to water deficit stress had interaction with atmospheric CO2 concentrations and nitrogen levels; this could be considered in breeding programs as well as planting management of sorghum.
... Toutefois, cette moyenne masque des variations importantes et la réponse à long terme peut différer sensiblement de la réponse à court terme (THORNLEY et CANNELL, 2000). Ainsi, dans l'expérience suisse de fumigation de CO 2 à l'air libre (FACE), en fertilisation azotée non limitante, la stimulation de la production du ray-grass anglais est passée en 10 ans de 7 à 32 % (LÜSCHER et AESCHLIMANN, 2006), alors qu'en azote limitant un effet non significatif a été observé (SCHNEIDER et al., 2004 ;LÜSCHER et AESCHLIMANN, 2006). Nous ne savons donc pas encore quelle est la part de l'effet fertilisant du CO 2 qui subsistera dans les conditions très variables de la production agricole mondiale (TUBIELLO et al., 2007 ;SOUSSANA et al., 2010). ...
Le changement climatique et ses effets sont un axe de travail majeur pour la recherche : l'agriculture peut contribuer à la lutte contre le réchauffement climatique mais devra également s'adapter. Cet article dresse une vue d'ensemble des premiers acquis concernant l'élevage et les fourrages et souligne les questions qui demeurent. De nombreux indicateurs confirment la réalité du changement climatique, qui devrait affecter prairies et système fourragers de plusieurs manières : par l'évolution moyenne des variables climatiques (température, précipitations et taux de CO2) mais aussi par leurs interactions et l'accroissement de leur variabilité. L'augmentation de la production fourragère avec l'augmentation du taux de CO2 est dépendante de nombreux autres facteurs et risque d'être compromise par les déficits hydriques estivaux. L'évolution à moyen terme de la végétation des prairies permanentes aura des incidences sur la qualité des fourrages et le bilan environnemental des prairies. Les espèces implantées comme la gestion des prairies devront être adaptées pour mieux résister aux extrêmes climatiques.
... In addition, ongoing and projected climate change will affect crop yield and quality worldwide, thus threatening global food security (Godfray et al., 2010;Sundström et al., 2014;Tubiello et al., 2007). Interactions between climatic variables such as temperature, radiation, rainfall, elevated CO2 and fertilization are widely documented, thereby justifying a tight monitoring of fertilization in the context of climate variability (Ross et al., 2004;Schneider et al., 2004). In oilseed rape, recent work has indicated additional, opposite and interactions effects between increased temperature during the grain filling period and S availability (Brunel-Muguet et al., 2015a). ...
Le colza est une oléoprotéagineuse exigeante en soufre (S) mais caractérisée par une faible efficience d’usage du S (EUS). La baisse des retombés atmosphériques soufrées, l'existence de fortes interactions entre les métabolismes soufrés et azotés et l’augmentation prédite des températures terrestres peuvent conduire à une altération des rendements et de la qualité des graines de colza. Dans ce contexte, en s’appuyant sur des approches in planta (conditions contrôlées et de plein champ) et in silico (expérimentations numériques via le modèle écophysiologique SuMoToRI "Sulphur Model Towards Rapeseed Improvement"), les principaux objectifs de cette thèse étaient d’étudier l’impact (i) de différentes stratégies de fertilisation S et N, (ii) du changement climatique et (iii) de la variabilité des paramètres « plante » du modèle sur la croissance ainsi que sur les composantes du rendement et la qualité des graines de colza.Cette étude a permis de confirmer les effets synergiques des apports de S et de N ainsi que leurs effets antagonistes lors d’apport excessif d’un des deux éléments sur l’EUS et l’EUN, soulignant l'importance d'équilibrer les apports S/N. Décaler l’apport en S a permis d’améliorer la qualité protéique des graines en augmentant l’abondance relative en napines (protéines de réserve des graines riches en cystéine). Deux indices de la qualité protéique des graines ont pu être proposés : (i) la teneur en S des graines, fortement corrélée avec l'abondance relative en napines et (ii) le ratio napine:cruciférine-30kDa (cruciférines : protéines de réserve pauvres en S), permettant d’apprécier l’équilibre des apports S/N. Les simulations réalisées avec le modèle ont montré que des ajustements de la fertilisation S devront s’opérer dans un contexte d’augmentation des températures et/ou de diminution du rayonnement incident conduisant à une baisse de la biomasse et à une augmentation du S stocké dans les feuilles. Ces résultats requestionnent les schémas conventionnels de fertilisation et l’utilisation d’idéotypes variétaux et culturaux adaptés aux schémas de fertilisation S et N ainsi qu’au dérèglement climatique.
... Soybean and wheat being C 3 crops, increased availability of carbon (C) for photosynthesis under elevated CO 2 conditions may affect the N assimilation in leaves (Bloom et al., 2014) and C and N cycling in soil system as residue decomposition is dependent upon C/N ratio of plant residues. Further, plant response to the CO 2 fertilization effect may be constrained under low availability of N in soil (Schneider et al., 2004;Li et al., 2017). ...
Changing climate has direct linkage with growth and metabolism in plants and is likely to alter nitrogen(N) uptake from fertilizers. Hence, a field study was conducted using 15N labeled urea in micro-plots to study the effects of elevated CO2 and/or temperature on fertilizer-N use in a soybean-wheat cropping sequence. Crops were grown in open top field chambers (OTCs) under two CO2 (386 and 558–561 ppmv), two temperature (ambient, 1.4-1.5°C above) and three N levels during 2017-18. The N treatments were 50,100 and150% of the recommended dose (N50, N100 andN150). Labeled (15N) urea (10% atom excess) was applied to soybean. Elevated CO2 and/or temperature showed significant effects on seed yield, total N uptake, fertilizer- N uptake, percent N derived from fertilizer (Ndff) and percent fertilizer-N use in the soybean-wheat crop sequence. Total N uptake in soybean significantly increased under elevated CO2 and/or temperature treatments, mainly due to higher N uptake in seeds. Effect of climate on fertilizer-N uptake and utilization in the soybean-wheat cropping sequence varied with level of N application. The 15N labelled fertilizer-N uptake and percent fertilizer-N use was significantly higher under co-elevation of both CO2 and temperature at N100 and N150, but, was mostly similar among the climate treatments at N50. In the soybean wheat crop sequence,13–40% of the applied fertilizer was used,with significantly higher use with co-elevation of both CO2 and temperature. Out of the applied fertilizer,20–51% got retained in the surface soil and 43–73% was traced in the soil-plant system. Significantly higher fertilizer-N use in the soybean-wheat crop sequence under coelevation of CO2 and temperature indicates increasing role of fertilizer-N to harvest the CO2 mediated enhancement in grain yield under the future climate conditions.
... However, an increase in the growth of plants will need an increased supply of essential plant nutrients (e.g., N, P), which are taken up from the available nutrition pool in soil (Edwards et al. 2005;Gentile et al. 2012;Jin et al. 2012Jin et al. , 2013. Therefore, elevated CO 2 -induced plant growth may influence the nutrients cycle, soil mineral composition, and the leachability of elements in ecosystems (Hungate et al. 2003;Luo et al. 2004;Schneider et al. 2004;Wang et al. 2013;Xu et al. 2013). For instance, Jin et al. (2015) reviewed the impacts of elevated CO 2 on the demand and utilization of P in plants and P acquisition from soil. ...
In the original publication of the article, the third author name has been misspelt. The correct name is given in this correction. The original version of this article was revised.
... However, an increase in the growth of plants will need an increased supply of essential plant nutrients (e.g., N, P), which are taken up from the available nutrition pool in soil (Edwards et al. 2005;Gentile et al. 2012;Jin et al. 2012Jin et al. , 2013. Therefore, elevated CO 2 -induced plant growth may influence the nutrients cycle, soil mineral composition, and the leachability of elements in ecosystems (Hungate et al. 2003;Luo et al. 2004;Schneider et al. 2004;Wang et al. 2013;Xu et al. 2013). For instance, Jin et al. (2015) reviewed the impacts of elevated CO 2 on the demand and utilization of P in plants and P acquisition from soil. ...
Carbon dioxide (CO2) capture and storage (CCS) plays a crucial role in reducing carbon emissions to the atmosphere. However, gas leakage from deep storage reservoirs, which may flow back into near-surface and eventually to the atmosphere, is a major concern associated with this technology. Despite an increase in research focusing on potential CO2 leakage into deep surface features and aquifers, a significant knowledge gap remains in the geochemical changes associated with near-surface. This study reviews the geochemical processes related to the intrusion of CO2 into near-surface environments with an emphasis on metal mobilization and discusses about the geochemical research approaches, recent findings, and current knowledge gaps. It is found that the intrusion of CO2(g) into near-surface likely induces changes in pH, dissolution of minerals, and potential degradation of surrounding environments. The development of adequate geochemical research approaches for assessing CO2 leakage in near-surface environments, using field studies, laboratory experiments, and/or geochemical modeling combined with isotopic tracers, has promoted extensive surveys of CO2-induced reactions. However, addressing knowledge gaps in geochemical changes in near-surface environments is fundamental to advance current knowledge on how CO2 leaks from storage sites and the consequences of this process on soil and water chemistry. For reliable detection and risk management of the potential impact of CO2 leakage from storage sites on the environmental chemistry, currently available geochemical research approaches should be either combined or used independently (albeit in a manner complementarily to one another), and the results should be jointly interpreted.
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... The likelihood that an ecosystem will suffer from PNL is highest in ecosystems where external inputs are low (Hu et al. 2006). In ecosystems that have shown indications of PNL, the N mineralization has decreased and composition of soil organic matter (SOM) has changed (Gill et al. 2002;2006;Schneider et al. 2004). However, there might be other feedback mechanisms preventing a development of PNL, e.g. ...
Nitrogen (N) is a fundamental element for life, and limiting in many terrestrial ecosystems. In non-N-fertilized ecosystems, the N inputs can be low, and the nutrient availability for plants is determined by the internal cycling of N. The N availability might alter with different factors, such as climate change, forest management practices, and tree species. Soil N cycling is investigated using stable isotopes, where the activity in the soil can be monitored over time. The overall aim of this thesis is to increase the understanding of the N cycle in natural and semi-natural ecosystems and the environmental factors important for nutrient cycling. The results show that all sites investigated in this thesis had higher NH 4 + turnover than NO 3-turnover. The mineralization rates were highest in the site with the lowest C:N ratio, and the lowest mineralization rates and the highest C:N ratio in the spruce forests, which demonstrate the importance of organic matter quality on gross N transformation rates. The N cycle responses to combined climate treatments were generally lower than responses to single climate treatments. For some processes, we observed opposing responses for eCO 2 as single and main treatment compared to the plots receiving the full treatment. This point to the importance of conducting multifactor climate change experiments, as many feedback controls are yet unknown. Gross nitrification was lowered with fertilization in a northern boreal forest, which is an interesting result in the light of the very low nitrous oxide (N 2 O) emissions from the investigated site, despite heavy annual fertilization of 50-70 kg ha-1. Moreover, the results from an experiment with soil of common origin and land history showed generally higher gross mineralization, immobilization and nitrification rates a beech stand compared to a spruce stand. The beech stand had also higher initial concentration of nitrate (NO 3-) which indicates a more NO 3-based N cycling. Finally, numerical modeling together with 15 N tracing is an improvement for simultaneously determining free amino acid (FAA) mineralization, peptide depolymerization and gross N mineralization rates, compared to analytical solutions.
