Soil Organic Matter Formation Research Articles

Can switchgrass increase carbon accrual in marginal soils? The importance of site selection

AbstractMost soil carbon (C) is in the form of soil organic matter (SOM), the composition of which is controlled by the plant–microbe–soil continuum. The extent to which plant and microbial inputs contribute to persistent SOM has been linked to edaphic properties such as mineralogy and aggregation. However, it is unknown how variation in plant inputs, microbial community structure, and soil physical and chemical attributes interact to influence the chemical classes that comprise SOM pools. We used two long‐term biofuel feedstock field experiments to test the influence of cropping systems (corn and switchgrass) and soil characteristics (sandy and silty loams) on microbial selection and SOM chemistry. Cropping system had a strong influence on water‐extractable organic C chemistry with perennial switchgrass generally having a higher chemical richness than the annual corn cropping system. Nonetheless, cropping system was a less influential driver of soil microbial community structure and overall C chemistry than soil type. Soil type was especially influential on fungal community structure and the chemical composition of the chloroform‐extractable C. Although plant inputs strongly influence the substrates available for decomposition and SOM formation, total C and nitrogen (N) did not differ between cropping systems within either site. We conclude this is likely due to enhanced microbial activity under the perennial cropping system. Silty soils also had a higher activity of phosphate and C liberating enzymes. After 8 years, silty loams still contained twice the total C and N as sandy loams, with no significant response to biofuel cropping system inputs. Together, these results demonstrate that initial site selection is critical to plant–microbe interactions and substantially impacts the potential for long‐term C accrual in soils under biofuel feedstock production.

315 Formation, persistence, and function of soil organic matter: how recent scientific advances apply in agroecosystems

Abstract Soil organic matter is fundamental to healthy and productive soils and building it is an effective means by which to draw down atmospheric greenhouse gas concentrations with added co-benefits. Scientific understanding of soil organic matter dynamics is constantly evolving, and the past decade has seen major advances and paradigm shifts. Soil organic matter creation from decaying plant litter is now thought to occur under two separate pathways, yielding two functionally different types: predominantly plant-derived, unprotected particulate organic matter (POM) and predominantly microbially-derived, mineral-associated organic matter (MAOM). The idea of naturally-occurring humic substances in soils has been largely abandoned, and long-term soil organic matter persistence is now understood to be driven mainly by mineral association. We will present the research behind these paradigm shifts, and show how considering POM and MAOM separately is key to understanding the mechanisms driving carbon accrual and persistence in soil, and therefore to guiding policy and management for soil carbon sequestration. We will present drivers of POM and MAOM contents, from individual fields to continents, including their capacity for sequestration and saturation in agricultural soils of the USA, and their responses to common management practices in agroecosystems.

Impact of surface soil manuring on particulate carbon fractions in relevant to nutrient stoichiometry in a Mollisol profile

Organic matter application significantly influences soil carbon storage and sequestration in cropland. The investigation of C sequestration through soil organic carbon (SOC) fractions in the soil profile and its relation to nutrient stoichiometry are essential for evaluating the effect of farming practice on soil fertility. This study aimed to examine the effect of cattle manure application on nutrient stoichiometry, which would be associated with soil C stock in SOC fractions, such as microbial biomass C (MBC), dissolved organic C (DOC) and water-soluble organic C (WSOC), coarse particulate organic carbon (cPOC), fine particulate organic carbon (fPOC) and the mineral-associated organic carbon (MOC). A four-year manure application experiment was conducted in a Mollisol, in which fertilization treatments comprised (1) NoF: the non-fertilizer control, (2) F: synthetic fertilizer, i.e. 20.3, 21.2 and 12.2 kg ha−1 of nitrogen (N), phosphorus (P) and potassium (K) in soybean, and 69.0, 28.1 and 6.1 kg ha−1 of N, P and K for corn on an annual basis, respectively, (3) F + M: synthetic fertilizer plus 15 Mg ha-1 of cattle manure and (4) F + 2M: synthetic fertilizer plus 30 Mg ha−1 of cattle manure in a soybean-corn rotation. SOC concentration in top 10 cm of soil was 18 % and 19 % higher under F + M than that under NoF and F, respectively. Whereas, F+2 M significantly increased SOC concentration by 32 %, 32 %, 14 %, and 11 % in 0−10, 10−20, 20−30, and 30−40 cm of soil depth compared to that in respective soil depth under NoF. Compared to NoF and F, manure application (F + M and F+2 M) significantly increased MBC in the top 30 cm of soil depth and DOC concentration in the top 20 cm of soil depth, but significantly decreased WSOC concentration in the top 20 cm of soil depth. F+2 M significantly increased cPOC concentration in the top 30 cm of soil depth compared to NoF and F. F + M significantly increased fPOC concentration in the top 50 cm of soil depth, compared to NoF, and F. F+2 M further increased the fPOC concentration in the top 30 cm of soil depth, compared to F + M. There was no difference among fertilization treatments in MOC concentration in any soil depth. There were significant exponential correlations of C concentration in POC fractions with soil N, P and N/P ratio. These results indicate that the combination of manure and synthetic fertilizer increased C accumulation in the POC fraction especially in the top 30 cm of soil depth. The change of nutrient stoichiometry was responsible to the accumulation of soil organic matter, highlighting that N, rather than P mainly regulated C accumulation in POC. Increasing input of N fertilizer combined with manure application would benefit the formation of soil organic matter in agricultural Mollisols.

