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Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids
Abstract
Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.
... This is in line with previous findings by Du et al. (2017) that reported more chemically recalcitrant OC retained in finer 53-250 µm and < 53 µm fractions in the biochar treated soils. Likewise, previous studies reported higher black carbon (revealed by benzenecarboxylic acids) in the organo-mineral complexes (reflecting by heavy fraction, > 2.0 g cm −3 and/or mineral particles, < 53 μm) than the coarse and/or light fractions (Chang et al. 2020;Glaser et al. 2000Glaser et al. , 2001. Despite the differences in the fractionation and biochar measured procedures compared to our study, these above works could still provide some implications and insights into the intrinsically refractory C interacted with the mineral particles to improve the stabilization and persistence. ...
... Importantly, growing concerns have shown that the ecosystem property rather than molecular structure alone (i.e., recalcitrance) dominated soil organic C stabilization (Schmidt et al. 2011;Lehmann and Kleber 2015). The black carbon-like structures are deemed considered long-term C sinks due to their high stability, particularly when these chemically recalcitrant C were incorporated into organo-mineral clusters (Chang et al. 2020;Glaser et al. 2000). Taken together, our data and others have further emphasized the importance of organo-mineral complexes for biochar particle stabilization. ...
Purpose
Biochar application to soil has gained great interest as a land-based climate change mitigation solution. However, it lacks long-term field assessment on the effectiveness of biochar compared with other widely applied land management—such as stubble retention—on soil organic carbon (SOC) accumulation and carbon (C) distribution and persistence in the soil matrix.
Materials and method
Here, we conducted a 9-year field trial in a temperate agroecosystem of North China to identify and quantify the location of C residing in the soil matrix (determined by two physical fractionation methods), as affected by land management—stubble removed (control), stubble returned at 15 t ha⁻¹ year⁻¹ (SR), two biochar doses at 4.5 t ha⁻¹ year⁻¹ (B4.5; equivalent to feedstock in SR) and 9.0 t ha⁻¹ year⁻¹ (B9.0).
Results and discussion
The results showed that biochar application significantly increased SOC in the free and occluded particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions. Compared to B4.5 and B9.0, SR was less effective in soil C accrual in the occluded POM and MAOM, although the largest increase of C occurred in the free POM (about 10–17 g kg–1 soil). Consistent with this, biochar rather than stubble retention significantly increased C in (i) coarse POM (i.e., unprotected POM, > 250 μm) by 190–210%, (ii) microaggregates (μAgg) by 56–70%, and (iii) MAOM in silt–clay fraction (iMAOM) by 4–12%, but the biochar dose effect was statistically insignificant. Importantly, biochar significantly increased chemically recalcitrant C in soils that were further protected in the μAgg and iMAOM fractions.
Conclusions
We conclude that biochar application was more beneficial for SOC accumulation and preservation than stubble management, particularly in the microaggregates and organo-mineral complexes, under the intensive cropping systems.
... The soils were mixed with deionized water at a ratio of 1:5 (w:v) and ultrasonically dispersed (output energy if 22 J mL −1 ). According to our previous study [26], the mixture was first passed through a 250 µm mesh sieve. The filtrate was further dispersed with an ultrasonic power of 47.5 W for 10 min. ...
The stabilization mechanism of soil organic matter (SOM) has received considerable attention. It is widely accepted that mineral sorption/protection is important for SOM stabilization. However, it remains unclear which organic carbon component is beneficial for mineral protection. We collected soil samples from a paddy field (TP) to compare with natural soil (NS). To illustrate the behavior of different SOM pools and their protection by particles, we separated the soils into different particle-size fractions and then removed the active minerals using an acid mixture (1 M HCl/10% HF). The different carbon pools were analyzed using stable carbon isotopes and lipid biomarkers. Our study showed that acid treatment evidently increased the extractability of free lipids, usually over 60%, which confirmed the predominant role of minerals in SOM protection. For NS, the δ13C values increased with decreasing soil particle sizes and soil depths, indicating that 13C-enriched SOM was selectively preserved. However, this trend disappeared after cultivation, which was mainly attributed to the combined effects of the input of 13C-depleted fresh SOM and decomposition of the preserved 13C-enriched SOM. Meanwhile, based on the degradation parameters of the overall lipid biomarkers, SOM showed higher degradation states in clay and silt fractions than in the sand fraction before cultivation. It is possible that the small particle-size fractions could selectively absorb highly degraded SOM. The clay-associated SOM showed a low degradation state, but its carbon content was low after cultivation. We propose that the previously protected SOM was degraded after cultivation and was replaced by relatively fresh SOM, which should be carefully monitored during SOM management.
... Meanwhile, available K was measured by ammonium acetate extraction (Knudsen et al., 1982), and available P was extracted with 0.5 M sodium bicarbonate before being measured with the molybdenum-antimony anti-spectrophotometric method (Olsen and Sommers, 1982). Finally, the benzene polycarboxylic acid (BPCA) analysis was conducted following previous studies (Chang et al., 2020;Chang et al., 2019), and detailed in the supporting information. ...
Application of biochar derived from biomass resources in soil is an encouraging method to decelerate global warming via carbon sequestration. While there has been extensive research on the priming effect of biochar on the mineralization of native soil organic carbon (nSOC), its correlation with soil organic carbon (SOC) structural variations remains poorly understood. A series of incubation experiments with soils amended with biochar prepared at 300 • C, 450 • C, and 600 • C were performed to explore the shifts in nSOC structure and reveal the interconnections among soil properties, SOC structural variation, and the priming effect induced by biochar. Biochar exhibited an "inhibitory concentration" on soil microorganisms, where 1 % biochar enhanced bacterial and fungal richness and diversity, while 3 % biochar reduced bacterial richness and diversity although soil nutrient conditions were improved. Biochar-treated soil had higher humic acid-and humin-like fractions (HAL and HML) but lower fulvic acid-like components (FAL). Moreover, bulk SOC, FAL, HAL, and HML of soils with 300 • C and 450 • C biochar contained more aromatic-type species (e.g., lignin). Such changes were related to improvement of the soil nutrient condition and shifts in microbial community structures, in addition to the "accumulation of biochar-derived OC in the nSOC pool". Stable carbon isotopes indicated that the inhibitory effect on SOC mineralization (− 94.5 to − 8.0 mg CO 2-C kg − 1 soil) induced by biochar was significantly explained by the change in FAL and HAL contents. The findings improved the understanding of the carbon sequestration potential of biochar in soils for SOC structural modification.
... The retention of biomolecules within smectite-type clay minerals is implicated in facilitating long-term preservation of organic matter in soils, which in turn influences ecosystem-scale carbon cycling (4,49,50). Our findings demonstrated that adsorption hierarchy was primarily due to electrostatic attraction in the biomoleculeclay interactions at the water-clay interfaces. ...