... While estimates of ecosystem-level N fixation rates are difficult to achieve, especially where contributions from diverse N-fixing processes are substantial (e.g. free-living microbes, bryophytes; Reed et al 2011), information about relative differences in fixation rates or the fraction of N in biomass derived from N fixation (Schneider et al 2004) can also be used as a valuable constraint for models. ...
A wide range of research shows that nutrient availability strongly influences terrestrial carbon (C) cycling and shapes ecosystem responses to environmental changes and hence terrestrial feedbacks to climate. Nonetheless, our understanding of nutrient controls remains far from complete and poorly quantified, at least partly due to a lack of informative, comparable, and accessible datasets at regional-to-global scales. A growing research infrastructure of multi-site networks are providing valuable data on C fluxes and stocks and are monitoring their responses to global environmental change and measuring responses to experimental treatments. These networks thus provide an opportunity for improving our understanding of C-nutrient cycle interactions and our ability to model them. However, coherent information on how nutrient cycling interacts with observed C cycle patterns is still generally lacking. Here, we argue that complementing available C-cycle measurements from monitoring and experimental sites with data characterizing nutrient availability will greatly enhance their power and will improve our capacity to forecast future trajectories of terrestrial C cycling and climate. Therefore, we propose a set of complementary measurements that are relatively easy to conduct routinely at any site or experiment and that, in combination with C cycle observations, can provide a robust characterization of the effects of nutrient availability across sites. In addition, we discuss the power of different observable variables for informing the formulation of models and constraining their predictions. Most widely available measurements of nutrient availability often do not align well with current modelling needs. This highlights the importance to foster the interaction between the empirical and modelling communities for setting future research priorities.
... On one hand, increasing N deposition has enhanced N supply to terrestrial ecosystems, attenuating N limitation (Xia and Wan 2008). On the other hand, elevated CO 2 may stimulate soil organic matter (SOM) decomposition (Schneider et al. 2004;Finzi et al. 2007;Müller et al. 2009), accelerate soil N turnover (Williams et al. 2000;Finzi et al. 2007;McKinley et al. 2009), increase biological N fixation (Hu et al. 2006;Liang et al. 2016), or allow plants to mine deeper soil N by increasing fine root production (McKinley et al. 2009;Iversen et al. 2011). These changes under elevated CO 2 may attenuate N limitation to plants by increasing soil N availability. ...
The stimulation of plant growth and biomass accumulation by elevated CO2 may be limited by soil nitrogen (N) availability. However, our understanding of the response of soil N cycling to elevated CO2 and when progressive N limitation occurs remains limited. Here, we used an open top chamber experiment to examine the effects of 10 years of elevated CO2 on ecosystem carbon (C) and N dynamics in a Quercus mongolica (oak) dominated system in northeastern China. Elevated CO2 increased oak biomass, C and N stocks and C/N by 26.4, 26.2, 16.5 and 8.6% respectively, which suggests increased plant N demand. Soil gross N mineralization, re-mineralization of microbial N and nitrification were accelerated likely due to increased photosynthesis (by 34.9%) and microbial biomass (by 24.2%) under elevated CO2. Thus, the supply of soil available N can sustain the tree growth stimulated by elevated CO2, and to date progressive N limitation has not happened. Nevertheless, both the annual increase of oak biomass, C and N stocks and C/N ratio and the seasonal variations of soil available N and microbial N concentrations, and net N transformation rates indicated that gradual N deficiency may be occurring and the CO2 fertilization effect has weakened with increasing treatment duration.
... In addition, ongoing and projected climate change will affect crop yield and quality worldwide, thus threatening global food security (Godfray et al., 2010;Sundström et al., 2014;Tubiello et al., 2007). Interactions between climatic variables such as temperature, radiation, rainfall, elevated CO 2 and fertilization are widely documented, thereby justifying a tight monitoring of fertilization in the context of climate variability (Ross et al., 2004;Schneider et al., 2004). In oilseed rape, recent work has indicated additional, opposite and interactions effects between increased temperature during the grain filling period and S availability (Brunel-Muguet et al., 2015a). ...
For the last few decades, environmental policies have led to drastic reduction in sulphur (S)-containing industrial emissions leading to reduced S inputs into the soil. This is of concern for oilseed rape (Brassica napus L.), which like most Brassica species is a high S-demanding crop. In this context, monitoring S fertilization has become a central issue. Moreover, ongoing and projected climate change will affect crop yield and quality worldwide, thus justifying the prediction of climate effects via modelling approaches so that crop management and practices can be adjusted. In this modelling study, the growth and S status of winter oilseed rape (WOSR) were investigated from the end of winter until the onset of pod formation under contrasting S supplies in a range of climatic conditions acquired for seven major WOSR-producing northern countries and under the four Representative Concentration Pathway (RCP) scenarios (i.e. RCP2.6, RCP4.5, RCP6.0, and RCP8.5). Simulations were performed with the process-based model SuMoToRI (Sulphur Model Towards Rapeseed Improvement) for past datasets (1948-2005) and projections (2015-2099). Simulation results indicated decreased plant biomass (mainly leaves) as temperatures increased (as expected under the increasingly negative scenarios as the century progresses) and as daily incident radiation decreased in contrast to the mobile S of leaves (mainly sulphate), which tended to accumulate as a consequence of reduced S sink (i.e. leaves) size. These simulations highlighted the increased risks of S over-fertilization, which can lead to environmental issues that mainly comprise S leaching due to high mobileS in leaves that senesce. Overall, this in silico study raises questions about the most suitable S fertilization strategies and associated farming practices for dealing with the expected adverse climatic conditions.
... Nitrogen addition enhances CO 2 effects on plant productivity. In ryegrass swards, compared to non-N control, N addition resulted in a greater yield response to eCO 2 (Schneider et al., 2004). Moreover, eCO 2 significantly increased N uptake of wheat (Butterly et al., 2016). ...
Nitrogen deficiency limits crop performance under elevated CO2 (eCO2), depending on the ability of plant N uptake. However, the dynamics and redistribution of N2 fixation, and fertilizer and soil N use in legumes under eCO2 have been little studied. Such an investigation is essential to improve the adaptability of legumes to climate change. We took advantage of genotype-specific responses of soybean to increased CO2 to test which N-uptake phenotypes are most strongly related to enhanced yield. Eight soybean cultivars were grown in open-top chambers with either 390 ppm (aCO2) or 550 ppm CO2 (eCO2). The plants were supplied with 100 mg N kg⁻¹ soil as ¹⁵N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO2 correlated highly (r = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO2 only led to small increases in the uptake of fertilizer-derived and soil-derived N during R5 to R8, and these increases did not correlate with enhanced yield. Elevated CO2 also decreased the proportion of seed N redistributed from shoot to seeds, and this decrease strongly correlated with increased yield. Moreover, the total N uptake was associated with increases in fixed-N per nodule in response to eCO2, but not with changes in nodule biomass, nodule density, or root length.
... Grasslands cover one third of the global terrestrial area (Suttie et al., 2005) and represent an important ecosystem that has been investigated in several grassland FACE experiments (Hovenden et al., 2006;Reich and Hobbie, 2012;Schneider et al., 2004). In a long-term FACE experiment in Giessen, Germany, extensively used grassland has been grown under eCO 2 since 1998, i.e. the eCO 2 concentration was on average 20% higher relative to the aCO 2 (Jäger et al., 2003). ...
Elevated CO2 (eCO2) reduces transpiration at the leaf level by inducing stomatal closure. However, this water saving effect might be offset at the canopy level by increased leaf area as a consequence of eCO2 fertilization. To investigate this bi-directional effect, we coupled a plant growth and a soil hydrological model. The model performance and the uncertainty in model parameters were checked using a 13 year data set of a Free Air Carbon dioxide Enrichment (FACE) experiment on grassland in Germany. We found a good agreement of simulated and observed data for soil moisture and total above-ground dry biomass (TAB) under ambient CO2 (∼395 ppm) and eCO2 (∼480 ppm). Optima for soil and plant growth model parameters were identified, which can be used in future studies. Our study presents a robust modelling approach for the investigation of effects of eCO2 on grassland biomass and water dynamics. We show an offset of the stomatal water saving effect at the canopy level because of a significant increase in TAB (6.5%, p < 0.001) leading to an increase in transpiration by +3.0 ± 6.0 mm, though insignificant (p = 0.1). However, the increased water loss through transpiration was counteracted by a significant decrease in soil evaporation (−2.1 ± 1.7 mm, p < 0.01) as a consequence of higher TAB. Hence, evapotranspiration was not affected by the increased eCO2 (+0.9 ± 4.9 mm, p = 0.5). This in turn led to a significantly better performance of the water use efficiency by 5.2% (p < 0.001). Our results indicate that mown, temperate grasslands can benefit from an increasing biomass production while maintaining water consumption at the +20% increase of eCO2 studied.
... However, time series on legume dynamics that could be used to test predictions of these models are fairly rare and often not very long (see Rabotnov 1977;Stewart & Haycock 1984;Frame & Newbold 1986;Schneider et al. 2004;Ammann et al. 2007 for some examples). Therefore, we do not have sufficient long-term information on such oscillations in the field, mainly in more natural systems. ...
Several dynamic models have shown that dynamics of legumes and grasses can result in periodic behaviour. These oscillations arise due to delays in nitrogen flows coupled with differences in ability to compete for light. However, long‐term time series on legume dynamics that could be used to test predictions of these models are almost non‐existent.
We examine legume oscillations in a semi‐natural mountain grassland using a long‐term (Tilde; 30 years) data series on aboveground biomass of individual species and on nitrogen and phosphorus content over time.
Using autocorrelation analysis, we show that there is a strong periodicity (with period of 8–9 years) of legume and grass biomass and nitrogen content in the grass biomass. These three variables are in fairly stable phase shifts relative to each other, with a grass peak followed by a peak in C : N ratio in grasses which is followed by a legume peak. Phosphorus content in either legume or grass biomass does not show synchronous cycling with legume or grass biomass or nitrogen content in grass.
Fitting a dynamic linear model to the data showed that legumes affect nitrogen content in grasses, and grass biomass both affects and is affected by nitrogen content. In contrast, there is no negative effect of grasses on legumes, indicating some other process must be responsible for the legume decline. Manuring, which was occasionally applied to the plots, also does not seem to affect the cycling. Second‐order term for legumes showed some evidence of self‐inhibitory effects in legumes, but phosphorus content in legumes shows no support for phosphorus limitation. The most likely explanation of the legume decline should be sought elsewhere (pathogens, soil biota etc.).