Soil organic matter in major pedogenic soil groups

Soil organic matter (SOM) accumulation is different in certain soil groups with differences in parent material, degree of weathering and mineral composition. These differences are modulated by climatic factors, but also by pedogenesis, in particular by the formation of reactive mineral surfaces, by soil aggregation, as well as by translocation processes such as eluviation and illuviation and different types of turbation. However, there is still a lack of conceptualization of how such processes and thus important Reference Soil Groups influence the composition and properties of OM. Here we summarize the basic processes of OM storage as they differ from soil group to soil group, in order to present a first overview of the processes of OM formation in the different terrestrial soils of the world. We distinguish between soils of different climatic zones, i.e. Cryosols in permafrost regions, soils of limited development (Cambisols), Podzols, Phaeozems, Chernozems, Kastanozems, and Luvisols in temperate climate zones, as well as Acrisols, Ferralsols, Plinthosols and Nitisols in the subtropics and tropics. We also include soils derived from a specific parent material (Andosols, Vertisols), as well as Anthrosols (paddy soils, Terra Preta, plaggen soils) as examples for human-made SOM accumulations. The compilation of the literature shows that research on OM is clearly focused on specific Reference Soil Groups in temperate climate zones and some man-made soils, while other soils such as Nitisols and Acrisols are clearly underrepresented. The contribution of the different soil groups to global organic carbon (OC) stocks varies, with large amounts of OC found for the first metre in Cryosols, Cambisols, and Podzols, due to the large land area they cover, followed by Acrisols and Ferralsols. In part, these differences can be attributed to differences in the formation of SOM, which we ascribe to three main mechanisms. We emphasize that in all major Reference Soil Groups, both the mechanism of sorptive conservation as well as the protection within the aggregates contribute to the storage of OM. However, the reactant partners and aggregate forming agents and therewith the intensity of these stabilisation processes vary among the Reference Soil Groups. As a result, there are differences in the SOM composition in the topsoil. Within the entire soil profile, however, pedogenic processes lead as third mechanism to soil-group-specific accrual of SOM in the subsoil, e.g. by means of illuviation, by cryo-/bio-, and peloturbation, as well as by management. We conclude that the specific pedogenic environment must be considered in the assessment of global SOM storage potentials and thus also in future global C models.

Turnover of gram-negative bacterial biomass-derived carbon through the microbial food web of an agricultural soil

Soil organic matter (SOM) represents an important terrestrial carbon reservoir in the biosphere, and microorganisms have been recognized as significant material contributors to the formation of SOM. However, the turnover of microbial biomass residues with respect to their detailed microbial food web remains elusive. To elucidate this turnover process, we traced the fate of Gram-negative (Gram−) microbial biomass carbon through the microbial food web over time, using the concentrations and isotopic compositions of biomarker phospholipid fatty acids (PLFA) in a soil incubation with isotopically (13C) labelled model bacterial cells (Escherichia coli). We found that after 120 days 46.5% of the recovered E. coli derived 13C remained in the soil, whereas 53.5% was emitted as 13CO2 (recovery 77.0% of the initially added 13C). The 13C in microbial biomass decreased to 5.3% of the initial value. This indicates that about 41% of the E. coli biomass in our experiment was transformed to SOM. The PLFA patterns over time demonstrate the pathway of 13C transformation associated with different groups of microorganisms along the incubation. The E. coli-derived 13C was utilized by fungi first, then the label was shifted from fungi to Gram-positive (Gram+) bacteria, arbuscular mycorrhizal fungi (AMF), actinomycetes, and to Gram− bacteria other than E. coli. Finally the carbon was transferred from all consumers to the next consumer level; this is reflected in ongoing loss of 13C-PLFA without a shift in the 13C-PLFA pattern. In summary, our study details the turnover process of microbial biomass residues via the microbial food web to necromass and finally to SOM. This supports the soil microbial carbon pump concept, i.e. carbon channelling and the assimilation of easily degradable carbon into microbial biomass and a significant contribution of these residues in SOM.