Clay minerals are implicated in the retention of biomolecules within organic matter in many soil environments. Spectroscopic studies have proposed several mechanisms for biomolecule adsorption on clays. Here, we employ molecular dynamics simulations to investigate these mechanisms in hydrated adsorbate conformations of montmorillonite, a smectite-type clay, with ten biomolecules of varying chemistry and structure, including sugars related to cellulose and hemicellulose, lignin-related phenolic acid, and amino acids with different functional groups. Our molecular modeling captures biomolecule–clay and biomolecule–biomolecule interactions that dictate selectivity and competition in adsorption retention and interlayer nanopore trapping, which we determine experimentally by NMR and X-ray diffraction, respectively. Specific adsorbate structures are important in facilitating the electrostatic attraction and Van der Waals energies underlying the hierarchy in biomolecule adsorption. Stabilized by a network of direct and water-bridged hydrogen bonds, favorable electrostatic interactions drive this hierarchy whereby amino acids with positively charged side chains are preferentially adsorbed on the negatively charged clay surface compared to the sugars and carboxylate-rich aromatics and amino acids. With divalent metal cations, our model adsorbate conformations illustrate hydrated metal cation bridging of carboxylate-containing biomolecules to the clay surface, thus explaining divalent cation-promoted adsorption from our experimental data. Adsorption experiments with a mixture of biomolecules reveal selective inhibition in biomolecule adsorption, which our molecular modeling attributes to electrostatic biomolecule–biomolecule pairing that is more energetically favorable than the biomolecule–clay complex. In sum, our findings highlight chemical and structural features that can inform hypotheses for predicting biomolecule adsorption at water–clay interfaces.
... Among them, the BPCA method destroys the fused ring aromatic structure through chemical oxidation to form a single small aromatic structural unit and then connects the carboxyl group at the bond-breaking position to form a series of BPCAs. This method is considered to be relatively accurate and suitable for the quantitative analysis of BC in the environment (Chang et al. 2020;Wiedemeier et al. 2015;Ziolkowski and Druffel 2010). In addition, the composition of BPCA monomers, including benzene tri-(B3CAs), tetra-(B4CAs), penta-(B5CAs), and hexacarboxylic acids (B6CA), can provide information of the degree of aromatic condensation of BC (Brodowski et al. 2005;Dittmar 2008;Glaser et al. 1998). ...
Biochar, a soil conditioner containing significant amounts of polycyclic aromatic hydrocarbons (PAHs), has gained widespread popularity in agricultural practices due to its advantages in improving soil fertility and carbon sequestration. While biochar may increase soil black carbon (BC) and PAH contents, the quantitative accumulation of BC and PAHs in different soil environments under varying biochar addition dosages remains poorly understood. Here, we investigated the content and composition of black carbon (evaluated using benzene polycarboxylic acids, BPCAs) and PAHs in soils treated with different biochar addition dosages from two long-term experimental farmlands in Ningxia (5-year) and Shandong (7- and 11-year), China. Results showed that increasing cumulative biochar dosage caused elevated contents of black carbon and PAHs, accompanied by decreases in their retention efficiencies. Contrasting retention was observed between sites, with the Shandong site characterized by higher retention efficiencies of BPCAs and lower retention efficiencies of PAHs, possibly owing to its higher temperature, more sandy soil texture, less irrigation, and lower sunlight intensity. Despite both black carbon and PAHs originating from biochar and sharing similar condensed aromatic structures, there was no significant correlation between the contents of black carbon and PAHs, indicating distinct behaviors and fates of these compounds. These findings emphasize the importance of optimizing biochar addition dosages and considering site-specific environmental factors for effective soil black carbon sequestration through biochar application.
... A higher H/C ratio indicates a lower aromaticity. As pyrolysis temperature rises, the H/C ratio in BC usually shows a descending curve, and a greater aromaticity in BC occurs (Chang et al., 2020). Nevertheless, a study demonstrated that the aromaticity of DBC derived at only 300 or 400°C is higher as compared to that from other DBCs derived at different temperatures (Chen et al., 2022a). ...
... A higher H/C ratio indicates a lower aromaticity. As pyrolysis temperature rises, the H/C ratio in BC usually shows a descending curve, and a greater aromaticity in BC occurs (Chang et al., 2020). Nevertheless, a study demonstrated that the aromaticity of DBC derived at only 300 or 400°C is higher as compared to that from other DBCs derived at different temperatures (Chen et al., 2022a). ...
Biochar (BC) is a sustainable and renewable carbonaceous material, and its soluble component, dissolved black carbon (DBC), is the key to understanding BC's geological and environmental processes. Although the relationship between the changes in DBC structure and its properties, functions, and associated environmental risks has been explored, a gap remains in our understanding of DBC's fate and behavior in the natural environment. Thus, in this review, we have highlighted the molecular and chemical compositions and the structural evolution of DBC during pyrolysis, the influence of DBC's physicochemical properties on its fate and transport, DBC's interaction with soil and its contaminants, and DBC stability in soil and water environments along with potential risks. Based on our in-depth assessment of DBC and its biogeochemical roles, we believe that future studies should focus on the following: (1) using advanced techniques to understand the chemical and molecular structure of DBC deeply and concisely and, thus, determine its fundamental role in the natural environment; (2) investigating the multi-functional properties of DBC and its interaction mechanisms; and (3) evaluating the environmental behaviors of and risks associated with DBC after BC application. In future, it is necessary to gain a deeper insight into the fate and transport of DBC with contaminants and study its associated risks under BC application in the environment.
... A higher H/C ratio indicates a lower aromaticity. As pyrolysis temperature rises, the H/C ratio in BC usually shows a descending curve, and a greater aromaticity in BC occurs (Chang et al., 2020). Nevertheless, a study demonstrated that the aromaticity of DBC derived at only 300 or 400°C is higher as compared to that from other DBCs derived at different temperatures (Chen et al., 2022a). ...
Biochar (BC) is a sustainable and renewable carbonaceous material, and its soluble component, dissolved black carbon (DBC), is the key to understanding BC's geological and environmental processes. Although the relationship between the changes in DBC structure and its properties, functions, and associated environmental risks has been explored, a gap remains in our understanding of DBC's fate and behavior in the natural environment. Thus, in this review, we have highlighted the molecular and chemical compositions and the structural evolution of DBC during pyrolysis, the influence of DBC's physicochemical properties on its fate and transport, DBC's interaction with soil and its contaminants, and DBC stability in soil and water environments along with potential risks. Based on our in-depth assessment of DBC and its biogeochemical roles, we believe that future studies should focus on the following: (1) using advanced techniques to understand the chemical and molecular structure of DBC deeply and concisely and, thus, determine its fundamental role in the natural environment; (2) investigating the multi-functional properties of DBC and its interaction mechanisms; and (3) evaluating the environmental behaviors of and risks associated with DBC after BC application. In future, it is necessary to gain a deeper insight into the fate and transport of DBC with contaminants and study its associated risks under BC application in the environment.
... It is thus resistant to biological decomposition and may be a good atmospheric C reservoir (Jamala and Oke 2013). Mineral associated C may also be associated with high clay content and high oxides (organo-mineral complexes) which help stabilise it against decomposition (Chang et al. 2020;Poirier et al. 2020). According to Fey (2010), the soils used in the current study are highly weathered with high concentrations of iron and aluminium oxides, which could contribute to the stabilisation of SOM. ...