Synthesis . Long‐term data support the existing the claim that legume dynamics are the key driver of nitrogen dynamics in nutrient‐poor semi‐natural grasslands. Grasses benefit from the nutrient enrichment due to legume cycling, but are a passive element and do not play a role in legume limitation. Apart from the role of nutrient cycling, these legume‐driven nutrient dynamics also constitutes processes by which long‐term richness of meadows is maintained.
... Therefore, the extent to which the emitted gas will affect the wheat quality should be determined using high-spectrum technology and biochemical component analysis in future research. In a long term, elevated CO 2 could alter soil C and nitrogen (N) cycling and then cause an indirect impact on plant growth (Schneider et al., 2004;Iversen et al., 2012;Black et al., 2016). Such feedback and effect should be continuously monitored and further studied. ...
In-situ monitoring of the environmental impact is essential for the verification of clean operation of carbon capture and storage (CCS) projects. We conducted an empirical study based on remotely sensed data and field observations from an enhanced oil recovery (EOR) site in China. Geostatistical analysis and general linear model regression were performed to detect the impact of fugitive CO2 emission from oil buffer tanks. Results show that the emitted CO2 resulted in CO2 enrichment about 25–100 m away from the buffer tanks. The spatial pattern and semivariogram parameters of normalized differential vegetation index (NDVI) in the CCS core operating area had not been altered significantly. The CO2 concentration is not a statistically significant explanatory factor for the variation of wheat growth in the present CCS-EOR site that located in Gaoqing County in the east of China. The impact of fugitive CO2 emission on wheat growth appears limited because of the instability and rapid diffusion of emitted CO2. However, we emphasize that these results were extracted from the in-situ monitoring that characterized by macro-level and short term. Long-term biology-based study at a micro-level is imperative for further understanding and determining the environmental impact of fugitive CO2 emission. Moreover, considering the incidental CO2 breakthrough and the unknown impact of emitted gas, including CO2 and hydrocarbons, on the quality of wheat plant and grain, long-time field monitoring, and improvement of production equipment and technique are essential to ensure the clean production of CCS-EOR.
... In this FACE experiment, mineral nutrient deficiency and growth limitation of host plants occurred under elevated atmospheric pCO2 Suter et al., 2002). Moreover, accumulation of soil organic matter under elevated pCO2 may have increased microbial immobilization of mineral nutrients (Zak et al., 2000;Schneider et al., 2004). Nutrient retention in soils and increased plant demand under elevated pCO2 both make nutritional symbioses more important. ...
• Altered environmental conditions may change populations of arbuscular mycorrhizal fungi and thereby affect mycorrhizal functioning. We investigated whether 8 yr of free-air CO2 enrichment has selected fungi that differently influence the nutrition and growth of host plants.
• In a controlled pot experiment, two sets of seven randomly picked single spore isolates, originating from field plots of elevated (60 Pa) or ambient CO2 partial pressure (pCO2), were inoculated on nodulated Trifolium repens (white clover) plants. Fungal isolates belonged to the Glomus claroideum or Glomus intraradices species complex, and host plants were clonal micropropagates derived from nine genets.
• Total nitrogen (N) concentration was increased in leaves of plants inoculated with fungal isolates from elevated-pCO2 plots. These isolates took up nearly twice as much N from the soil as isolates from ambient-pCO2 plots and showed much greater stimulation of biological N2 fixation. The morpho-species identity of isolates had a more pronounced effect on N2 fixation and on root length colonized than isolate identity.
• We conclude that rising atmospheric pCO2 may select for fungal strains that will help their host plants to meet increased N demands.
... The lower and upper n (i.e., n and N) in Refs. Schneider et al. (2004), McCarthy et al. (2010) and Reich and Hobbie (2013) divided into forest, grassland, and cropland to explore possible differences between ecosystem types. For the second data set, 15 available time series of plant growth were collected from 7 decadal FACE experiments ( Table 1). ...
The nitrogen (N) cycle has the potential to regulate climate
change through its influence on carbon (C) sequestration. Although extensive
research has explored whether or not progressive N limitation (PNL) occurs
under CO2 enrichment, a comprehensive assessment of the processes that
regulate PNL is still lacking. Here, we quantitatively synthesized the
responses of all major processes and pools in the terrestrial N cycle with
meta-analysis of CO2 experimental data available in the literature. The
results showed that CO2 enrichment significantly increased N
sequestration in the plant and litter pools but not in the soil pool,
partially supporting one of the basic assumptions in the PNL hypothesis that
elevated CO2 results in more N sequestered in organic pools. However,
CO2 enrichment significantly increased the N influx via biological N
fixation and the loss via N2O emission, but decreased the N efflux via
leaching. In addition, no general diminished CO2 fertilization effect
on plant growth was observed over time up to the longest experiment of 13
years. Overall, our analyses suggest that the extra N supply by the
increased biological N fixation and decreased leaching may potentially
alleviate PNL under elevated CO2 conditions in spite of the increases
in plant N sequestration and N2O emission. Moreover, our syntheses
indicate that CO2 enrichment increases soil ammonium (NH4+)
to nitrate (NO3−) ratio. The changed NH4+/NO3−
ratio and subsequent biological processes may result in changes in soil
microenvironments, above-belowground community structures and associated
interactions, which could potentially affect the terrestrial biogeochemical
cycles. In addition, our data synthesis suggests that more long-term
studies, especially in regions other than temperate ones, are needed for
comprehensive assessments of the PNL hypothesis.
... Many CO 2 enrichment studies have found small or negligible stimulation effects on productivity, especially when C 4 grasses are dominant [28,64]. Few field studies have quantified combined eCO 2 +N enr stimulation effects; the three we found for grass-dominated communities that did [65][66][67] all based their values on current-year biomass production (≈NPP a ). Stimulation effects ranged from 7 to 42% in the three studies. ...
The primary productivity of coastal wetlands is changing dramatically in response to rising atmospheric carbon dioxide (CO2) concentrations, nitrogen (N) enrichment, and invasions by novel species, potentially altering their ecosystem services and resilience to sea level rise. In order to determine how these interacting global change factors will affect coastal wetland productivity, we quantified growing-season carbon assimilation (≈gross primary productivity, or GPP) and carbon retained in living plant biomass (≈net primary productivity, or NPP) of North American mid-Atlantic saltmarshes invaded by Phragmites australis (common reed) under four treatment conditions: two levels of CO2 (ambient and +300 ppm) crossed with two levels of N (0 and 25 g N added m−2 yr−1). For GPP, we combined descriptions of canopy structure and leaf-level photosynthesis in a simulation model, using empirical data from an open-top chamber field study. Under ambient CO2 and low N loading (i.e., the Control), we determined GPP to be 1.66 ± 0.05 kg C m−2 yr−1 at a typical Phragmites stand density. Individually, elevated CO2 and N enrichment increased GPP by 44 and 60%, respectively. Changes under N enrichment came largely from stimulation to carbon assimilation early and late in the growing season, while changes from CO2 came from stimulation during the early and mid-growing season. In combination, elevated CO2 and N enrichment increased GPP by 95% over the Control, yielding 3.24 ± 0.08 kg C m−2 yr−1. We used biomass data to calculate NPP, and determined that it represented 44%–60% of GPP, with global change conditions decreasing carbon retention compared to the Control. Our results indicate that Phragmites invasions in eutrophied saltmarshes are driven, in part, by extended phenology yielding 3.1× greater NPP than native marsh. Further, we can expect elevated CO2 to amplify Phragmites productivity throughout the growing season, with potential implications including accelerated spread and greater carbon storage belowground.
... 4are shown. The lower and upper n (i.e., n and N) in Refs.Schneider et al., 2004; McCarthy et al., 2010; Reich and Hobbie, 2013 mean without and with N addition, respectively. The lower and upper o (i.e., o and O) in Ref.Talhelm et al., 2012 mean without and with O 3 treatment, respectively. ...
Nitrogen (N) cycle has the potential to regulate climate change through its influence on carbon (C) sequestration. Although extensive researches have been done to explore whether or not progressive N limitation (PNL) occurs under CO2 enrichment, a comprehensive assessment of the processes that regulate PNL is still lacking. Here, we quantitatively synthesized the responses of all major processes and pools in terrestrial N cycle with meta-analysis of CO2 experimental data available in the literature. The results showed that CO2 enrichment significantly increased N sequestration in plant and litter pools but not in soil pool. Thus, the basis of PNL occurrence partially exists. However, CO2 enrichment also significantly increased the N influx via biological N fixation, but decreased the N efflux via leaching. In addition, no general diminished CO2 fertilization effect on plant growth over time was observed. Overall, our analyses suggest that the extra N supply by the increased biological N fixation and decreased leaching may potentially alleviate PNL under elevated CO2 conditions. Moreover, our synthesis showed that CO2 enrichment increased soil ammonium (NH4+) but decreased nitrate (NO3-). The different responses of NH4+ and NO3-, and the consequent biological processes, may result in changes in soil microenvironment, community structures and above-belowground interactions, which could potentially affect the terrestrial biogeochemical cycles and the feedback to climate change.
... The need for extra N supply under elevated CO 2 is indicated by the work of Reich et al. (2006) who found that there was a 20-25 % increase in plant biomass by elevated CO 2 with enriched N, in comparison with only 8-12 % with an insufficient N supply. The impact of elevated CO 2 on the N cycle in ecosystems, and on soil N mineralization and immobilization, and organic matter decomposition and turnover have been well studied (Hungate et al., 2003;Luo et al., 2004;Schneider et al., 2004;Wang et al., 2013;Xu et al., 2013). In comparison, the impact of elevated CO 2 on interactions between soil P supply and plant growth need further interpretation. ...
Increasing attention is being focused on the influence of rapid increases in atmospheric CO2 concentration on nutrient cycling in ecosystems. An understanding of how elevated CO2 affects plant utilization and acquisition of phosphorus (P) will be critical for P management to maintain ecosystem sustainability in P-deficient regions.
This review focuses on the impact of elevated CO2 on plant P demand, utilization in plants and P acquisition from soil. Several knowledge gaps on elevated CO2-P associations are highlighted.
Significant increases in P demand by plants are likely to happen under elevated CO2 due to the stimulation of photosynthesis, and subsequent growth responses. Elevated CO2 alters P acquisition through changes in root morphology and increases in rooting depth. Moreover, the quantity and composition of root exudates are likely to change under elevated CO2, due to the changes in carbon fluxes along the glycolytic pathway and the tricarboxylic acid cycle. As a consequence, these root exudates may lead to P mobilization by the chelation of P from sparingly soluble P complexes, by the alteration of the biochemical environment and by changes to microbial activity in the rhizosphere. Future research on chemical, molecular, microbiological and physiological aspects is needed to improve understanding of how elevated CO2 might affect the use and acquisition of P by plants.
© The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.