Management of Grazed Landscapes to Increase Soil Carbon Stocks in Temperate, Dryland Grasslands

Management of the temperate, grazed grasslands in New Zealand for more than a century has led to swards dominated by ryegrass/clover, which, with inputs of inorganic fertilisers, are highly productive for grazing animals. In the last 20 years the widespread introduction of irrigation to dryland areas on flat land has increased productivity further. However, these intensive practices decrease soil carbon stocks. In contrast, there is limited evidence that improved management of dryland, grazed, hill country grasslands can lead to increases in soil carbon stocks. To address global needs for food security and climate change mitigation, priority actions to increase soil carbon stocks need to focus on improved management practices to increase carbon inputs and retention in soils identified as having high potential for increasing carbon storage. While there are limited data from New Zealand studies, international observations suggest that soil carbon stocks can be increased by enhancing below-ground carbon inputs from plants with deep roots, using swards with diverse species, and moderate grazing rather than harvesting biomass. However, there is less certainty about the processes regulating the formation and decomposition of soil organic matter and their dependence on soil physical properties and microbial accessibility. Scaling findings from plot studies to forecast long-term changes in soil carbon stocks at the landscape scale can be done using models but new approaches are required to integrate the impacts of multiple concurrent practices associated with grazing management.

Gram-positive bacteria control the rapid anabolism of protein-sized soil organic nitrogen compounds questioning the present paradigm

The cycling of especially large size organic nitrogen (N) from plants into stable microbial derived soil organic carbon (C) and N pools is understudied, in spite of organic N composing 90% of soil N and the intimate link between organic N and soil C stabilization. We investigated the fate of peptide-size and protein-size organic N fractions in soils from two long-term field experiments markedly differing in conditions for microorganisms. We combined amino acid stable isotope probing (AA-SIP) fingerprinting with PLFA-SIP to trace organic N into the soil microbial biomass. Contrary to the present paradigm, we found for both soils that greater molecular size did not protect against decomposition of these compounds neither did protection via strong sorption to the soil mineral phase. Instead, we found strong evidence that gram-positive bacteria are the key actors in the decomposition of protein-sized nitrogen compounds and that amino acids bound in large organic nitrogen compounds directly contribute to the build-up of bacterial tissue. We conclude that when large organic nitrogen compounds are dissolved, turnover occurs rapidly, irrespective of molecular size, and the bacterial incorporation of these rapid cycling compounds makes an important contribution to soil organic matter formation.

Transformation of low molecular compounds and soil humic acid by two domain laccase of Streptomyces puniceus in the presence of ferulic and caffeic acids.

The two-domain bacterial laccases oxidize substrates at alkaline pH. The role of natural phenolic compounds in the oxidation of substrates by the enzyme is poorly understood. We have studied the role of ferulic and caffeic acids in the transformation of low molecular weight substrates and of soil humic acid (HA) by two-domain laccase of Streptomyces puniceus (SpSL, previously undescribed). A gene encoding a two-domain laccase was cloned from S. puniceus and over-expressed in Escherichia coli. The recombinant protein was purified by affinity chromatography to an electrophoretically homogeneous state. The enzyme showed high thermal stability, alkaline pH optimum for the oxidation of phenolic substrates and an acidic pH optimum for the oxidation of K4[Fe(CN)6] (potassium ferrocyanide) and ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt). Phenolic compounds were oxidized with lower efficiency than K4[Fe(CN)6] and ABTS. The SpSL did not oxidize 3.4-dimethoxybenzoic alcohol and p-hydroxybenzoic acid neither in the absence of phenolic acids nor in their presence. The enzyme polymerized HA—the amount of its high molecular weight fraction (>80 kDa) increased at the expense of low MW fraction (10 kDa). The addition of phenolic acids as potential mediators did not cause the destruction of HA by SpSL. In the absence of the HA, the enzyme polymerized caffeic and ferulic acids to macromolecular fractions (>80 kDa and 10–12 kDa). The interaction of SpSL with HA in the presence of phenolic acids caused an increase in the amount of HA high MW fraction and a two-fold increase in the molecular weight of its low MW fraction (from 10 to 20 kDa), suggesting a cross-coupling reaction. Infrared and solution-state 1H-NMR spectroscopy revealed an increase in the aromaticity of HA after its interaction with phenolic acids. The results of the study expand our knowledge on the transformation of natural substrates by two-domain bacterial laccases and indicate a potentially important role of the enzyme in the formation of soil organic matter (SOM) at alkaline pH values.

