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Litter decomposition is the main source of soil organic carbon (SOC) pool, regarding as an important part of terrestrial ecosystem C dynamics. The turnover of SOC is mainly regulated by extracellular enzymes secreted by microorganisms. However, the response mechanism of soil C-degrading enzymes and SOC in litter decomposition remains unclear. To clarify how SOC fraction dynamics respond to C-degrading enzymes in litter decomposition, we used field experiments to collect leaf litter and SOC fractions from the underlying layer in Robinia pseudoacacia plantations on the Loess Plateau. Our results showed that SOC, easily oxidizable organic C, dissolved organic C, and microbial biomass C increased significantly during the decomposition process. Litter decomposition significantly decreased soil hydrolase activity, but slightly increased oxidase activity. Correlation analysis results showed that SOC fractions were significantly positively correlated with the litter mass, lignin, soil moisture, and oxidase activity, but significantly negatively correlated with cellulose content and soil pH. Partial least squares path models revealed that soil C-degrading enzymes can directly or indirectly affect the changes of soil C fractions. The most direct factors affecting the SOC fractions of topsoil during litter decomposition were litter lignin and cellulose degradation, soil pH, and C-degrading enzymes. Furthermore, regression analysis showed that the decrease of SOC stability in litter decomposition was closely related to the decrease of soil hydrolase to oxidase ratio. These results highlighted that litter degradation-induced changes in C-degrading enzyme activity significantly affected SOC fractions. Furthermore, the distribution of soil hydrolases and oxidases affected the stability of SOC during litter decomposition. These findings provided a theoretical framework for a more comprehensive understanding of C turnover and stabilization mechanisms between plant and soil. [ABSTRACT FROM AUTHOR]

Chemical structure of organic matter (OM) pools in the arable layers of agrochernozems (non-eroded, eroded, and depositional has been studied by solid-state 13C-NMR spectroscopy. As has been shown, two competing processes simultaneously take place in the erosional zone: decomposition of the OM of the underlying horizon exposed due to erosion, and the stabilization of fresh OM having entered with the crop residues (OM dynamic replacement). Analytical data suggest that the OM dynamic replacement in the erosional zone efficiently compensate for the OM decomposition, as is evidenced by the highest C/N ratio of all studied OM pools in the eroded agrochernozem along with the absence of statistically significant differences in the integral characteristics of their chemical structure. However, a continuous removal of the upper soil layer from the eroded agrochernozem with each erosion event does not fully compensate for the quantitative OM losses there. The most labile OM part can be mineralized during transportation of the eroded material to the depositional zone. Accordingly, the OM entering the depositional zone is to a greater degree transformed as compared with that of the eroded agrochernozem. Nevertheless, characteristic of the depositional agrochernozem is an increased accumulation of organic carbon in the bulk soil and all examined OM pools. Correspondingly, the continuous OM inputs from the slope position subject to erosion with its subsequent burial after each consecutive erosion event, as well as the repacking/aggregation of the newly deposited OM, efficiently contribute to the deposition of organic carbon in the depositional zone. [ABSTRACT FROM AUTHOR]