... However, when assuming fixed agricultural practices, we found plant C:N increases indicating increased N limitations under elevated [CO 2 ]. This is consistent with experimental findings and simulations that showed a possible change in herbage quality with higher concentration of water-soluble carbohydrates and lower concentrations of N at a given yield (Schneider et al., 2004;Lüscher et al., 2006;Perring et al., 2010). ...
The Pasture Simulation model (PaSim) was used to simulate agronomic and environmental services at 12 grassland sites in France, comparing near past climate conditions (1970-1999) and projections for near (2020-2049) and far future (2070-2099). Modelled climate change impacts were assessed using statistical and threshold-based analysis on managed (intensive, extensive) grasslands (permanent, sown), comparing shallow and deep soils, and for an ensemble of emission scenarios, climate models, and downscaling/initialization methods. Results show a significant drift of all sites towards more arid climates by the end of the century. Significant reductions in annual water drainage and herbage protein content are projected, together with new opportunities for annual and seasonal herbage production, in spring and winter especially. Simulated future conditions also show an increased interannual and seasonal variability of production. Contrary to expectations, current natural conditions reflecting large water availability are not expected to reduce future risks of negative climate change impacts on grassland systems. Notably, milk production from aftermath grazing of permanent grasslands established on deep soils at Mirecourt (humid site) is estimated to fall down below one-third of the current median value in four out of 30 years (similar to 13%) for 2070-2099, whereas similar shortfalls were not observed with the baseline climate. Balancing risks and opportunities, permanent extensive grasslands can be viewed as a trade-off between the continuity of grassland service provision and the mitigation of greenhouse gas emissions. (C) 2012 Published by Elsevier B.V.
... consumption. On a global scale, if fewer N-limited ecosystems are prone to PNL, sustained enhanced NPP can be expected, as has indeed been found in several long-term eCO 2 studies (McCarthy et al. 2010;Reich 2009;Schneider et al. 2004), which will also increase litter production and formation of new SOM (Drake et al. 2011;van Groenigen et al. 2014). The increased litter input has been shown to balance the faster SOM decomposition under eCO 2 , leading to unchanged SOM content (van Groenigen et al. 2014). ...
Nitrogen (N) limitation of terrestrial ecosystems is a crucial factor for predicting how these ecosystems respond and feedback to climate change. Nitrogen availability for plants in terrestrial ecosystems depends on the internal soil N cycle and inputs to the ecosystem via biological N2 fixation. We reviewed the effect of elevated atmospheric CO2 concentrations (eCO2) on gross soil N transformations to advance our understanding of ecosystem responses to eCO2. Overall, neither gross mineralization nor gross nitrification was altered by eCO2. However, emerging from ecosystem specific analysis, we propose a new conceptual model for eCO2 effects on gross mineralization based on ecosystem nutrient status: gross mineralization is only stimulated in N limited ecosystems, but unaffected in phosphorus limited ecosystems. Moreover, the ratio of ammonium oxidation to immobilization is decreased under eCO2, indicating a tighter N cycle with reduced ecosystem N losses. This new conceptual model on N cycle responses to eCO2 should be tested in the future in independent experiments and it provides a new concept for refining mechanistic models of ecosystem responses to climate change.
Temporal stability of primary productivity is the key to stable provisioning of ecosystem services to human beings. Yet, the effects of various global changes on grassland stability remain ambiguous.
Here, we conducted a comprehensive meta‐analysis based on 1070 multi‐year paired observations from 173 studies, to examine the impacts of various global changes on productivity, community stability and plant diversity of grasslands on a global scale. The global change drivers include nitrogen (N) addition, phosphorus (P) addition, N and P addition, precipitation increase, precipitation decrease, elevated CO2 and warming.
Global change drivers generally had stronger impacts on grassland productivity than on temporal stability, except for precipitation changes. Community temporal stability was reduced by N addition, N and P addition and precipitation decrease, but was increased by precipitation increase and remained unchanged under P addition, elevated CO2 and warming. In addition, species richness decreased under N addition, N and P addition and precipitation decrease. At the plant functional group level, N and P addition reduced grasses' stability and precipitation increase enhanced forbs' stability.
Nutrient additions decreased community stability via increasing the inter‐annual variation more than the mean of primary productivity, while precipitation changes mainly affected community temporal stability via changing mean productivity. The negative impacts of global change drivers (i.e. N and P addition, warming) on community temporal stability increased with the degree of species loss but decreased with increasing stability of grasses. Moreover, the negative impacts of nutrient addition and precipitation decrease on community stability was lessened while the positive effect of precipitation increase on community stability was enhanced in grasslands with higher historical precipitation variability, greater soil fertility and longer experimental duration.
Synthesis. Our findings demonstrate that N‐based nutrient additions and drought destabilise grassland productivity, while precipitation increase enhances community stability. Impacts of global changes on community productivity and stability are mediated by species richness, plant functional group, site‐specific environmental conditions (i.e. climate, soil) and experimental duration, which deserve more attention in grassland management practices under future global change scenarios.
Intensified drought stress threatens plant growth and productivity, while elevated CO2 (e[CO2]) alleviates the negative impact of drought stress on plants through alteration in water use and improvement in plant growth. In the terrestrial ecosystem, crops are particularly sensitive to drought and benefit from e[CO2]. To cope with the drier and CO2-enriched climate, plants have evolved various adaptive strategies. Water-dependent crops can benefit from e[CO2] but are species-dependent and depend on the intensities and durations of drought stress. In this chapter, we summarized drought impact on crops, crop performance under e[CO2], as well as their interactions in physiological, biochemical, and molecular levels.
Increasing levels of atmospheric CO2 may change C and N dynamics in pasture ecosystems. The present study was conducted to examine the impact of four years of CO2 enrichment on soil and root composition and soil N transformation in natural pastureland. Plots of open-top growth chambers were continuously injected with ambient CO2 (350 µL L–1) and elevated CO2 (625 µL L–1). Soil cores exposed to ambient and elevated CO2 treatment were incubated and collected each year. Net N-mineralization rates in soil (NH4+-N plus NO3ˉ–-N), in addition to total C and N content (%) of soil and root tissues were measured. Results revealed that elevated CO2 caused a significant reduction in soil NO3 (P < 0.05), however, no significant CO2 effect was found on total soil C and N content (%). Roots of plants grown under elevated CO2 treatment had higher C/N ratios. Changes in root C/N ratios were driven by changes in root N concentrations as total root N content (%) was significantly reduced by 30% (P < 0.05). Overall, findings suggest that the effects of CO2 enrichment was more noticeable on N content (%) than C content (%) of soil and roots; elevated CO2 significantly affected soil N-mineralization and total N content (%) in roots, however, no substantial change was found in C inputs in CO2-enriched soil.
We applied two approaches to model grassland yield and nitrogen (N) content. The first was a series of regression equations; the second was the Century dynamic model. The regression model was generated from data from eighty-nine experimental sites across Europe, distinguishing between five climatic regions. The Century model was applied to six sites across these regions. Both approaches estimated mean grassland yields and N content reasonably well, though the root mean squared error tended to be lower for the dynamic model. The regression model achieved better correlations between observed and predicted values. Both models were more sensitive to uncertainties in weather than in soil properties, with precipitation often accounting for the majority of model uncertainty. The regression approach is applicable over large spatial scales but lacks precision, making it suitable for considering general trends. Century is better applied at a local level where more detailed and specific analysis is required.
Climate change and its impact are a major subject of research: Agriculture can help prevent global warming but will have to adapt in order to do so. Climate change is expected to affect grassland and forage systems in different ways: based on the evolution of average climatic variables (temperature, rainfall, and CO2 levels), interactions and increased variability. Increase in forage production, as a result of higher CO2 levels, depends on several factors and could be compromised by summer droughts. Medium-term evolution of vegetation in permanent grassland will affect the quality of forage and the environmental balance of grassland. Established species and grassland management will have to be adapted in order to better resist climatic extremes. © 2013 Association Francaise pour la Production Fourragere. All rights reserved.
Recent studies have highlighted great challenges of transgene silencing for transgenic plants facing climate change. In order to understand the impacts of elevated CO2 on exogenous Bacillus thuringiensis (Bt) toxins and transgene expression in transgenic rice under different levels of N-fertilizer supply, we investigated the biomass, exogenous Bt toxins, Bt-transgene expression and methylation status in Bt rice exposed to two levels of CO2 concentrations and nitrogen (N) supply (1/8, 1/4, 1/2, 1 and 2 N). It is elucidated that the increased levels of global atmospheric CO2 concentration will trigger up-regulation of Bt toxin expression in transgenic rice, especially with appropriate increase of N fertilizer supply, while, to some extent, the exogenous Bt-transgene expression is reduced at sub-N levels (1/4 and 1/2N), even though the total protein of plant tissues is reduced and the plant growth is restricted. The unpredictable and stochastic occurrence of transgene silencing and epigenetic alternations remains unresolved for most transgenic plants. It is expected that N fertilization supply may promote the expression of transgenic Bt toxin in transgenic Bt rice, particularly under elevated CO2.
Terrestrial carbon and nitrogen cycles are tightly coupled by plant tissue stoichiometry and plant controls on ecosystem N retention and fixation. Their co-evolution under rising CO2 and global environmental change is one of the key uncertainties in future Earth system projections. Free Air CO2 Enrichment experiments provide crucial insights into ecosystem responses to elevated CO2, but due to the paucity of such experiments, upscaling and generalisations are often difficult to make. Here, we investigate observed responses in the SwissFACE experiment using a new ecosystem model for the coupled C and N cycles that embodies functional balance theory. In the model, above versus belowground allocation is balanced under daily light conditions and N availability to achieve C and N acquisition in a ratio corresponding to its requirement for new growth. A model setup with plant control on N fixation, based on the relative efficiencies of active uptake versus fixation, is compared with a no-fixation setup, reflecting the SwissFACE field experiment conducted separately with N-fixing and non-fixing monocultures.
Model predictions based on functional balance theory are generally consistent with observed responses. The strong observed aboveground growth enhancement in N-fertilised plots is caused by the relief of N limitation and reduced requirement for fine roots. Under elevated CO2 and low N levels, resource limitation shifts belowground and therefore leads to smaller leaf growth enhancements but enhanced fine root production. On time scales beyond the experiment, this limitation is predicted to be relieved by re-balancing of net N mineralisation and enhanced N retention. These dynamics are different in N-fixing plant communities where temporally increased N limitation after the CO2 step increase is relieved by higher rates of N fixation.
Such a model-data analysis supports the interpretation of empirical results and serves as a validation for a new type of global vegetation models based on ecological theory.
Fungi relieve nitrogen limitation
Rising concentrations of atmospheric CO 2 stimulate plant growth; an effect that could reduce the pace of anthropogenic climate change. But plants also need nitrogen for growth. So far, experimental nitrogen addition has had equivocal effects on the magnitude of CO 2 fertilization. Terrer et al. explain that the impact of nitrogen on plant growth depends on the relationship between nitrogen availability and symbioses with mycorrhizal soil fungi. Only plants with ectomycorrhizal fungi associated with their roots can overcome nitrogen limitation.