Root herbivory and soil carbon cycling: Shedding “green” light onto a “brown” world

Roots play an important role in the formation and turnover of soil organic matter (SOM). Biological processes in the rhizosphere have the capacity to regulate the input of root materials to soil and the fate of root-derived carbon (C) in soil. While some aspects of rhizosphere ecology have received considerable research attention, one potentially important biotic driver, root herbivory, has been largely overlooked. In this study, we first review the literature on root herbivory, revealing that belowground herbivores can have substantial effects on the quality, quantity, and timing of belowground plant inputs to soil. Next, we introduce a conceptual framework in which root herbivory presents a distinct conduit that transfers C from living plants (the “green” world) to soil (the “brown” world). We also argue that direct and indirect interactions between root herbivores and the rhizosphere microbiome and other soil fauna can have significant impacts on soil microbial activities and soil C cycling. Finally, we outline key questions and topics for future research that incorporate belowground plant-herbivore interactions into soil biogeochemistry in an effort to improve our understanding of C flow in the rhizosphere and sharpen future predictions of soil C storage under changing environments.

Soil microbial carbon pump: Mechanism and appraisal

The soil microbial carbon pump (MCP) conceptualizes a sequestration mechanism based on the process of microbial production of a set of new organic compounds, which carry the carbon from plant, through microbial anabolism, and enter into soil where it can be stabilized by the entombing effect. Understanding soil MCP and its related entombing effect is essential to the stewardship of ecosystem services, provided by microbial necromass in the formation and stabilization of soil organic matter as well as its resilience and vulnerability to global change. The mechanism and appraisal of soil MCP, however, remain to be elucidated. This lack of knowledge hampers the improvement of climate models and the development of land use policies. Here, I overview available knowledge to provide insights on the nature of the soil MCP in the context of two main aspects, i.e., internal features and external constraints that mechanistically influence the soil MCP operation and ultimately influence microbial necromass dynamics. The approach of biomarker amino sugars for investigation of microbial necromass and the methodological limitations are discussed. Finally, I am eager to call new investigations to obtain empirical data in soil microbial necromass research area, which urgently awaits synthesized quantitative and modeling studies to relate to soil carbon cycling and climate change.

The formation of protected SOM facilitated by labile C input via artificial roots

Plant root-derived labile carbon (C) delivered to soils can regulate the dynamics and turnover of soil organic matter (SOM); however, it remains largely unclear how inputs of individual labile components (e.g., glucose) affect the formation of physicochemical-protected SOM. In a firmly controlled rhizosphere system, we added a glucose solution through artificial roots to soils collected from two subalpine coniferous forests (an approximately 200-year-old spruce-fir forest and an approximately 70-year-old spruce plantation) with the soils subsequently incubated over 25 days. The results showed that the addition of glucose significantly increased the concentrations of aluminum (Al) and iron (Fe) bound in both metal-organic complexes (MOCs) and short-range order phases (SROs) by 45%, 68%, 31% and 38%, respectively (soil-averaged), which indicated that glucose addition enhanced the formation of physicochemical-protected SOM. The induced protection of SOM mainly resulted from the stimulation of interactions of the microbial residues with soil minerals and/or metal cations, as indicated by the concurrently increased microbial communities and zeta potential after glucose addition. Moreover, the glucose-induced absolute changes in MOCs at the spruce-fir site were higher than those at the spruce plantation site, whereas the changes in SROs exhibited an opposite trend to that observed for MOCs. This discrepancy is presumably due to the higher organic matter content with more extractable metals at the spruce-fir site being more beneficial for supporting microbial stabilization of glucose and subsequent necromass sequestration as MOCs. In addition, more clay minerals at the spruce plantation site had larger surface areas and more binding sites to bind microbial-derived organic molecules as SROs. Collectively, our findings provide robust evidence that the input of labile C to soils could facilitate the formation of physicochemical-protected SOM, which is potentially involved in transformation into microbial residues and subsequent interaction with metal cations and/or minerals and may also be mediated by soil properties. The formation of protected SOM may potentially offset native SOM decomposition and thus has ecologically important implications for long-term terrestrial C storage.