While conservation agriculture (CA) has largely been successful in many areas, some reports suggest that certain farmers have not realised the benefits they had hoped for, especially in Africa. The benefits of CA could depend on the cropping sequences involved. This study determined the short-term effects of wheat (Triticum aestivum L.)/ maize (Zea mays L.) and wheat/ soybean (Glycine max L.) cropping sequences on fractions of soil organic carbon (SOC), inorganic phosphorus and other soil quality parameters in the 0–200 and 200–400 mm depths on CA farms. Water-soluble carbon (C) and available phosphorus (P) (NaHCO3 Pi) were significantly higher and NaOH I Pi was lower in maize/wheat than in soybean/wheat sequences. SOC, extractable P, and NaHCO3 Pi were significantly higher in the 0–200 mm than in the 200–400 mm depth. Extractable P correlated positively with particulate organic carbon (OC) fractions under both sequences. In addition, soil pH, exchangeable potassium (K), calcium (Ca), magnesium (Mg) and cation exchange capacity (CEC) were significantly higher while acid saturation and Ca:Mg was lower in maize/wheat than in soybean/wheat sequences. The findings imply that short-term cropping sequences do not affect SOC sequestration but a wheat crop preceded by maize could benefit from higher soil pH and labile C fractions making P more available, with the additional benefit of available K in these CA systems.
... Previous studies have shown that PAHs were mainly adsorbed on soil organic matter (SOM) Yu et al. 2014) or organic matter-mineral complexes (Chang et al. 2020) and usually accumulated more in SOM-rich topsoil (He et al. 2009;Wilcke 2000). Moreover, the SOM was mainly distributed in soil aggregates. ...
Soil contamination by polycyclic aromatic hydrocarbons (PAHs) is an increasing problem in many countries, impacting the ecological environment’s sustainable development. This study investigated the effects of fluoranthene (Fla) on soil aggregate stability. A possible mechanism for the interaction of Fla with soil aggregates was proposed by characterizing the aggregate structure. The results showed that Fla significantly improved the aggregate stability in the concentration range of 0–30.0 mg/kg. The content of macro-aggregates reached the maximum value at 10 mg/kg of Fla, which increased by 24.25% compared with the control group, while the content of large-aggregates decreased by 12.11%. Meanwhile, the mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 56.63% and 37.66%, respectively. However, the macro-aggregates zeta potential value and specific surface area (SSA) decreased by 12.68% and 13.61%, respectively. The cracks of macro-aggregates were also significantly reduced. In addition, Fla-based free radicals were detected on the macro-aggregates. The absorption peak of the C–O group significantly increased, indicating that Fla may be covalently bound to the aggregates by aromatic ether bonds, which is a possible mechanism for the interaction between Fla and aggregates. This study provides theoretical support for revealing the effects of PAHs on soil.
... This interaction between clay and OM hinders the decomposition of the latter and further stabilizes it in sediments (Saidy et al., 2013), thereby promoting the long-term OM preservation in sediment material (Singh et al., 2016). Clay minerals are believed to play a paramount role in this OM preservation process (Singh et al., 2016;Blattmann et al., 2019), because they closely associate with OM to form a clay-OM complex, which is an important form of stable OM (Parfitt et al., 1997;Barré et al., 2014;Chang et al., 2020;Kopittke et al., 2020). For example, OM intercalated to the interlayer space of swelling clay minerals (e.g., montmorillonite and mixed-layer illite/smectite) is notably stable, with preservation time lasting 2000 to 10,000 years (Dubbin et al., 2014). ...
The sequestration of algal organic matter (AOM) in clays constitute important carbon sinks in lake sediments. The composition of AOM was proven recently to critically influence its preservation in sediments. Yet it remains unclear which AOM components and types of clay minerals contribute to AOM burial in lake sediments. Here we investigated the AOM fractionation in kaolinite and illite via their adsorption, to ascertain the distribution of AOM components on these clay surfaces. The results showed that the AOM adsorbed quantity was 2-fold higher for illite than kaolinite. The adsorption isotherm data of illite and kaolinite were well fitted by a Freundlich model. The three-dimensional excitation emission matrix fluorescence spectroscopy (3D EEM) with parallel factor analysis (PARAFAC) modeling identified the AOM component under fractionation. The AOM solution mainly consisted of humic acid-like, tyrosine-like, and tryptophan-like components. Notably, large amounts of the hydrophilic tyrosine-like components were adsorbed onto the surface of both kaolinite and illite, whereas the hydrophobic tryptophan-like components were seldom adsorbed onto neither of the clay materials, while humic acid-like components were only adsorbed onto illite. Therefore, illite is more suitable for the preservation of AOM. Both transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) and the X-ray photoelectron spectroscopy (XPS) analysis further confirmed the high adsorption of AOM on the surface of illite; i.e., the molar content of carbon in AOM adsorbed to illite was 22.04%, almost a double of that adsorbed to kaolinite (11.59%). Overall, our results demonstrate the selective preservation of AOM on clay surfaces and highlight the role of clay minerals as sequesters of organic matter in eutrophic lakes.
Background Strongly varying timescales of pyrogenic carbon (PyC) degradation have been observed across depositional settings. To date, PyC degradation in UK peatlands has had limited investigation. Aims This study aims to evaluate how PyC recalcitrance relates to differing production characteristics, fuels and duration of exposure in UK peatlands. Methods PyC samples produced from key peatland vegetation types were exposed on a peatland surface to assess molecular (by Fourier-transform infrared), leachable carbon (water-extractable organic carbon) and elemental (C, H, N, O) changes occurring over a year. Key results PyC degradation phases were observed: (1) very rapid (≤1 month) loss of leachable carbon; (2) longer-term (1–12 months) changes to PyC characteristics indicative of soil interactions. ‘Severity’ had a significant effect on all measured variables. Conclusions This study indicates that PyC is susceptible to changes within short timescales in UK peatlands, particularly low-temperature PyC, but that stabilisation through soil matrix interactions may occur over longer periods (>1 year). Implications The findings indicate that UK peatland wildfire carbon cycling research should consider early pulses of carbon to the wider environment, as well as longer-term C storage in PyC.
Investigators are debating on the positive and negative priming effects of biochar on native soil organic carbon (SOC), which is largely attributed to the technical barrier of identifying biochar contribution to the apparently measured SOC or mineralized CO2. We combined benzene polycarboxylic acids (BPCAs) molecular biomarkers and soil particle density fractionation to identify biochar contributions to the carbon content in three representative allitic soils in Yunnan. The soil-biochar mixture was incubated for one-month to avoid significant biodegradation of biochar. The results showed that BPCAs were mainly distributed in free light fractions (fLF) up to 87 % of the total BPCAs contents after one month incubation. Recognition of BPCAs in occluded light fractions (oLF) and heavy fractions (HF) suggested a significant interaction between biochar and soil mineral particles. In addition, the percentage of B6CA is comparable or even higher in HF than in fLF or oLF. Thus, biochar-mineral interactions may be an additional stabilization mechanism besides the condensed aromatic structures in biochar. The apparently measured carbon contents increased after biochar application, and both positive and negative priming effects to native SOC were observed after deducting biochar contents based an accurate calculation from BPCAs. The most native SOC depletion (positive priming effects) was noted for the soil with the most favored biochar embedding in soil mineral compositions. This study emphasized that combining BPCAs molecular biomarkers and soil particle density fractionation could accurately quantify different carbon pools, and thus facilitate a comprehensive understanding on the stabilization and turnover of biochar in soils.