Science , this issue p. 72
Recent scientific evidence shows that crop yields in many Sub Saharan Africa (SSA) countries are likely to be severely affected by climate change. Reliance on rainfall in this region increases the vulnerability of cereal systems to climate change and variability. In large parts of SSA, maize (Zea mays L.) is the principal staple crop, covering a total of nearly 27 M ha, and yet maize yields remain the lowest in the world, stagnated at less than 2 Mg ha−1. Calculated and simulated analyses for SSA show that crop yields will decline by more than 10 % by 2055. The effect of climate change on crop yields is mainly attributed to: increased frequency of extreme events; effects of elevated CO2 (where studies project crop yield increases of 5–20 % at 550 ppm CO2); interactions of elevated CO2 with temperature and rainfall as well as with soil nutrients; and increased vulnerability to weed competition, insect pests, and diseases. However, several studies show that rainfall and water availability limit agricultural production more than temperature in SSA. The projected rainfall would increase by 2–4 % in Eastern Africa, but decrease by 5 % in Southern Africa during the main crop growing seasons. Temperatures are likely to increase throughout SSA by 2050, but the combination of increasing temperatures and low seasonal rainfall in Southern Africa suggest this region will be particularly vulnerable. Some of the crop models used for predicting the effect of climate change on yields are limited by their ability to predict effects of climatic events that lie outside the range of present-day variability. In addition, comparisons between models for the same setting have sometimes given differing results. This review paper shows that, for most of the SSA countries, the data required for assessing long-term effect of climate change on crop yield are lacking, that most of the models do not cater to assessment at the household level, and that no single approach can be considered as adequate. Therefore, a clear need exists for collaboration among different scientific disciplines for the development of agriculture in SSA in a changing climate.
Keywords
Climate change Crop yields Sub-Saharan Africa
Statistical relationships were established between the fate of C and N from 47 types of crop residues and their biochemical characteristics during a soil incubation at 15°C. The incubations were carried out under nonlimiting N in order to differentiate the effects of biochemical characteristics of residues from those of soil N availability. Depending on the residue, the apparent mineralization of residue C after 168 d varied from 330 to 670 g kg-1 of added C. Mineralization kinetics were described using a two-compartment decomposition model that decomposes according to first-order kinetics. Amounts of C mineralized after 7 d and the decomposition rate coefficient of the labile fraction were related mainly to the soluble C forms of the residue. No statistical relationship was established between the N concentration of residues and their decomposition in the soil. The incorporation of crop residues into soil led to various soil mineral N dynamics. Two residues caused net N mineralization from the time of their incorporation, whereas all the others induced net N immobilization (1-33 g N kg-1 of added C). After 168 d, only residues with a C/N ratio <24 induced a surplus of mineral N compared with the control soil. The mineral N dynamics were related mainly to the organic N concentration of the residues and to their C/N ratio. At the start of incubation, these dynamics were also influenced by the presence of polyphenols in the plant tissues. Finally, this study showed the need to include the biochemical quality of crop residues in any C and N transformation models that describe decomposition. In contrast, the N concentration or C/N ratio of the residues are sufficient to predict the net effects of crop residues on soil mineral N dynamics.
Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 µmol mol−1 for 10 years using Free Air CO2Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source : sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (Vc,max) and the maximum rate of electron transport (Jmax). Immediately prior to the periodic (every 4–8 weeks) cuts of the L. perenne stands, Vc,max and Jmax, were significantly lower in elevated than in ambient [CO2] in the low-nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation in Vc,max and Jmax in the later years in either nitrogen treatment.
Crops of tomorrow are likely to grow under higher levels of atmospheric CO2. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO2 concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO2 concentration will affect roots and shoots of crop plants proportionally. Since elevated CO2 can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO2 on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability. Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO2 and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO2 enrichment will add substantially to the global change response data base.
Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO2 levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO2 can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO2. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO2. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO2. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO2 most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO2 responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO2. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO2. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [`(n)]\bar \nu , and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO2 except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGRmax).
The concentration of carbon dioxide (CO2) in the Earth's
atmosphere is rising rapidly, with the potential to alter many ecosystem
processes. Elevated CO2 often stimulates photosynthesis,
creating the possibility that the terrestrial biosphere will sequester
carbon in response to rising atmospheric CO2 concentration,
partly offsetting emissions from fossil-fuel combustion, cement
manufacture, and deforestation,. However, the responses of intact
ecosystems to elevated CO2 concentration, particularly the
below-ground responses, are not well understood. Here we present an
annual budget focusing on below-ground carbon cycling for two grassland
ecosystems exposed to elevated CO2 concentrations. Three
years of experimental CO2 doubling increased ecosystem carbon
uptake, but greatly increased carbon partitioning to rapidly cycling
carbon pools below ground. This provides an explanation for the
imbalance observed in numerous CO2 experiments, where the
carbon increment from increased photosynthesis is greater than the
increments in ecosystem carbon stocks. The shift in ecosystem carbon
partitioning suggests that elevated CO2 concentration causes
a greater increase in carbon cycling than in carbon storage in
grasslands.
Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.
The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil-water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.
The formation and death of tillers was studied in swards of perennial ryegrass. These swards differed in the length of the growing period and in the amount of N applied after each cut. Tillers died faster after a 6-weekly growing period than after a 4-weekly period. Some plants were far more affected than others, often resulting in the complete death of plants and thus in open spaces in the sward. Shading experiments suggested that plant deaths resulted from mutual shading. Small plants are shaded by taller ones and have a lower reserve content and a therefore higher death rate. High cutting frequencies or low N application rates could prevent open spaces from appearing or could be used to rehabilitate an open sward provided it was not yet infested with unwanted species. (Abstract retrieved from CAB Abstracts by CABI’s permission)
The International Panel on Climate Change (IPCC) predicted, from current trends in emissions and uptake, that the atmospheric CO2 concentration (Ca) would rise from its 1990 level of 354 μmol mol-1 to ca. 530 μml-1 mol-1 by 2050 and to over 700 μmol mol-1 by the end of the next century; i.e. a doubling of the present concentrations (Watson et al., 1990). This increase in the concentration of CO2 and other heteroatomic gas molecules in the troposphere is expected to lead to an increase in mean global temperatures of ca. 3°C by 2050 and ca. 4°C by 2100 (Watson et al., 1990). Tropospheric O3 concentrations in western Europe are expected to rise in parallel with CO2 concentrations from ca. 50 nmol mol-1 in 1990 at ca. 0.5% p.a. (Dizengremel, 1992) and are likely to depress concentrations of RubisCO in leaves which develop in these atmospheres (Pell, 1992). Rising tropospheric CO2 and O3 concentrations will modify the response of photosynthesis to a wide range of environmental variables, in particular temperature (Long, 1991). Any consideration of the effects of rising CO2 concentrations on photosynthetic production must therefore incorporate the interactive effects of rising temperature and rising tropospheric O3 concentration (Farage et al., 1991; Long, 1991; Long & Hutchin, 1991)
A field trial was conducted on 8 sites for 3 years to compare the productivity and nitrogen (N) uptake of ageing sown leys (5–12 years old) with newly sown diploid perennial ryegrass at a range of N-fertilizer rates (0, 125, 250, 375 and 500 kg N ha−1) and with a diploid-tetraploid mixture of perennial ryegrasses (at 250 kg N ha−1 only). All plots were cut at 6-weekly intervals. Mean annual DM yields at 500 kg N ha−1 were 12.3 and 13.2 t ha−1 for ageing and newly sown swards, respectively.
Soil Ecology is designed to meet the increasing challenge faced by
today's environmental scientists, ecologists, agriculturalists, and
biotechnologists for an integrated approach to soil ecology. It
emphasizes the interrelations among plants, animals, and microbes, by
first establishing the fundamental physical and chemical properties of
the soil habitat and then functionally characterizing the major
components of the soil biota and some of their most important
interactions. The fundamental principles underpinning soil ecology are
established and this then enables an integrated approach to explore and
understand the processes of soil nutrient (carbon, nitrogen, and
phosphorus) cycling and the ecology of extreme soil conditions such as
soil-water stress. Two of the most topical aspects of applied soil
ecology are then selected. First, the ecology of soil pollution is
examined, focusing on acid deposition and radionuclide pollution.
Second, manipulation of soil ecology through biotechnology is discussed,
illustrating the use of pesticides and microbial inocula in soils and
pointing toward the future by considering the impact of genetically
modified inocula on soil ecology.
Fluxes of nitrous oxide, methane and carbon dioxide were measured from soils under ambient (350 µL L-1) and enhanced (600 µL L-1) carbon dioxide partial pressures (pCO2) at the ‘Free Air Carbon Dioxide Enrichment’ (FACE) experiment, Eidgenössische Technische Hochschule (ETH), Eschikon, Switzerland in July 1995, using a GC housed in a mobile laboratory. Measurements were made in plots of Lolium perenne maintained under high N input. During the data collection period N fertiliser was applied at a rate of 14 g m-2 of N. Elevated pCO2 appeared to result in an increased (27%) output of N2O, thought to be the consequence of enhanced root-derived available soil C, acting as an energy source for denitrification. The climate, agricultural practices and soils at the FACE experiment combined to give rise to some of the largest N2O emissions recorded for any terrestrial ecosystem. The amount of CO2–C being lost from the control plot was higher (10%) than for the enhanced CO2 plot, and is the reverse of that predicted. The control plot oxidised consistently more CH4 than the enhanced plot, oxidising 25.5 ± 0.8 µg m-2 hr-1 of CH4 for the control plot, with an average of 8.5 ± 0.4 µg m-2 hr-1 of CH4 for the enhanced CO2 plot. This suggests that elevated pCO2 may lead to a feedback whereby less CH4 is removed from the atmosphere. Despite the limited nature of the current study (in time and space), the observations made here on the interactions of elevated pCO2 and soil trace gas release suggest that significant interactions are occurring. The feedbacks involved could have importance at the global scale.
Previous experiments have shown that the anatomy and chemical composition of leaves of inherently fast‐ and slow‐growing grass species, grown at non‐limiting nitrogen supply, differ systematically. The present experiment was carried out to investigate whether these differences persist when the plants are grown at an intermediate or a very low nitrogen supply. To this end, the inherently fast‐growing Poa annua L. and Poa trivialis L., and the inherently slow‐growing Poa compressa L. and Poa pratensis (L.) Schreb. were grown hydroponically at three levels of nitrate supply: at optimum (RGR max ) and at relative addition rates of 100 and 50 mmol N (mol N) ⁻¹ d ⁻¹ (RAR 100 and RAR 50 ), respectively.