Natural 13C abundances in different organic matter (OM) pools in the water-stable macro- and free microaggregates of Haplic Chernozem under contrasting land uses (steppe and long-term bare fallow) are described. The 13C fractionation at individual stages in the formation of OM pools is relatively constant regardless of the level of soil structural organization. The proposed conceptual scheme, allowing for quantification of the carbon (C) fluxes in the system of aggregate/OM pool, illustrates this assertion. As is shown, the main C fluxes in the OM pools go from the free OM (LFfr) to the residue fraction (Res) through microaggregates within water-stable aggregates (mWSAs), the components of which are the occluded OM (LFocc) and clay fraction (Clay). With a high degree of probability, C migrates from macroaggregates (WSAma) to free microaggregates (WSAmi). However, a higher probability of the C flux from the mWSA into the Res as compared with the direct C fluxes from the LFocc and Clay supports the hypothesis that the Res is to a greater degree represented by pieces/fragments (50–1 μm in size) of disintegrated mWSAs. Regardless of the size, the WSAs contain labile OM (as the components of mWSAs) along with LFfr (only in macroaggregates) and stable OM (Res). The labile OM (LFocc and Clay) within WSAmi displays a lower degree of microbial transformation (a "lighter" isotopic signature) compared to that in WSAma, which is a result of its better "physical" protection against microbial attacks. However, the most stable OM pool, concentrated in the Res within WSAmi, is enriched in 13C as compared with that in WSAma. Considering that the Res determines the total isotopic composition of C in WSAs, the OM of free microaggregates is generally characterized by a higher degree of microbial processing compared to that of macroaggregates. Free microaggregates are pieces of disintegrated macroaggregates. [ABSTRACT FROM AUTHOR]

The impact of cropping systems on soil aggregation and associated carbon (C) and nitrogen (N) stabilization in relation to soil's antecedent C level has not been well addressed, which is essential for prioritization of agricultural soils for C and N sequestration. In this background, the present study investigated the influence of maize-wheat (MW) and soybean-wheat (SW) cropping systems and continuous fallow (CF) on soil aggregation and associated C and N storage in four soils of same type and texture (sandy loam textured Typic Ustorthents) but different antecedent C levels: a low-C soil (Soil 1, 5.6 g C kg–1), two medium-C soils (Soil 2, 9.0 g C kg–1 and Soil 3, 9.6 g C kg–1) and a high-C soil (Soil 4, 12.9 g C kg–1). While in low-C soil, MW outperformed the SW in increasing aggregate mean weight diameter (MWD) by 13% and C and N preservation capacities of macroaggregate fractions by 5–52%; in medium-C soils, the opposite occurred where SW showed 4–9% increased MWD and 8–76% increased C and N preservation capacity over MW. Contrarily in high-C soil, the two cropping systems behaved similarly; these decreased the aggregate MWD by 6–10% and the macroaggregate-preserved C and N by 8–39% compared to the CF. Changes in macroaggregate C and N storage were significantly related to bulk soil C and N levels (R2 = 0.65–0.85, p < 0.05). Conclusively, the selection of cropping systems to improve aggregate C and N storage must preconsider the antecedent soil C level; because the magnitude and direction of cropping impacts on C and N storage depend on soil's antecedent C level. For a greater physical stabilization of sequestered C and N, in the Entisols of northwest India, MW and SW may be promoted to low- and medium-C soils, respectively. [ABSTRACT FROM AUTHOR]

Aims: Mineral-associated organic matter, mainly derived from microbial by-products, persists longer in soil compared to particulate organic matter (POM). POM is highly recalcitrant and originates largely from decomposing root and shoot litter. Theory suggests that root traits and growth dynamics should affect carbon (C) accumulation into these different pools, but the specific traits driving this accumulation are not clearly identified. Methods: Twelve herbaceous species were grown for 37 weeks in monocultures. Root elongation rate (RER) was measured throughout the experiment. At the end of the experiment, we determined morphological and chemical root traits, as well as substrate induced respiration (SIR) as a proxy for microbial activity. Carbon was measured in four different soil fractions, following particle-size and density fractionation. Results: Root biomass, RER, root diameter, hemicellulose content and SIR (characteristic of N2-fixing Fabaceae species), were all positively correlated with increased C in the coarse silt fraction. Root diameter and hemicellulose content were negatively correlated with C in the POM fraction, that was greater under non N2-fixing Poaceae species, characterized by lignin-rich roots with a high carbon:nitrogen ratio that grew slowly. The accumulation of C in different soil pools was mediated by microbial activity. Conclusions: Our results show that root traits determine C input into different soil pools, mediated primarily by microbial activity, thus determining the fate of soil organic C. We also highlight that C in different soil pools, and not only total soil organic C, should be reported in future studies to better understand its origin, fate and dynamics. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics and remain underrepresented in more recent compartmental models that separate SOM into discrete pools with differing turnover times. Thus, a gap currently exists between the emergent understanding of SOM dynamics and our ability to improve terrestrial biogeochemical projections that rely on the existing models. In this opinion paper, we portray the SOM paradigm as a triangle composed of three nodes: conceptual theory, analytical measurement, and numerical models. In successful approaches, we contend that the nodes are connected—models capture the essential features of dominant theories while measurement tools generate data adequate to parameterize and evaluate the models—and balanced—models can inspire new theories via emergent behaviors, pushing empiricists to devise new measurements. Many exciting advances recently pushed the boundaries on one or more nodes. However, newly integrated triangles have yet to coalesce. We conclude that our ability to incorporate mechanisms of microbial decomposition and physicochemical protection into predictions of SOM change is limited by current disconnections and imbalances among theory, measurement, and modeling. Opportunities to reintegrate the three components of the SOM paradigm exist by carefully considering their linkages and feedbacks at specific scales of observation. [ABSTRACT FROM AUTHOR]