Biochar inevitably goes through long-term aging under biotic and abiotic processes in the environment, which results in various changes in its physicochemical properties. However, the traditional characterization methods based on particle separation cannot effectively monitor biochar in complex matrixes. Molecular markers, especially benzene polycarboxylic acids (BPCAs), can be used to directly identify the source and properties of biochar. In this study, biochars were prepared using corn straw (CS) and pinewood (PW) and were oxidized with HNO3/H2SO4 to simulate the aging processes. Molecular markers of lignin-derived phenols showed that PW has more vanillyl unit and thus more stable than CS. The overall BPCAs content and the relative content of mellitic acid (B6CA) both increased with pyrolysis temperature, indicating increased aromatic condensation/aromaticity. The pristine CS biochar has a higher BPCAs content compared to PW biochar. HNO3/H2SO4 treatment greatly decreased the lignin components and more vanillyl and cinnamyl units were removed from CS biochar than PW biochar. In addition, BPCAs contents decreased by 41–60 mg/g for CS biochar, while increased by 86–133 mg/g for PW biochar after HNO3/H2SO4 oxidation. This is owing to the release of the condensed aromatic structures in CS biochars, but the concentration of the condensed aromatic structures in PW biochars after oxidation. These results showed that PW biochars are more stable than CS biochars. The application of the molecular markers can help understanding the dynamic change of biochar in the environment.
The elucidation of interactions between the dissolved black carbon (DBC) in biochar and hydrophobic organic contaminants (HOCs) is crucial for controlling the environmental behavior of HOCs. The complicated chemical structures of DBCs result in diverse interaction mechanisms between DBCs and HOCs, which were driven by different chemical structures in DBCs. In the present study, ten DBCs were extracted from rice straw and corncob biochars and their chemical structures were characterized and analyzed. The binding of phenanthrene (Phen) with DBC were studied through fluorescence quenching experiments. DBCs with low concentration (1 mg C/L) were found to complex with high amounts of Phen per unit mass. No significant difference was found in the amount of the bound Phen per unit amount of DBC when the concentration of DBC increased beyond >5 mg C/L. The dominant mechanisms involved in the binding of Phen by DBCs are speculated to be hydrophobic interactions, π-π electron donor-acceptor (EDA), and chemical partition, which was driven by the fatty carbon chain, aromatic rings, and quinone groups or ester groups, respectively. This study elucidates the interactions between DBC and Phen, which is of great significance for understanding the environmental behavior of HOCs.
Polycyclic aromatic hydrocarbons (PAHs) in soil can be recalcitrant to solvent extraction after aging. We showed in this study that mixing a small amount of water in the extracting solvent during microwave-assisted extraction (MAE) can release recalcitrant PAHs, resulting in significant improvement in the analyzed concentrations. The improvement factor (F) for the total of 16 priority PAHs (∑PAH16) listed by the United States Environmental Protection Agency was 1.44–1.55 for field soils. By comparing the F values for different soil organic components, we demonstrated that the recalcitrant PAHs were primarily associated with biochar, humic acid (HA), and humin (HM), with the F values for ∑PAH16 of 1.94, 6.62, and 4.59, respectively. The results showed that the recalcitrant PAHs comprised a sequestered fraction and a desorption-limited fraction. NMR spectra showed that water worked alone at elevated temperature to promote hydrolysis of biochar and destroy the macromolecular structure, thus causing the release of the otherwise sequestered PAHs during MAE. The substantial reduction in F values for HA and HM after demineralization indicated sequestration of PAHs in organic-mineral complexes, which can be destroyed by hot water treatment. The release of the sequestered fraction was nonselective and independent of compound hydrophobicity. In comparison, the release of the desorption-limited fraction was positively affected by the hydrophobicity of PAHs and was facilitated by the presence of water in the extracting solvent. The results of this study provide important insights into the sequestration and release of recalcitrant PAHs in soil.
Magnetite-coating biochar (MBC) is a promising remediator for antibiotic contamination. Accurate models describing the sorption affinity are required to better understand the role of minerals. In this study, the presence of magnetite led to the improvements of oxygen-containing groups (i.e. C=O) and regulation of π-systems within BC. Based on Dubinin-Ashtakhov (DA) model, the differences of site energy (Em) and sorption heterogeneity (σe⁎) led to the variances between sorption capacities of sulfonamides (SAs). The positive correlations between Em and the oxygen content or pore volume of MBCs indicated that π-π interactions, H-bonding, and pore-filling may act as the high energy sites. Moreover, σe⁎ was related to the distribution of magnetite on BC and their porosities. These results suggested that compared to BCs, the coating minerals improved the π-interaction assisted H-bonding and proton configuration of antibiotic when sorbing on MBC. The negative correlations between the Em of different SAs with their molecular sizes and solubilities resulted from steric effects and competition with water, which further confirmed the proposed high energy sites on MBCs. This study provided the insightful information of site energy distribution and understanding of fate and transport of organic pollutants on BC when the iron minerals were embedded or coated.
Black carbon (BC), ubiquitous in soils, plays an important role in global carbon cycles, the radiative heat balance of the Earth, pollutant fate, emissions of greenhouse gas, soil fertility, soil microbial community, and ecosystem stability. However, information on BC in topsoils of the northeastern Qinghai-Tibet Plateau is limited. Therefore, this study performed field sampling and analyzed contents of total BC and soot BC in topsoils. The results indicated that the contents of total BC in all soil samples ranged from 0.504 to 74.381 g kg⁻¹ with an average value of 5.152 g kg⁻¹, whereas those of soot BC were in the range of 0.400–15.200 g kg⁻¹ with a mean value of 1.719 g kg⁻¹. Contents of BC were significantly correlated with those of total carbon and total organic carbon. Soil types affected the distribution of soil BC. The contents of total BC in the loam soils were larger than those in the clay soils, whereas soot BC was more easily enriched in the clay soils. Total BC was negatively correlated with Ca, and soot BC was negatively correlated with Ti. The contents of soil BC in functional areas, such as agricultural and pastoral areas, industrial areas, and mining areas, were significantly higher than those in other areas, illustrating that anthropogenic activities drastically affected the distribution of soil BC. This study exhibits the fundamental information on soil BC in the northeastern Qinghai-Tibet Plateau to provide important knowledge on global soil carbon sink.
Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to physical isolation and chemical stabilization at the organic-mineral interface. Instrumentation for probing this interface is limited. As a result, much of the micron-and molecular-scale knowledge about organic-mineral interactions remains largely qualitative. Here we report the use of force spectroscopy to directly measure the binding between organic ligands with known chemical functionalities and soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that chemistry of both the organic ligand and mineral contribute to values of binding free energy and that changes in pH and ionic strength produce significant differences in binding energies. These direct measurements of molecular binding provide mechanistic insights into organo-mineral interactions, which could potentially inform land-carbon models that explicitly include mineral-bound C pools.
Pyrogenic carbon (PyC; includes soot, char, black carbon, and biochar) is produced by the incomplete combustion of organic matter accompanying biomass burning and fossil fuel consumption. PyC is pervasive in the environment, distributed throughout the atmosphere as well as soils, sediments, and water in both the marine and terrestrial environment. The physicochemical characteristics of PyC are complex and highly variable, dependent on the organic precursor and the conditions of formation. A component of PyC is highly recalcitrant and persists in the environment for millennia. However, it is now clear that a significant proportion of PyC undergoes transformation, translocation, and remineralization by a range of biotic and abiotic processes on comparatively short timescales. Here we synthesize current knowledge of the production, stocks, and fluxes of PyC as well as the physical and chemical processes through which it interacts as a dynamic component of the global carbon cycle.