As expected, at the lowest N supply, the potentially fast‐growing species grew at the same rate as the inherently slow‐growing ones. Similarly, the differences in leaf area ratio (LAR, leaf area:total dry mass), specific leaf area (SLA, leaf arear:leaf dry mass) and leaf mass ratio (LMR, leaf dry mass:total dry mass) disappeared. Under optimal conditions, the fast‐growing species differed from the slow‐growing ones in that they had a higher N concentration. There were no significant differences in C concentration. With decreasing N supply, the total N concentration decreased and the differences between the species disappeared. The total C concentration increased for the fast‐growing species and decreased for the slow‐growing ones, i.e. the small, but insignificant, difference in C concentration between the species at RGR max increased with decreasing N supply.
The chemical composition of the leaves at low N supply, analysed in more detail by pyrolysis–mass spectrometry, showed an increase in the relative amounts of guaiacyl lignin, cellulose and hemicellulose, whereas those of syringyl lignin and protein decreased.
The anatomy and morphology of the leaves of the four grass species differing in RGR max were analysed by image‐processing analysis. The proportion of the total volume occupied by mesophyll plus intercellular spaces and epidermis did not correlate with the amount of leaf mass per unit leaf area (specific leaf mass, SLM) at different N supply. The higher SLM at low N supply was caused partly by a high proportion of non‐veinal sclerenchymatic cells per cross‐section and partly by the smaller volume of epidermal cells.
We conclude that the decrease in relative growth rate (and increase in SLM) at decreasing N supply is partly due to chemical and anatomical changes. The differences between the fast‐ and slow‐growing grass species at an optimum nutrient supply diminished when plants were growing at a limiting nitrogen supply.
The following arguments are outlined and then illustrated by the response of the Hurley Pasture Model to [CO2] doubling in the climate of southern Britain. 1. The growth of N-limited vegetation is determined by the concentration of
N in the soil mineral N pools and high turnover rates of these pools (i.e., large input and output fluxes) contribute positively
to growth. 2. The size and turnover rates of the soil mineral N pools are determined overwhelmingly by N cycling into all
forms of organic matter (plants, animals, soil biomass and soil organic matter — `immobilisation' in a broad sense) and back
again by mineralisation. Annual system N gains (by N2 fixation and atmospheric deposition) and losses (by leaching, volatilisation, nitrification and denitrification) are small
by comparison. 3. Elevated [CO2] enriches the organic matter in plants and soils with C, which leads directly to increased removal of N from the soil mineral
N pools into plant biomass, soil biomass and soil organic matter (SOM). ‘Immobilisation’ in the broad sense then exceeds mineralisation.
This is a transient state and as long as it exists the soil mineral N pools are depleted, N gaseous and leaching losses are
reduced and the ecosystem gains N. Thus, net immobilisation gradually increases the N status of the ecosystem. 4. At the same
time, elevated [CO2] increases symbiotic and non-symbiotic N2 fixation. Thus, more N is gained each year as well as less lost. Effectively, the extra C fixed in elevated [CO2] is used to capture and retain more N and so the N cycle tracks the C cycle. 5. However, the amount of extra N fixed and
retained by the ecosystem each year will always be small (ca. 5–10 kg N ha-1 yr-1) compared with amount of N in the immobilisation-mineralisation cycle (ca. 1000 kg N ha-1 yr-1). Consequently, the ecosystem can take decades to centuries to gear up to a new equilibrium higher-N state. 6. The extent
and timescale of the depletion of the mineral N pools in elevated [CO2] depends on the N status of the system and the magnitude of the overall system N gains and losses. Small changes in the large
immobilisation—mineralisation cycle have large effects on the small mineral N pools. Consequently, it is possible to obtain
a variety of growth responses within 1–10 year experiments. Ironically, ecosystem models — artificial constructs — may be
the best or only way of determining what is happening in the real world.
The challenge to identify the biospheric sinks for about half the total carbon emissions from fossil fuels must include a consideration of below-ground ecosystem processes as well as those more easily measured above-ground. Recent studies suggest that tropical grasslands and savannas may contribute more to the 'missing sink' than was previously appreciated, perhaps as much as 0.5 Pg (= 0.5 Gt) carbon per annum. The rapid increase in availability of productivity data facilitated by the Internet will be important for future scaling-up of global change responses, to establish independent lines of evidence about the location and size of carbon sinks.
IN short-term experiments under productive laboratory conditions, native herbaceous plants differ widely in their potential to achieve higher yields at elevated concentrations of atmospheric carbon dioxide1–8. The most responsive species appear to be large fast-growing perennials of recently disturbed fertile soils7,8. These types of plants are currently increasing in abundance9 but it is not known whether this is an effect of rising carbon dioxide or is due to other factors. Doubts concerning the potential of natural vegetation for sustained response to rising carbon dioxide have arisen from experiments on infertile soils, where the stimulus to growth was curtailed by mineral nutrient limitations2,3,10. Here we present evidence that mineral nutrient constraints on the fertilizer effect of elevated carbon dioxide can also occur on fertile soil and in the earliest stages of secondary succession. Our data indicate that there may be a feedback mechanism in which elevated carbon dioxide causes an increase in substrate release into the rhizosphere by non-mycorrhizal plants, leading to mineral nutrient sequestration by the expanded microflora and a consequent nutritional limitation on plant growth.
Sward structure and tiller density are involved in the resistance of sown grasslands to the deterioration by ageing, and tiller density is known to be strongly affected by nitrogen. Tall fescue swards were sown in 1986 (old swards) and in 1990 (young swards) on the same site under an oceanic mountain climate. They were fertilised with 150 kg N ha−1 year−1 (N-poor swards) and with 350 kg N ha−1 year−1 (N-rich swards). All swards were cut simultaneously four times a year. In each sward type, the horizontal distribution of the tillers was mapped in ten quadrats, using grids of 10×15 square cells (2×2 cm), after each cut and throughout the growing seasons of 1992 and 1993. These growing seasons had reduced summer droughts after a dry year. The maps showed the heterogeneity of grass patches in a quadrat size (6 dm2): areas without any tiller were adjacent to high-density areas (100 tillers dm−2 or more). The observed stand structures cannot be shown as a distribution of separate tufted plants. The high nitrogen level increased the mean tiller density, mainly by increasing the high density areas, and favoured the tiller aggregation. A 4-years ageing was equivalent to nitrogen starvation. The gap areas were poorly affected by the experimental treatments. They were reduced after 1 year of humid growing conditions. Ageing tall-fescue swards deteriorate through decreasing local competitiveness rather than through expanding openings.
The length and weight per unit length of individual internodes of mature reproductive tillers of S. 24 and S. 23 perennial ryegrass and S. 345 and S. 143 cocksfoot were examined under three manurial treatments. At the lower level of nitrogen application successive internodes generally increased in length from the base upward. With additional nitrogen there was an increased proportion of tillers in which internodes near the base of the stem were longer than those immediately above them; lateheading tillers were most affected.
Nitrogen increased the weight per unit length of upper internodes of all four varieties; the effect was most marked in S. 23 ryegrass. In the two ryegrasses, nitrogen reduced the weight per unit length of the basal internode.
At maturity the upper internodes had lower dry-matter digestibility values than the lower. Nitrogen reduced the dry-matter digestibility of upper and lower internodes.
The results are discussed in relation to selection criteria in variety synthesis.
The influence of N availability on C sequestration under prolonged elevated CO2 in terrestrial ecosystems remains unclear. We studied the relationships between C and N dynamics in a pasture seeded to Lolium perenne after 8 years of elevated atmospheric CO2 concentration (FACE) conditions. Fertilizer-15N was applied at a rate of 140 and 560 kg N ha2−1 y2−1 and depleted 13C-CO2 was used to increase the CO2 concentration to 60 Pa pCO2. The 13C–15N dual isotopic tracer enabled us to study the dynamics of newly sequestered C and N in the soil by aggregate size and fractions of particulate organic matter (POM), made up by intra-aggregate POM (iPOM) and free light fraction (LF). Eight years of elevated CO2 did not increase total C content in any of the aggregate classes or POM fractions at both rates of N application. The fraction of new C in the POM fractions also remained largely unaffected by N fertilization. Changes in the fractions of new C and new N (fertilizer-N) under elevated CO2 were more pronounced between POM classes than between aggregate size classes. Hence, changes in the dynamics of soil C and N cycling are easier to detect in the POM fractions than in the whole aggregates. Within N treatments, fractions of new C and N in POM classes were highly correlated with more new C and N in large POM fractions and less in the smaller POM fractions. Isotopic data show that the microaggregates were derived from the macro-aggregates and that the C and N associated with the microaggregates turned over slower than the C and N associated with the macroaggregates. There was also isotopic evidence that N immobilized by soil microorganisms was an important source of N in the iPOM fractions. Under low N availability, 3.04 units of new C per unit of fertilizer N were sequestered in the POM fractions. Under high N availability, the ratio of new C sequestered per unit of fertilizer N was reduced to 1.47. Elevated and ambient CO2 concentrations lead to similar 15N enrichments in the iPOM fractions under both low and high N additions, clearly showing that the SOM-N dynamics were unaffected by prolonged elevated CO2 concentrations.
Emissions of N2O were measured during the growth season over a year from grass swards under ambient (360 μL L−1) and elevated (600 μL L−1) CO2 partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m−2 yr−1) and low (14 g N m−2 yr−1) rates of fertilizer application, split over 5 re-growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO2 increased annual emissions of N2O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N2O emissions) of 179 and 111 g CO2 equivalents m−2, respectively, compared with the GWP of ambient pCO2 swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO2Lolium swards were attributed to greater below-ground C allocation under elevated pCO2 providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N2O-N m−2 yr−1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO2. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO2Lolium swards measured during the first two re-growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re-growths (April–June 2001) and the third, fourth and fifth re-growths (late June–October 2000), with available soil NO3− and rainfall explaining 70%, and soil water content explaining 72% of the variability in N2O in these periods, respectively. Caution is therefore required when extrapolating from short-term measurements to predict long-term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO2 may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N2O, thereby exacerbating the forcing effect of elevated CO2 on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO2 concentrations on global warming.
Plant response to elevated atmospheric CO 2 may depend on the carbon sink strength, determined by the availability of resources other than CO 2 , and the developmental stage.
In a 2‐yr field experiment with model swards of Lolium perenne , the effect of CO 2 enrichment (FACE) on yield and allocation of dry mass (DM) and N were examined under three N fertilization treatments during vegetative and reproductive growth.
During vegetative growth, in the highest N treatment, the greatest increase in DM yield occurred at elevated CO 2 ; there was no change in DM allocation. By contrast, at low N, residual biomass, but not yield, increased under CO 2 enrichment, and the tillers were shorter. During reproductive growth, under CO 2 enrichment DM yield increased similarly across all N treatments; there was no change in DM and N partitioning. The mean weight and height of the reproductive tillers increased.
At high N availability, or during reproductive growth, L. perenne swards overcome carbon‐sink limitation and show a strong yield response to elevated CO 2 . Biomass allocation and the height of the plants, in response to elevated CO 2 , clearly depend on N fertilization and developmental stage.