Background and aims: Legumes are used in forage agriculture to replace mineral N fertilizer by biological N fixation. We hypothesised that N addition through biological N fixation may have different effects on soil organic matter (SOM)112 quantity and composition compared to mineral N fertilization. In this study we introduced lucerne into grasslands and aimed to examine the resulting effects on C stocks and the molecular C signatures of roots and soil. Methods: Lucerne, cocksfoot and tall fescue were grown for five years as monocultures or mixtures. Grass monocultures were N-fertilised (SNF), while lucerne monoculture and mixtures received N through biological N fixation (BNF). For root and soil samples from 30 cm depth, we analysed quantity and composition of (1) non-cellulosic neutral carbohydrates after acid hydrolysis and (2) lignin after CuO oxidation. Results: Our data showed species-specific chemical signatures for roots extracted from soil. Lucerne presence increased the N content of roots and lowered their lignin/N ratio. Contrasting root input over four growing seasons did not impact SOM stocks, as they were similar in all treatments. We observed different chemical signatures for soil under mixtures as compared to lucerne monoculture. In particular, the state of SOM degradation was enhanced under mixtures. Conclusion: A greater input of higher quality litter to soils receiving BNF did not increase SOM storage. When lucerne was grown in mixture with grass, we observed lignin and carbohydrate signatures that indicated a more advanced state of degradation as compared to monocultures. Therefore, we suggest that the introduction of lucerne into grassland influences OM degradation more than its stabilization. After four growing seasons, SOM molecular signatures were not significantly influenced by type of N fertilization and showed little difference among the treatments despite species specific root composition. [ABSTRACT FROM AUTHOR]

Co-composting biowastes such as manures and biosolids can be used to stabilize carbon (C) without impacting the quality of these biowastes. This study investigated the effect of co-composting biowastes with alkaline materials on C stabilization and monitored the fertilization and revegetation values of these co-composts. The stabilization of C in biowastes (poultry manure and biosolids) was examined by their composting in the presence of various alkaline amendments (lime, fluidized bed boiler ash, flue gas desulphurization gypsum, and red mud) for 6 months in a controlled environment. The effects of co-composting on the biowastes' properties were assessed for different physical C fractions, microbial biomass C, priming effect, potentially mineralizable nitrogen, bioavailable phosphorus, and revegetation of an urban landfill soil. Co-composting biowastes with alkaline materials increased C stabilization, attributed to interaction with alkaline materials, thereby protecting it from microbial decomposition. The co-composted biowastes also increased the fertility of the landfill soil, thereby enhancing its revegetation potential. Stabilization of biowastes using alkaline materials through co-composting maintains their fertilization value in terms of improving plant growth. The co-composted biowastes also contribute to long-term soil C sequestration and reduction of bioavailability of heavy metals. [ABSTRACT FROM AUTHOR]