New conceptual models that highlight the importance of environmental, rather than molecular, controls on soil organic matter affect interpretations of organic matter (OM) persistence across terrestrial and aquatic boundaries. We propose that changing para- digms in our thinking about OM decomposition explain some of the uncertainties surrounding the fate of land-derived carbon (C) in marine environments. Terrestrial OM, which historically has been thought to be chemically recalcitrant to decay in soil and aquatic environments, dominates inputs to rivers yet is found in trace amounts in the ocean. We discuss three major transformations in our understanding of OM persis- tence that influence interpretations of the fate of aquatic OM: (1) a shift away from an emphasis on chemical recalcitrance as a primary predictor of turnover; (2) new interpretations of radiocarbon ages, which affect predictions of reactivity; and (3) the recognition that most OM leaving soils in dissolved form has been microbially processed. The first two explain rapid turnover for terrigenous OM in aquatic ecosystems once it leaves the soil matrix. The third suggests that the presence of terrestrial OM in aquatic ecosystems may be underestimated by the use of plant biomarkers. Whether these mechanisms occur in isolation of each other or in combination, they provide insight into the missing terrestrial C signature in the ocean. Spatially and temporally varying transforma- tions of OM along land–water networks require that common terrestrial source indicators be interpreted within specific environmental contexts. We identify areas of research where collaborations between aquatic and terrestrial scientists will enhance quanti- fication of C transfer from soils to inland water bodies, the ocean, and the atmosphere. Accurate estimates of OM processing are essential for improving predictions of the response of vulnerable C pools at the interface of soil and water to changes in climate and land use.
Lignocellulose biodegradation, an essential step in terrestrial carbon cycling, generally involves removal of the recalcitrant lignin barrier that otherwise prevents infiltration by microbial polysaccharide hydrolases. However, fungi that cause brown rot of wood, a major route for biomass recycling in coniferous forests, utilize wood polysaccharides efficiently while removing little of the lignin. The mechanism by which these basidiomycetes breach the lignin remains unclear. We used recently developed methods for solubilization and multidimensional (1) H-(13) C solution-state NMR spectroscopy of ball-milled lignocellulose to analyse aspen wood degraded by Postia placenta. The results showed that decay decreased the content of the principal arylglycerol-β-aryl ether interunit linkage in the lignin by more than half, while increasing the frequency of several truncated lignin structures roughly fourfold over the level found in sound aspen. These new end-groups, consisting of benzaldehydes, benzoic acids and phenylglycerols, accounted for 6-7% of all original lignin subunits. Our results provide evidence that brown rot by P. placenta results in significant ligninolysis, which might enable infiltration of the wood by polysaccharide hydrolases even though the partially degraded lignin remains in situ. Recent work has revealed that the P. placenta genome encodes no ligninolytic peroxidases, but has also shown that this fungus produces an extracellular Fenton system. It is accordingly likely that P. placenta employs electrophilic reactive oxygen species such as hydroxyl radicals to disrupt lignin in wood.
Black Carbon (BC) quantification methods are reviewed, including new Rock-Eval 6 data on BC reference materials. BC has been reported to have major impacts on climate, human health and environmental quality. Especially for risk assessment of persistent organic pollutants (POPs) it is important to account for risk reduction caused by BC, as suggested for POP safety assessment in the framework of the new European Community Regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). Four major classes of BC quantification methods are reviewed including application to BC reference materials. Methods include chemical oxidation, thermal oxidation, molecular marker, optical methods and Rock-Eval analyses. Residual carbon from Rock-Eval 6 analysis correlated well with BC data from 'gentle' methods like optical and molecular marker methods, which capture a major part of the BC continuum including labile fractions (e.g. char). In contrast, the temperature at which 50% of the organic matter was oxidized (T(50%)) in an oxidation-only Rock-Eval analysis, correlated well with data from chemothermal oxidation (CTO), which captures only refractory BC fractions (e.g. soot). Rock-Eval analysis can further be used for BC characterization through deconvolution of the dominant peaks of the thermogram and appears to be a powerful tool in BC analysis.
Black carbon (BC) enters the ocean through aerosol and river deposition. BC makes up 12 to 31 percent of the sedimentary organic
carbon (SOC) at two deep ocean sites, and it is 2400 to 13,900 carbon-14 years older than non-BC SOC deposited concurrently.
BC is likely older because it is stored in an intermediate reservoir before sedimentary deposition. Possible intermediate
pools are oceanic dissolved organic carbon (DOC) and terrestrial soils. If DOC is the intermediate reservoir, then BC is 4
to 22 percent of the DOC pool. If soils are the intermediate reservoir, then the importance of riverine carbon in the ocean
carbon cycle has been underestimated.
Biochar function in soil is based on properties such as sorption characteristics, and these are expected to change throughout the life cycle of the biochar. Because biochar particles cannot easily be separated from soil particles, this change is seldom investigated. Biochar-related molecular markers, such as benzene polycarboxylic acids (BPCAs) are promising tools for studying the properties of biochars in complex environmental matrices. In this study, biochars were derived from corn straw and pine wood sawdust at 200-500 °C, and their aging was simulated with NaClO. Biochar properties were characterized by elemental analysis, BET surface characterization and BPCA molecular marker analysis. Chemical oxidation decreased the surface area (SA) but increased the O content of biochars. The oxidation decreased the amount of biochars, with a mass loss in the range of 10-55%. A similar mass loss was also observed for BPCAs and was negatively related to both the pyrolysis temperature and the extent of the condensed structure (higher aromaticity). The biochar amounts were calculated quantitatively using the sum of BPCA contents, with a conversion factor (the ratio of biochar amount to BPCA content) in the range of 3.3-5.5, and were negatively related to the B5CA content. Three model pollutants, namely, bisphenol A (BPA), sulfamethoxazole (SMX), and phenanthrene (PHE), were chosen to study the sorption characteristics of biochar before and after oxidation. Chemical oxidation generally increased SMX sorption but decreased PHE sorption. The nonlinear factor n, based on Freundlich equation modeling, was negatively related to B6CA for all three chemicals. The BPCA molecular markers, especially B5CA and B6CA, were correlated to the biochar properties before and after oxidation and are thus a potentially useful technique for describing the characteristics of biochar in the environment.