The uptake of labelled and unlabelled N by wheat was measured in pot and field experiments with 15N-labelled fertilizer. Soils from two sites on the same series were used in the pot experiment; one had been bare-fallowed for 22 years and contained 1.6% organic C, the other had been under grass for many years and contained 3.8% organic C. Fertilizer N increased the uptake of unlabelled soil N in both soils, i.e. there was a positive ‘added nitrogen interaction’ (ANI). There was no ANI in the field experiment. A simulation model is used to show how positive ANIs can arise as a result of ‘pool substitution’—labelled inorganic fertilizer N standing proxy for unlabelled inorganic soil N that would otherwise have been immobilized. In the low-organic fallow soil, pool substitution accounted for the whole of the observed ANI and fertilizer N did not enhance either gross or net mineralization of soil N. Pool substitution also operated in the high organic grassland soil, but here net mineralization of soil N increased with increasing additions of fertilizer, giving rise to a ‘real’ ANI in addition to the larger ‘apparent’ ANI caused by pool substitution. This increase in net mineralization is probably caused by a decrease in immobilization of N as fertilizer N additions increase, not by an increase in gross mineralization of soil N. For pool substitution to operate, fertilizer N and soil inorganic N must occupy the same pool. This occurred in the pot experiment but not in the field experiment, where fertilizer and soil inorganic N remained separate and there was no ANI. When pool substitution occurs, fertilizer use efficiency is predictably lower as measured by the isotopic method than as measured by the conventional non-isotopic procedure.
The rhizosphere is one of the key fine-scale components of C cycles. This study was undertaken to improve understanding of the potential effects of atmospheric CO2 increase on rhizosphere processes. Using C isotope techniques, we found that elevated atmospheric CO2 significantly increased wheat plant growth, dry mass accumulation, rhizosphere respiration, and soluble C concentrations in the rhizosphere. When plants were grown under elevated CO2 concentration, soluble C concentration in the rhizosphere increased by approximately 60%. The degree of elevated CO2 enhancement on rhizosphere respiration was much higher than on root biomass. Averaged between the two nitrogen treatments and compared with the ambient CO2 treatment, wheat rhizosphere respiration rate increased 60% and root biomass only increased 26% under the elevated CO2 treatment. These results indicated that elevated atmospheric CO2 in a wheat-soil system significantly increased substrate input to the rhizosphere due to both increased root growth and increased root activities per unit of roots. Nitrogen treatments changed the effect of elevated CO2 on soil organic matter decomposition. Elevated CO2 increased soil organic matter decomposition (22%) in the nitrogen-added treatment but decreased soil organic matter decomposition (18%) without nitrogen addition. Soil nitrogen status was therefore found to be important in determining the directions of the effect of elevated CO2 on soil organic matter decomposition.
Elevated atmospheric CO2 increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO2on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO2 levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO2 in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO2 concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO2 on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO2 on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO2 enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO2, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.
To test inter- and intraspecific variability in the responsiveness to elevated CO2, 9–14 different genotypes of each of 12 perennial species from fertile permanent grassland were grown in Lolium perenne swards under ambient (35 Pa) and elevated (60 Pa) atmospheric partial pressure of CO2 (pCO2) for 3 years in a free air carbon dioxide enrichment (FACE) experiment. The plant species were grouped according to their
functional types: grasses (L. perenne, L. multiflorum, Arrhenatherum elatius, Dactylis glomerata, Festuca pratensis, Holcus lanatus, Trisetum flavescens), non-legume dicots (Rumex obtusifolius, R. acetosa, Ranunculus friesianus), and legumes (Trifolium repens, T. pratense). Yield (above a cutting height of 4.5 cm) was measured three times per year. The results were as follow. (1) There were
highly significant differences in the responsiveness to elevated pCO2 between the three functional types; legumes showed the strongest and grasses the weakest yield increase at elevated pCO2. (2) There were differences in the temporal development of responsiveness to elevated pCO2 among the functional types. The responsiveness of the legumes declined from the first to the second year, while the responsiveness
of the non-legume dicots increased over the 3 years. During the growing season, the grasses and the non-legume dicots showed
the strongest response to elevated pCO2 during reproductive growth in the spring. (3) There were no significant genotypic differences in responsiveness to elevated
pCO2. Our results suggest that, due to interspecific differences in the responsiveness to elevated pCO2, the species proportion within fertile temperate grassland may change if the increase in pCO2 continues. Due to the temporal differences in the responsiveness to elevated pCO2 among species, complex effects of elevated pCO2 on competitive interactions in mixed swards must be expected. The existence of genotypic variability in the responsiveness
to elevated pCO2, on which selection could act, was not found under our experimental conditions.
Trifolium repens L. and Lolium perenne L. were grown in monocultures and bi-species mixture in a Free Air Carbon Dioxide Enrichment (FACE) experiment at elevated (60 Pa) and ambient (35 Pa) CO2 partial pressure (pCO2) for three years. The effects of defoliation frequencies (4 and 7 cuts in 1993; 4 and 8 cuts in 1994/95) and nitrogen fertilization (10 and 42 g m–2 y–1 N in 1993; 14 and 56 g m–2 y–1 N in 1994/95) on the growth response to pCO2 were investigated.
There were significant interspecific differences in the CO2 responses during the first two years, while in the third growing season, these interspecific differences disappeared. Yield of T. repens in monocultures increased in the first two years by 20% when grown at elevated pCO2. This CO2 response was independent of defoliation frequency and nitrogen fertilization. In the third year, the CO2 response of T. repens declined to 11%. In contrast, yield of L. perenne monocultures increased by only 7% on average over three years at elevated pCO2. The yield response of L. perenne to CO2 changed according to defoliation frequency and nitrogen fertilization, mainly in the second and third year. The ratio of root/yield of L. perenne increased under elevated pCO2, low N fertilizer rate, and frequent defoliation, but it remained unchanged in T. repens. We suggest that the more abundant root growth of L. perenne was related to increased N limitation under elevated pCO2.
The consequence of these interspecific differences in the CO2 response was a higher proportion of T. repens in the mixed swards at elevated pCO2. This was evident in all combinations of defoliation and nitrogen treatments. However, the proportion of the species was more strongly affected by N fertilization and defoliation frequency than by elevated pCO2. Based on these results, we conclude that the species proportion in managed grassland may change as the CO2 concentration increases. However, an adapted management could, at least partially, counteract such CO2 induced changes in the proportion of the species. Since the availability of mineral N in the soil may be important for the species’ responses to elevated pCO2, more long-term studies, particularly of processes in the soil, are required to predict the entire ecosystem response.
The effect of an elevated partial pressure of CO2 (pCO2) on carbohydrate concentrations in source leaves and pseudo-stems (stubble) of Lolium perenne L. (perennial ryegrass) during regrowth was studied in a regularly defoliated grass sward in the field. The free air carbon dioxide enrichment (FACE) technology enabled natural environmental conditions to be provided. Two levels of nitrogen (N) supply were used to modulate potential plant growth. Carbohydrate concentrations in source leaves were increased at elevated pCO2, particularly at low N supply. Elevated leaf carbohydrate concentrations were related to an increased structural carbon (C) to N ratio and thus reflected an increased C availability together with a N-dependent sink limitation. Immediately after defoliation, apparent assimilate export rates (differences in the carbohydrate concentrations of young source leaves measured in the evening and on the following morning) showed a greater increase at elevated pCO2 than at ambient pCO2; however, replenishment of carbohydrate reserves was not accelerated. Distinct, treatment-dependent carbohydrate concentrations in pseudo-stems suggested an increasing degree of C-sink limitation from the treatment at ambient pCO2 with high N supply to that at elevated pCO2 with low N supply. During two growing seasons, no evidence of a substantial change in the response of the carbohydrate source in L. perenne to elevated pCO2 was found. Our results support the view that the response of L. perenne to elevated pCO2 is restricted by a C-sink limitation, which is particularly severe at low N supply.
Emissions of N2O and N2 were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO2 at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m−2 as 15NH415NO3 or 14NH415NO3 (1 at.% excess 15N). Total denitrification (N2O+N2) was increased under elevated pCO2 with emissions of 6.2 and 19.5 mg 15N m−2 measured over 22 d from ambient and elevated pCO2 swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO2 provides the energy for denitrification. Nitrification was the predominant N2O producing process under ambient pCO2 whereas denitrification was predominant under elevated pCO2. The N2-to-N2O ratio was often higher under elevated pCO2 suggesting that previous estimates of gaseous N losses based only on N2O emissions have greatly underestimated the loss of N by denitrification.
Effects of ambient and elevated atmospheric CO2 concentrations (350 and 700 μl l−1) on net carbon input into soil, the production of root-derived material and the subsequent microbial transformation were investigated. Perennial ryegrass plants (L. perenne L.) were labelled in a continuously labelled 14C-CO2 atmosphere to follow carbon flow through the plant and all soil compartments. After 115 days, root biomass was 41% greater at elevated CO2 than at ambient CO2 and this root biomass seemed to be the driving force for the increase of 14C-labelled carbon in all compartments examined, i.e. carbon in the soil solution, soil microbial biomass and soil residue. After incubation for 230 days at 14°C, roots grown at elevated CO2 decomposed slower (14%) than roots grown at ambient CO2. Increasing the incubation temperature of the roots grown at elevated CO2 by 2°C could not compensate for this delay in decomposition. In addition, ‘elevated CO2’ root-derived material (14C-labelled soil microorganisms plus 14C-labelled soil residue) decomposed significantly slower (29%) than ‘ambient CO2′ root-derived material. At the end of the incubation experiment, the ratio between 14C-labelled microorganisms and total 14CO2 evolved showed no difference among root incubation and incubation of root-derived material. Thus, the substrate use efficiency of microorganisms, involved with decomposition of roots and root-derived material, seems not to be affected by an increase in atmospheric CO2 concentrations. Therefore, the lower decomposition rate at elevated CO2 is not due to a change in the internal metabolism of microorganisms.
Elevated concentration of atmospheric carbon dioxide will affect carbon cycling in terrestrial ecosystems. Possible effects include increased carbon input into the soil through the rhizosphere, altered nutrient concentrations of plant litter and altered soil moisture. Consequently, the ongoing rise in atmospheric carbon dioxide might indirectly influence soil biota, decomposition and nutrient transformations.N-mineralisation and activities of the enzymes invertase, xylanase, urease, protease, arylsulfatase, and alkaline phosphatase were investigated in spring and summer in calcareous grassland, which had been exposed to ambient and elevated CO2 concentrations (365 and 600 μl l−1) for six growing seasons.In spring, N-mineralisation increased significantly by 30% at elevated CO2, while there was no significant difference between treatments in summer (+3%). The response of soil enzymes to CO2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly by 32 and 21%. In summer, differences of activities between CO2 treatments were greatest in the case of urease and protease (+21 and +17% at elevated CO2).The stimulation of N-mineralisation and enzyme activities at elevated CO2 was probably caused by higher soil moisture and/or increased root biomass. We conclude that elevated CO2 will enhance below-ground C- and N-cycling in grasslands.