Alum water treatment sludge is composed of amorphous hydroxyl-Al, which has variable charge surfaces with a large Brunauer-Emmett-Teller (BET) surface area (103 m g) capable of specific adsorption of organic matter molecules, phosphate, and heavy metals. The effects of adding dried, ground, alum water treatment sludge (10 % w/ w) to the feedstock for composting municipal green waste alone, green waste plus poultry manure, or green waste plus biosolids were determined. Addition of water treatment sludge reduced water soluble C, microbial biomass C, CO evolution, extractable P, and extractable heavy metals during composting. The decrease in CO evolution (i.e., C sequestration) was greatest for poultry manure and least for biosolid composts. The effects of addition of water treatment sludge to mature green waste-based poultry manure and biosolid composts were also determined in a 24-week incubation experiment. The composts were either incubated alone or after addition to a soil. Extractable P and heavy metal concentrations were decreased by additions of water treatment sludge in all treatments, and CO evolution was also reduced from the poultry manure compost over the first 16-18 weeks. However, for biosolid compost, addition of water treatment sludge increased microbial biomass C and CO evolution rate over the entire 24-week incubation period. This was attributed to the greatly reduced extractable heavy metal concentrations (As, Cr, Cu, Pb, and Zn) present following addition of water treatment sludge, and thus increased microbial activity. It was concluded that addition of water treatment sludge reduces concentrations of extractable P and heavy metals in composts and that its effect on organic matter stabilization is much greater during the composting process than for mature compost because levels of easily decomposable organic matter are initially much higher in the feedstock than those in matured composts. [ABSTRACT FROM AUTHOR]

Models of ecosystem carbon (C) balance generally assume a strong relationship between NPP, litter inputs, and soil C accumulation, but there is little direct evidence for such a coupled relationship. Using a unique 50-year detrital manipulation experiment in a mixed deciduous forest and in restored prairie grasslands in Wisconsin, combined with sequential density fractionation, isotopic analysis, and short-term incubation, we examined the effects of detrital inputs and removals on soil C stabilization, destabilization, and quality. Both forested sites showed greater decline in bulk soil C content in litter removal plots (55 and 66 %) compared to increases in litter addition plots (27 and 38 % increase in surface soils compared to controls). No accumulation in the mineral fraction C was observed after 50 years of litter addition of the two forested plots, thus increases in the light density fraction pool drove patterns in total C content. Litter removal across both ecosystem types resulted in a decline in both free light fraction and mineral C content, with an overall 51 % decline in mineral-associated carbon in the intermediate (1.85-2.4 g cm) density pool; isotopic data suggest that it was preferentially younger C that was lost. In contrast to results from other, but younger litter manipulation sites, there was with no evidence of priming even in soils collected after 28 years of treatment. In prairie soils, aboveground litter exclusion had an effect on C levels similar to that of root exclusion, thus we did not see evidence that root-derived C is more critical to soil C sequestration. There was no clear evidence that soil C quality changed in litter addition plots in the forested sites; δC and ΔC values, and incubation estimates of labile C were similar between control and litter addition soils. C quality appeared to change in litter removal plots; soils with litter excluded had ΔC values indicative of longer mean residence times, δC values indicative of loss of fresh plant-derived C, and decreases in all light fraction C pools, although incubation estimates of labile C did not change. In prairie soils, δC values suggest a loss of recent C4-derived soil C in litter removal plots along with significant increases in mean residence time, especially in plots with removal of roots. Our results suggest surface mineral soils may be vulnerable to significant C loss in association with disturbance, land use change, or perhaps even climate change over century-decadal timescales, and also highlight the need for longer-term experimental manipulations to study soil organic matter dynamics. [ABSTRACT FROM AUTHOR]