The lipids extractable by organic solvents are important components of soil organic matter (SOM) and were used to trace the sources and degradation of SOM. Previous studies have suggested soil mineral protection of lipids, which might decrease the efficiency of extraction of some lipid compounds by organic solvents. Therefore, in this study we applied a mild acid treatment to remove most of the reactive mineral particles without altering SOM chemical structures. The SOM information provided by lipid biomarkers was different before and after acid treatment in this research. Because of the various chemical contents of lipids, the presence of reactive minerals might alter lipid biomarker signals considerably and could lead to the wrong conclusions. Based on lipid biomarker information obtained through acid‐aided extraction, we identified that the source for alkanoic acids was different from that for alkanols and alkanes. Alkanoic acids were derived from both original vegetation (bamboos) and new inputs from fresh rubber tree tissues, whereas alkanols and alkanes were mainly from the original vegetation. Various biomarker indices indicated an extended degradation of lipids after intensive rubber tree cultivation. The leaching of short‐chain aliphatic lipids and the new input of alkanoic acids in the surface soil were also considered to avoid the incorrect orientation of biomarker information. This study suggested that although the new input from rubber trees was evident in cultivated soil, cultivation activities might enhance the degradation of SOM and accelerate its turnover. The cycling of SOM thus needs to be investigated carefully to protect the ecosystems affected by intense human activity.
Highlights
• The extractability of lipids is enhanced after removal of reactive minerals.
• Various biomarkers indicate enhanced lipid degradation after cultivation.
• New input of SOM from rubber trees is suggested by alkanoic acid biomarkers.
• Cultivation activities accelerate SOM degradation and turnover.
Contamination of soils with persistent organic pollutants (POPs), such as organochlorine pesticide, polybrominated diphenyl ethers, halohydrocarbon, polycyclic aromatic hydrocarbons (PAHs) is of increasing concern. Microbial degradation is potential mechanism for the removal of POPs, but it is often restricted by low bioavailability of POPs. Thus, it is important to enhance bioavailability of POPs in soil bioremediation. A series of reviews on bioavailability of POPs has been published in the past few years. However, bioavailability of POPs in relation to soil organic matter, minerals and soil microbes has been little studied. To fully understand POPs bioavailability in soil, research on interactions of POPs with soil components and microbial responses in bioavailability limitation conditions are needed. This review focuses on bioavailability mechanisms of POPs in terms of sorption, transport and microbial adaptation, which is particularly novel. In consideration of the significance of bioavailability, further studies should investigate the influence of various bioremediation strategies on POPs bioavailability.
The behavior of iron (Fe)-bound organic carbon (OC) under anoxic conditions in natural soils and sediments represents a critical knowledge gap for understanding the biogeochemical cycles of OC and Fe. In this study, we investigated the dynamics of Fe and OC in four forest soils in the presence of the dissimilatory Fe-reducing
bacterium, Shewanella oneidensis MR-1. Over an 8-day reduction period, 3.8–9.9% of total OC was released to solution in conjunction with the reduction of 12.5–37.7% of reactive Fe. The fraction of OC released was correlated with the fraction of Fe reduced, indicating that the reductive release was the controlling factor for the mobilization of OC upon the anaerobic microbial reaction. During the reduction, the fractions of poorly crystalline Fe oxides decreased, coupling with an increase in the relative abundance of crystalline Fe oxides. Lability of OC (as reflected bywater-extractable OC content) increased after microbial reduction, indicating the decreased stability of OC because of changes in mineral-OC interactions and the conformation of mineral-OC complexes. The reduction of Fe was closely related to bulk soil electron accepting capacity (0.15–0.34 mmol e−/mol C). Our findings demonstrate that the redox reactions of Fe,modified by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.
The stability and size distribution of soil aggregates is profoundly affected by land use, but the influencing mechanisms of land use are not clear. A study was carried out to investigate and attempted to interpret the effects of land use on soil aggregates from types of land use and soil properties in soil samples and size fractions of soil aggregates. Soil samples were taken from 9 sites under paddy, forest, and upland in southern China. The wet-sieving method was used to obtain 6 size fractions of soil aggregates: >5, 5–2, 2–1, 1–0.5, 0.5–0.25, and <0.25 mm. The stability and size distribution of soil aggregates was measured as mean weight diameter (MWD), the percentage of water-stable aggregate (WSA) and the percentage of each size fraction (PSA). The quantities of soil organic carbon (SOC), humic substances, dithionite-citrate-bicarbonate (DCB) and oxalate extractable iron (Fe) and aluminum (Al) oxides were also measured. The results showed that types of land use solely explained 66.6% variation of soil aggregates; SOC, DCB-extractable Fe and Al oxides, and oxalate-extractable Al oxide caused 84.3% variation, in which SOC contributed 29.0%, Fe and Al oxides contributed 33.8%, and their interactions contributed 21.4%. The multiple linear regression and partial correlation analysis showed that soil organic matter and Fe and Al oxides had significant effects but played different roles on the stability and size distribution of soil aggregates. The study suggests that land use affects the stability and size distribution of soil aggregates through the integration of soil organic matter and Fe and Al oxides.
For the assessment of black carbon (BC), its oxidation to benzene polycarboxylic acids (BPCAs) is an established method. However, doubts about biological precursors remain and not all published data were obtained at low carbon concentration. We hypothesised that a considerable proportion of BC may be produced during sample treatment in the presence of a high amount of organic carbon (OC). We thereforere tested whether and to which degree (i) BC-free material from stems of Zea mays L. (maize straw) and leaves of Capsicum annuum L. (bell pepper), and specificically that (ii) cyclic and non-cyclic carbon forms (chlorophyllin, ellagic acid and β-carotene) afford BPCAs when method protocols are overloaded with a sample above the recommended amount of 5 mg OC. The results showed that an amount (< 2 g/kg OC) of three to four times of carboxylated BPCAs may be formed even at low sample weight (< 5 mg OC), thereby falsely representing biological BC production. When this threshold was exceeded, all BPCA forms were detected. The artificial BPCA production yield in g OC increased with increasing amount of OC (R2 ⩾ 0.81), adding up to 8.7 g/kg OC (19.7 g BC/kg OC) artificial production. We therefore strongly recommend that a threshold of 5 mg OC sample concentration be maintained in future studies and that future BC assessments be restricted to BPCAs with five and six carboxyl groups. This constrains the application of the BPCA method for organic rich samples and for samples expected to containa a relatively low amount of BC.
Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3-5 are examined here. Our results show that 1,4-hydroquinone reduces Fe(III) in acidic conditions, generating semiquinone radicals ( ) that can oxidize Fe(II) back to Fe(III). The oxidation rate of Fe(II) by increases with increase in pH due to the speciation change of with its deprotonated form ( ) oxidizing Fe(II) more rapidly than the protonated form ( ). Although oxygenation of Fe(II) is negligible at pH < 5, O2 still plays an important role in iron redox transformation by rapidly oxidizing to form benzoquinone ( ). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron- Suwannee River Fulvic Acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution rapidly oxidize in the presence of dioxygen while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen with the result that steady state semiquinone concentrations in SRFA solutions are 2-3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in steady state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states with iron turnover frequencies in solutions containing SRFA 10-20 times higher than observed in solutions of 1,4-hydroquinone.