Two key questions regarding the effects of elevated atmospheric CO2 on soil microbial biomass are, (a) will future levels of elevated CO2 affect the amount of microbial biomass in soil? and (b) how will any observed changes impact on C-flux from soils? These questions were addressed by examining soil microbial biomass, and in situ estimations of soil respiration in grassland soils exposed to free air carbon dioxide enrichment (60 Pa). Corresponding measurements of plant litter mass loss were taken using litter bags, ensuring that ambient litter was decomposed in ambient soil, and elevated CO2 grown litter was decomposed in soils exposed to elevated CO2. Significantly greater levels of microbial biomass (p < 0.05, paired t-test) were detected in soils exposed to elevated CO2 (1174.1 compared to 878.9 μg N g−1 dry soil for ambient CO2 exposed soils). This corresponded with a significant increase (p < 0.005, paired t-test) in in situ soil respiration from the elevated CO2 acclimatised soils (28.7 compared to 20.4 μmol CO2 m2 h−1 from soils exposed to ambient CO2). However, when soil respiration was calculated per unit of microbial biomass, no differences in activity per unit biomass were detected (approx. 0.02 μmol CO2 m2 h−1 unit biomass−1), suggesting that increased soil microbial biomass, rather than increased activity was responsible for the observed differences. The mass loss of litter was greater in the elevated CO2 acclimatised soils (p < 0.05, ANOVA), even though the initial nutrient ratios of the litter were not significantly different.
1 Possible patterns in nutrient resorption efficiency (% of the leaf nutrient pool resorbed) from senescing leaves of perennials were examined at both the intra- and the interspecific level. Most of the data used originated from studies with evergreen and deciduous shrubs and trees. 2 Combining all data, mean nutrient resorption efficiency was 50% for N (n = 287) and 52% for P (n = 226). N resorption efficiency of evergreen shrubs and trees (47%) was significantly lower than in deciduous shrubs and trees (54%), whereas P resorption efficiency did not differ significantly between these growth-forms (51 and 50%, respectively). Although nutrient resorption is an important nutrient conservation mechanism at the species level, it does not differ strongly between growth-forms. 3 Mean N and P concentrations in leaves of deciduous shrubs and trees were about 60% higher than in evergreen species. There were only small differences in mean resorption efficiency and nutrient concentrations in leaf litter of deciduous species were therefore much higher than in evergreens. This implies that, in comparison with deciduous species, the low nutrient concentrations in mature leaves of evergreens contribute far more to nutrient conservation than does nutrient resorption. 4 Relations between leaf nutrient status and leaf nutrient resorption were absent or very weak. Assuming that leaf nutrient status reflects nutrient availability, this implies that nutrient resorption is only weakly controlled by nutrient availability. 5 At the intraspecific level, nutrient resorption was not very responsive to increased nutrient availability. There was no response in 63% of the experiments analysed (covering 60 spp.), whereas in 32% there was a decrease in N resorption in response to increased nutrient availability. For P (37 species analysed) there was no response in 57% of the cases and in 35% of the cases P resorption decreased upon enhanced nutrient supply. Evergreen shrubs and trees showed especially low responsiveness. 6 This review shows that there are no clear nutritional controls on nutrient resorption efficiency. Future research should focus on the biochemical basis of variation in nutrient resorption efficiency and on the factors, other than nutrient availability, that control nutrient resorption efficiency.
We tested a conceptual model describing the influence of elevated atmospheric CO 2 on plant
production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is
based on the observation that in nutrient-poor soils, plants (C3) grown in an elevated CO 2 atmosphere
often increase production and allocation to belowground structures. We predicted that greater
belowground C inputs at elevated CO 2 should elicit an increase in soil microbial biomass and increased
rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass
production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field
season at ambient and twice-ambient (i.e., elevated) atmospheric CO 2 concentrations. Plants were
grown in a sandy subsurface soil i) at ambient CO 2 with no open top chamber, ii) at ambient CO 2 in an
open top chamber, and iii) at twice-ambient CO 2 in an open top chamber. Plants were fertilized with
4.5 g N m -2 over a 47 d period midway through the growing season. Following 152 d of growth, we
quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested
for a significant CO 2 effect on plant growth and soil C and N dynamics by comparing the means of the
chambered ambient and chambered elevated CO 2 treatments.
Rates of photosynthesis in plants grown at elevated CO 2 were significantly greater than those
measured under ambient conditions. The number of roots, root length, and root length increment were
also substantially greater at elevated CO:. Total and belowground biomass were significantly greater at
elevated CO:. Under N-limited conditions, plants allocated 50-70% of their biomass to roots. Labile C
in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient
treatments; there were no significant differences between labile C pools in the bulk soil of ambient and
elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of
plants grown at elevated CO 2 compared to that in the ambient treatment. Moreover, a short-term
laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk
soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO 2 concentrations can
have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments
conducted for longer periods of time will be necessary to test the potential for negative feedback due to
altered leaf litter chemistry.
Reduced soil N availability under elevated CO2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N2-fixing system. We studied the effects of Trifolium repens (an N2-fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO2 under FACE conditions. 15N-labeled fertilizer was applied at a rate of 140 and 560 kg Nha -1 yr -1 and the CO2 concentration was increased to 60 Pa pCO2 using 13C-depleted CO2. The total soil C content was unaffected by elevated CO2, species and rate of 15N fertilization. However, under elevated CO2, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer-N (fN) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO2, had a significant effect on fN values of the total soil N pool. The fractions of newly sequestered C (fC) differed strongly among intra-aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO2, the ratio of fertilizer-N per unit of new C decreased under T.repens compared with L. perenne. The L. perenne system sequestered more 15N fertilizer than T. repens: 179 vs. 101 kg N ha -1 for the low rate of N fertilization and 393 vs. 319 kg N ha -1 for the high N-fertilization rate. As the loss of fertilizer-15N contributed to the 15N-isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N2 fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non-N2-fixing L. perenne system. This suggests that N2 fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.
Diss. no. 12329 nat. sc. SFIT Zurich. Literaturverz.
Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term.. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 mumol mol(-1) for 10 years using Free Air CO2 Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source: sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (V-c,V-max) and the maximum rate of electron transport (J(max)). Immediately prior to the periodic (every 4-8 weeks) cuts of the L. perenne stands, V-c,V-max and J(max), were significantly lower in elevated than in ambient [CO2] in the low-nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation in V-c,V-max and J(max) in the later years in either nitrogen treatment.
> Abstract The increase in atmospheric CO2 content alters C3 plant photosynthetic rate, leading to changes in rhizodeposition and other root activities. This may influence the activity, the biomass, and the structure of soil and rhizosphere microbial communities and therefore the nutrient cycling rates and the plant growth. The present paper focuses on bacterial numbers and on community structure. The rhizospheres of two grassland plants, Lolium perenne (ryegrass) and Trifolium repens (white clover), were divided into three fractions: the bulk soil, the rhizospheric soil, and the rhizoplane-endorhizosphere. The elevated atmospheric CO2 content increased the most probable numbers of heterotrophic bacteria in the rhizosphere of L. perenne. However, this effect lasted only at the beginning of the vegetation period for T. repens. Community structure was assessed after isolation of DNA, PCR amplification, and construction of cloned 16S rDNA libraries. Amplified ribosomal DNA restriction analysis (ARDRA) and colony hybridization with an oligonucleotide probe designed to detect Pseudomonas spp. showed under elevated atmospheric CO2 content an increased dominance of pseudomonads in the rhizosphere of L. perenne and a decreased dominance in the rhizosphere of T. repens. This work provides evidence for a CO2-induced alteration in the structure of the rhizosphere bacterial populations, suggesting a possible alteration of the plant-growth-promoting-rhizobacterial (PGPR) effect.http://link.springer-ny.com/link/service/journals/00248/bibs/38n1p39.html
Plant scientists have long debated whether plants or microorganisms are the superior competitor for nitrogen in terrestrial ecosystems. Microorganisms have traditionally been viewed as the victors but recent evidence that plants can take up organic nitrogen compounds intact and can successfully acquire N from organic patches in soil raises the question anew. We argue that the key determinants of 'success' in nitrogen competition are spatial differences in nitrogen availability and in root and microbial distributions, together with temporal differences in microbial and root turnover. Consequently, it is not possible to discuss plant-microorganism competition without taking into account this spatiotemporal context.
Understanding rhizosphere processes in relation to increasing atmospheric CO2 concentrations is important for predicting the response of forest ecosystems to environmental changes, because rhizosphere
processes are intimately linked with nutrient cycling and soil organic matter decomposition, both of which feedback to tree
growth and soil carbon storage. Plants grown in elevated CO2 substantially increase C input to the rhizosphere. Although it is known that elevated CO2 enhances rhizosphere respiration more than it enhances root biomass, the fate and function of this extra carbon input to
the rhizosphere in response to elevated CO2 are not clear. Depending on specific plant and soil conditions, the increased carbon input to the rhizosphere can result
in an increase, a decrease, or no effect on soil organic matter decomposition and nutrient mineralization. Three mechanisms
may account for these inconsistent results: (1) the “preferential substrate utilization” hypothesis; (2) the “priming effect”
hypothesis; and (3) the “competition” hypothesis, i.e., competition for mineral nutrients between plants and soil microorganisms.
A microbial growth model is developed that quantitatively links the increased rhizosphere input in response to elevated CO2 with soil organic matter decomposition. The model incorporates the three proposed mechanisms, and simulates the complexity
of the rhizosphere processes. The model also illustrates mechanistically the interactions among nitrogen availability, substrate
quality, and microbial dynamics when the system is exposed to elevated CO2.
Source-sink relations in Lolium perenne L. as reflected by carbohydrate
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Pudoc, Wageningen. Fischer BU, Frehner M, Hebeisen T et al. (1997) Source-sink relations in Lolium perenne L. as reflected by carbohydrate
Yield response of Lolium perenne swards to free air CO 2 enrichment increased over six years in a high N input system on fertile soil
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Daepp M, Suter D, Almeida JPF et al. (2000) Yield response of Lolium perenne swards to free air CO 2 enrichment increased over six years in a high N input system on fertile soil. Global Change Biology, 6, 805–816.
Effect of high nitrogen supply on sward deterioration and root mass
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Climate change: the potential to affect ecosystem functions through changes in amount and quality of litter. In: Driven by Nature: Plant Litter Quality and Decomposition Denitrification in grass swards is increased under elevated atmospheric CO 2
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Developmental characteristics of grass varieties in relation to their herbage production. 5. Effects of nitrogen treatment on the relative development of upper and lower internodes of mature reproductive tillers of Dactylis glomerata and Lolium perenne
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Root system adjustments
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The global carbon sink
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The decomposition of Lolium perenne in soils exposed to elevated CO2
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