The number of studies focused on the transformation and sequestration of soil organic carbon (C) has dramatically increased in recent years due to growing interest in understanding the global C cycle. While it is readily accepted that terrestrial C dynamics are heavily influenced by the catabolic and anabolic activities of microorganisms, the incorporation of microbial biomass components into stable soil C pools (via microbial living cells and necromass) has received less attention. Nevertheless, microbial-derived C inputs to soils are now increasingly recognized as playing a far greater role in stabilization of soil organic matter than previously believed. Our understanding, however, is limited by the difficulties associated with studying microbial turnover in soils. Here, we describe the use of an Absorbing Markov Chain (AMC) to model the dynamics of soil C transformations among three microbial states: living microbial biomass, microbial necromass, and C removed from living and dead microbial sources. We find that AMC provides a powerful quantitative approach that allows prediction of how C will be distributed among these three states, and how long it will take for the entire amount of initial C to pass through the biomass and necromass pools and be moved into atmosphere. Further, assuming constant C inputs to the model, we can predict how C is eventually distributed, along with how much C sequestrated in soil is microbial-derived. Our work represents a first step in attempting to quantify the flow of C through microbial pathways, and has the potential to increase our understanding of the microbial role in soil C dynamics. [ABSTRACT FROM AUTHOR]

Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary. [ABSTRACT FROM AUTHOR]

The contribution of soil microbial residues to stable carbon pools may be of particular importance in the tropics where carbon residence times are short and any available carbon is rapidly utilized. In this study we investigated the vertical distribution of microbially-derived amino sugars in two tropical forests on contrasting meta-sedimentary and serpentinite parent materials in the lowlands of Mt. Kinabalu, Borneo. Despite their similar climate, vegetative cover, and general microbial community structure, the two soils were chemically and physically distinct. We found that both parent material and depth significantly influenced the pool sizes of microbial residues in the two soils. In particular, the soil derived from sedimentary parent material had greater amino sugar contents, glucosamine to galactosamine ratios, and percentage of total soil carbon that is amino sugar derived, than the soil derived from serpentinite substrate. We speculate that residue stabilization was linked to soil iron oxide content, with significant difference in amino sugars contribution to total soil carbon at depth in the serpentinite-derived soil versus that derived from sedimentary parent material. Based on observed patterns of amino sugar content and relative abundance we suggest that near the surface of both soils vegetation and litter input determines the composition and quantity of microbial residues. With increasing depth the influence of vegetation declines and production and stabilization of microbial amino sugars becomes driven by soil matrix characteristics. These differences in stabilization mechanism and carbon dynamics with depth may be particularly critical in deep weathered tropical soils. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) pools are important in maintaining soil productivity and influencing the CO2 loading into the atmosphere. An attempt is made here to investigate into the dynamics of pools of SOC viz., total organic carbon ( C tot), oxidisable organic carbon ( C oc) and its four different fractions such as very labile ( C frac 1), labile ( C frac 2), less labile ( C frac 3) and non-labile ( C frac 4), microbial biomass carbon ( C mic), mineralizable carbon ( C min), and particulate organic carbon ( C p) in relation to crop productivity using a 34 year old rice ( Oryza sativa L)–wheat ( Triticum aestivum L)–jute ( Corchorus olitorius L) cropping system with different management strategies (no fertilization, only N, NP, NPK and NPK + FYM) in the hot humid, subtropics of India. A fallow treatment was also included to compare the impact of cultivation vis-à-vis no cultivation. Cultivation over the years caused a net decrease, while balanced fertilization with NPK maintained the SOC pools at par with the fallow. Only 22% of the C applied as FYM was stabilized into SOC, while the rest got lost. Of the analysed pools, C frac 1, C mic, C p and C min were influenced most by the treatments imposed. Most of the labile pools were significantly correlated with each other and with the yield and sustainable yield index (SYI) of the studied system. Of them, C frac1, C min, C mic and C p explained higher per cent variability in the SYI and yield of the crops. Results suggest that because of low cost and ease of estimation and also for upkeeping environmental conditions, C frac1 may be used as a good indicator for assessment of soil as to its crop productivity. [ABSTRACT FROM AUTHOR]