Mineral topsoils possess large organic carbon (OC) contents but there is only limited knowledge on the mechanisms controlling the preservation of organic matter (OM) against microbial decay. Samples were taken from the uppermost mineral topsoil horizon (0 to 5cm) of seven sites under mature deciduous forest showing OC contents between 69 and 164gkg-1 and a wide range in mineral characteristics. At first, organic particles and the water-extractable OM were removed from the soil samples. Thereafter, Na-pyrophosphate extractable organic matter (OM(PY)), assumed to be indicative for OM bound via cation mediated interactions, and the OM remaining in the extraction residue (OM(ER)), supposed to be indicative for OM occluded in mechanically highly stable micro-aggregates, were sequentially separated and quantified. The composition of OM(PY) and OM(ER) was analyzed by FTIR and their stability by 14C measurements. The OC remaining in the extraction residues accounted for 38 to 59% of the bulk soil OC (SOC) suggesting a much larger relevance of OM(ER) for the OM dynamic in the analyzed soils as compared with OM(PY) that accounted for 1.6 to 7.5% of the SOC. The FTIR analyses revealed a lower relative proportion of CO groups in OM(ER) compared to OM(PY) indicating differences in the degree of microbial processing between these fractions. Correlation analyses suggest an increase in the stability of OM(PY) with the soil pH and contents of Na-pyrophosphate soluble Fe, Al, and Mg and an increase in the stability of OM(ER) with the soil pH and the contents of clay and oxalate-soluble Fe and Al. Despite the detected influence of soil mineral characteristics on the turnover of OM(PY) and OM(ER), the δ14C signatures indicated mean residence times less than 100years. The presence of less stabilized OM in these fractions can be derived from methodological uncertainties and/or the fast cycling compartment of mineral-associated OM.
Lignin mineralization represents a critical flux in the terrestrial carbon (C) cycle, yet little is known about mechanisms and environmental factors controlling lignin breakdown in mineral soils. Hypoxia is thought to suppress lignin decomposition, yet potential effects of oxygen (O2 ) variability in surface soils have not been explored. Here, we tested the impact of redox fluctuations on lignin breakdown in humid tropical forest soils during ten-week laboratory incubations. We used synthetic lignins labeled with (13) C in either of two positions (aromatic methoxyl or propyl sidechain Cβ ) to provide highly sensitive and specific measures of lignin mineralization seldom employed in soils. Four-day redox fluctuations increased the percent contribution of methoxyl C to soil respiration relative to static aerobic conditions, and cumulative methoxyl C mineralization was statistically equivalent under static aerobic and fluctuating redox conditions despite lower soil respiration in the latter treatment. Contributions of the less labile lignin Cβ to soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O2 exposure, and tended to decline during periods of O2 limitation, resulting in lower cumulative Cβ mineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the Cβ - and methoxyl-labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O2 exposure. Oxygen fluctuations are thought to be sub-optimal for canonical lignin-degrading microorganisms. However, O2 fluctuations drove substantial Fe reduction and oxidation, and reactive oxygen species generated during abiotic Fe oxidation might explain the elevated contribution of lignin to C mineralization. Iron redox cycling provides a potential mechanism for lignin depletion in soil organic matter. Couplings between soil moisture, redox fluctuations, and lignin breakdown provide a potential link between climate variability and the biochemical composition of soil organic matter. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
In order to be able to avoid artefacts during fractionation of soil into primary particles, knowledge is required about the soil organic matter (SOM) behaviour when dispersing soil aggregates from different environments. This study was designed to investigate dispersion behaviour of macro- and microaggregates of native grassland epipedons from different climatic regions. Samples were collected from eight grassland epipedons to a depth of 10 cm along a climosequence in the prairie from Central Saskatchewan, Canada, to Southern Texas, USA. The samples were sieved to 2 mm and ultrasonically dispersed at 0, 1, 5, 15, 25, and 75 kJ, with partial dispersion also at 2 and 3 kJ. Carbon and nitrogen analysis was performed on the 5 kJ), however, disrupted particulate organic matter. Ultrasonic dispersion at 75 kJ was different for different ultrasonic instruments, suggesting that the calorimetric calibration was not very suitable for standardisation of ultrasonic energy input. For minimisation of artefacts, it is suggested that particulate SOM should be removed after weak dispersion (≥3 kJ), and, prior to subsequent aggregate disruption at 22 kJ, input energy should be checked at intervals by ultrasonic pressure measurements to determine whether the tip of the probe has deteriorated.
Summary The great stability of black carbon (BC) in soils may not be solely attributable to its refractory structure but also to poor accessibility when physically enveloped by soil particles. Our aim was to elucidate the intensity of physical entrapment of BC within soil aggregates. For this purpose, the A horizon of a forest, and of a grassland soil, and of three soils under tillage, were sampled at the experimental station Rotthalmünster, Germany. Black carbon was assessed in water-stable aggregates and aggregate-density fractions using benzene polycarboxylic acids as specific markers. The greatest BC concentrations made up 7.2% of organic carbon and were found in the 2 mm). This pattern has been sustained even after tillage. The C-normalized BC concentrations were significantly greater (P −3, but 22–24% in the OPOM fraction with a density of 1.6–2.0 g cm−3. This suggests that BC possibly acted as a binding agent or was selectively enriched during decomposition of protected SOM, or both. Physical inclusion, particularly within microaggregates, could therefore contribute to the long mean-residence times of soil-inherent BC.
Soil organic matter (SOM) processes in dynamic landscapes are strongly
influenced by soil erosion and sedimentation. We determined the
contribution of physical isolation of organic matter (OM) inside
aggregates, chemical interaction of OM with soil minerals, and molecular
structure of SOM in controlling storage and persistence of SOM in
different types of eroding and depositional landform positions. By
combining density fractionation with elemental and spectroscopic
analyses, we showed that SOM in depositional settings is less
transformed and better preserved than SOM in eroding landform positions.
However, which environmental factors exert primary control on storage
and persistence of SOM depended on the nature of the landform position
considered. In an annual grassland watershed, protection of SOM by
physical isolation inside aggregates and chemical association of organic
matter (complexation) with soil minerals, as assessed by correlation
with radiocarbon concentration, were more effective in the poorly
drained, lowest-lying depositional landform positions, compared to
well-drained landform positions in the upper parts of the watershed.
Results of this study demonstrated that processes of soil erosion and
deposition are important mechanisms of long-term OM stabilization.
DNA bound on clay minerals, sand, and humic acids has been shown, both in vitro and in situ, to be capable of transforming bacteria and to resist degradation by nucleases, which could result in the crypticity of genes in soil and other natural habitats. To determine where DNA is bound on clay minerals, which may help to explain how bound DNA becomes resistant to degradation by nucleases but retains the ability to transform competent cells, chromosomal DNA from Bacillus subtilis bound on montmorillonite (M) and kaolinite (K) was examined by X-ray diffractometry and transmission and scanning electron microscopy. X-ray diffraction analysis showed that the basal spacings of M and K were not altered, indicating that this DNA did not significantly intercalate the clays. Scanning and transmission electron microscopy showed that the binding of this DNA was primarily on the edges of M and K, although some binding was also apparent on the planar surfaces. Based on the results of these studies, it is postulated that:
1.extension from the edges of the clays enables the unbound end of DNA to interact with receptor sites on competent cells and result in their transformation; and2.binding on clays alters the electron distribution and/or conformation of DNA, which reduces its hydrolysis by nucleases.
There is evidence of black carbon (BC) contributing to stable humus in the soil environment but its quantity and fate are poorly examined. We used benzene polycarboxylic acids (BPCAs) as markers to assess the contents and distribution of BC in soils and soil fractions of the long-term field experiments in Halle (Haplic Phaeozem), Bad Lauchstädt (Haplic Chernozem), Rotthalmünster (Haplic Luvisol) and an additional site near Bad Lauchstädt (Haplic Phaeozem), Germany. Black carbon comprised 11.9–13.2% of organic C in the top soils (0–10 cm) of the black soils located around Halle and Bad Lauchstädt while it explained only 2.7% of organic carbon in the Haplic Luvisol of Rotthalmünster. The BPCA pattern at Bad Lauchstädt and Halle suggested that two thirds of BC were of fossil origin. In general, BC contents (in g kg− 1 soil) decreased with increasing soil depth. The C-normalized BC concentrations, however, increased and reached 35 ± 7% of organic carbon at a depth of 87–114 cm. With increasing depth BC was increasingly localized in the coarse silt and sand fractions and the heavy mineral fraction. This indicated that BC was connected to the minerals of this size and preserved there. Inorganic fertilization for > 30 years did not affect BC contents.
Black carbon (BC), the ubiquitous stable product of incomplete combustion, is believed to be a potential sink for atmospheric CO2 and therefore a contributor to the Earth’s radiative heat balance. Nevertheless, analytical procedures to measure BC are inconsistent, giving a non-systematic variation by factors of 14–571 for estimates of its content in soil. We hypothesized that the HCl used to isolate benzene polycarboxylic acids (BPCAs) as markers for BC helps form these compounds, which could then cause an overestimation of the BC content of the soil. We found that indeed up to 90% of BPCA yields may be attributed to this HCl pre-treatment. To correct this error we developed a revised method that uses BPCAs as BC markers but which allows us to eliminate any confusion in the results. This aim was achieved by digestion with 4 M trifluoroacetic acid (TFA). After oxidation with HNO3, the BPCAs were purified using a cation exchange resin and derivatized to form the trimethylsilyl derivatives. Analyses were performed using a gas chromatograph (GC) equipped with a flame ionization detector (FID); constant linearity was obtained at ⩾7 ng BPCA injection amount and peak purity was determined using mass spectrometry (MS). The recovery of the BPCAs averaged 93.5 ± 5.1% for pure standards and 95.0 ± 3.6% for spiked charred plant material. The contribution of BPCAs from aspergillin to soil organic carbon was estimated to be negligible. No close correlation between the results obtained with the original method and our revised procedure was observed.
The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO2 measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (13C, 14C) measurements after hot-water extraction of OM.
Soils contain significant amounts of black carbon (BC) from biomass and fossil fuel combustion. However, its origin, morphology, and chemistry have remained obscure. Here, we examined BC in particle-size and density fractions of the surface soil of a Haplic Chernozem using a scanning electron microscope (SEM) coupled to an energy-dispersive X-ray spectrometer (EDX) in order to investigate the morphological and chemical properties of BC as a function of its origin and fate in soils. The results showed that BC did not only occur as well-defined but also as SEM-amorphous particles. The BC particles exhibited different morphologies ranging from spherical to irregular shapes and from smooth to rough surfaces. Particles with similar morphologies were found in different soil fractions, indicating that BC from different sources is present in the soil, dominated by soot-BC from coal (and oil) combustion and char-BC from coal combustion and biomass burning. The identity of BC was ascertained by atomic O/C ratios ≤ 0.33. Within a BC particle, the O/C ratio increased from interior to exterior surfaces. The mean degree of oxidation increased significantly with an increase in the size of the particle fraction and an increase in the density of the fraction. The presence of inherently light BC in heavy mineral fractions as well as SEM-visible mineral associations with BC particles provided evidence that the partially oxidized BC chemically interacted with the mineral phase, presumably resulting in a protection of the enclosed BC against further decomposition in soil.
Recent findings have confirmed the importance of black carbon (BC) in the global biogeochemical cycles of carbon and oxygen through its important contribution to the slowly cycling organic carbon (OC) pool. Yet, most BC determination methods published to date measure operationally defined BC fractions, oftentimes with a high potential for artifacts and a lack of specificity for one of the two major forms of the BC continuum, soot/graphitic BC (GBC) and char/charcoal BC (CBC). This paper describes a method that reduces the potential for artifacts to accurately and selectively measure the concentration of GBC in complex mineral and organic matrixes. Marine and lacustrine sediments, river sediments, suspended particles, and a marine plankton sample were first demineralized with a mixture of hydrochloric (HCl) and hydrofluoric (HF) acids to expose any biochemical entrapped in a mineral matrix. The hydrolyzable organic matter fraction (mostly proteins and carbohydrates) was then removed with 02-free trifluoroacetic acid and HCl, after which the non-GBC, non-hydrolyzable OC fraction was finally removed by thermal oxidation at 375 degrees C for 24 h. The specificity of the method for GBC was assessed with pure CBC and GBC samples. Detection limit and GBC recovery in spiked samples were 10 mg kg(-1) and approximately 85%, respectively. Typical GBC concentrations measured in a series of natural samples ranged from <10 mg kg(-1) in marine plankton to 0.19% in a riverine sample. These concentrations were lower by as much as 3 orders of magnitude than those obtained by thermal oxidation without demineralization and removal of hydrolyzable organic matter. The improvements presented in this work allow for the accurate and precise measurement of GBC in complex organic and mineral matrixes by eliminating the interference caused by the presence of CBC, residual non-BC OC and minerals, or by the formation of condensation products that could account for as much as 4-6% of total OC. Combined to stable and radioisotope analysis, this improved method should permit quantitative assessments of the role and dynamics of GBC in the global geochemical cycles of carbon and oxygen.
Black carbon (BC) is a complex continuum of partly charred organic matter predominantly consisting of condensed aromatic and graphitic moieties and it has high potential for long‐term carbon sequestration in soils and sediments. There has been common agreement that BC is exclusively formed by incomplete combustion of organic matter, while non‐pyrogenic sources are negligible. In this study, we investigated the stable carbon isotope signature of benzenepolycarboxylic acids (BPCAs) as molecular markers for BC to test if there is also a significant contribution of non‐pyrogenic carbon to this fraction in soils. BPCAs were formed by hot nitric acid oxidation of different soils and analyzed by three different procedures: (i) elemental analysis – isotope ratio mass spectrometry (EA‐IRMS) of bulk BPCAs and gas chromatography – combustion – isotope ratio mass spectrometry (GC‐C‐IRMS) of (ii) BPCA trimethylsilyl (TMS) derivatives, and (iii) BPCA methyl derivatives. Best accuracy and precision of isotope measurements were obtained by EA‐IRMS of bulk BPCAs although this method has a risk of contamination by non‐BC‐derived compounds. The accuracy and precision of GC‐C‐IRMS measurements were superior for methyl derivatives (±0.1‰ and 0.5‰, respectively) to those for TMS derivatives (+3.5‰ and 2.2‰, respectively).
Comparison of BPCA δ ¹³ C values of soil samples prior to and after laboratory and field incubations with both positive and negative ¹³ C labels at natural and artificial abundances revealed that up to 25% of the isolated BC fraction in soils had been produced in situ , without fire or charring. Commonly applied methods to quantify BC exclusively formed by pyrogenic processes may thus be biased by a significant non‐pyrogenic fraction. Further research is encouraged to better define isolated BC fractions and/or understand mechanisms for non‐pyrogenic BC production in soils. Copyright © 2008 John Wiley & Sons, Ltd.
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