Cellulose dominantly affects soil fauna in the decomposition of forest litter: A meta-analysis

Litter decomposition is widely acknowledged as a vital connection that links aboveground and belowground ecological processes, including the formation of soil organic matter and nutrient cycling in forest ecosystems. Soil fauna consumes significant quantities of litter and plays a critical role in the decomposition of soil organic matter. However, our understanding of the influences of soil fauna on the decomposition of forest litter remains elusive. Using a meta-analysis of 479 paired observations garnered from 49 studies, we found that, on average, soil fauna contributes 30.9 ± 6.8% (mean and standard error) to the forest litter decomposition rate. Moreover, the positive effects of soil fauna on litter decomposition increased with the cellulose concentration of the initial litter, and in warmer and moister climates. Our results suggest that the role of soil fauna in litter decomposition will even be more critical in view of ongoing global environmental change.

Coarse mineral-associated organic matter is a pivotal fraction for SOM formation and is sensitive to the quality of organic inputs

Recent models of soil organic matter (SOM) formation propose that the coarse mineral-associated fraction (coarse-MAOM) could be a pivotal fraction bridging the gap between fresh organic matter inputs and stabilized, mineral-associated SOM. However, little is known about the effect of organic input chemistry and soil texture on the carbon (C) and nitrogen (N) dynamics of this fraction. We studied SOM dynamics in a factorial field experiment comparing 10 combinations of crop residue management (retained or exported) with different fertilizer sources, including solid poultry, liquid dairy and liquid swine manure, a mineral fertilizer (NPK), and a control without N (PK) in two soils with contrasted texture. After 9 y of management, we compared the distribution of C and N within the light fraction (LF), the coarse-MAOM, and the fine mineral-associated organic matter fraction (fine-MAOM) of surface (0–5 cm) soil. We found that retention of mature crop residues increased soil C content through an increase in LF and coarse-MAOM but did not influence N content in any fractions. Conversely, the fertilizer source influenced C and N accumulation in the coarse- and fine-MAOM in the silty clay soil where greater values were generally found with poultry and liquid dairy manures. Similar trends were found in the sandy loam soil, but statistically significant differences were only found for C in the coarse-MAOM fraction. The coarse-MAOM fraction had a much lower C:N ratio (15–16) than LF (26–36) and appeared generally responsive to the different organic inputs. Despite its limited size (∼10% of total soil organic carbon), coarse-MAOM appeared to represent a central location for the formation of new SOM and for the accumulation of C and N following fresh organic inputs. We therefore propose that it is considered in models as a pivotal fraction of SOM formation in arable soils.

Nitrogen enrichment affects soil enzymatic stoichiometry via soil acidification in arid and hot land

Soil enzymatic stoichiometry reflects the patterns of soil microbial resource allocation for the acquisition of nutrients from different forms of soil organic matter. Nitrogen enrichment derived from human activities greatly affects soil biological properties and chemical properties. However, the mechanism for changing soil enzymatic stoichiometry due to nitrogen enrichment remains unclear. We established a factorial field experiment in an arid-hot valley to investigate the effects of nitrogen addition and plant species on the enzymatic stoichiometries for leucine aminopeptidase (LAP), acid phosphatase (AP), β-1,4-glucosidase (BG) and β-N-acetyl glucosaminidase (NAG). We found that nitrogen addition consistently reduced LAP activities but elevated AP activities beneath different grass species. Plant species had no effects on soil enzymatic stoichiometries although it significantly influenced the absolute values of soil enzyme activities. Nitrogen addition increased ln(BG):ln(LAP), but decreased ln(BG):ln(AP), ln(LAP):ln(NAG) and ln(LAP):ln(AP). Furthermore, by using structure equation modelling analyses, we found that the effects of N addition on enzymatic stoichiometries were mediated by soil acidification and microbial community structures. Our findings highlight that soil acidification alters microbial community structures and the patterns of microbial resource allocation for nutrient acquisition, both of which in turn play important roles in shaping soil enzymatic stoichiometry under the scenario of nitrogen enrichment.

Combination of energy limitation and sorption capacity explains 14C depth gradients

During the last decade, a paradigmatic shift regarding which processes determine the persistence of soil organic matter (SOM) took place. The interaction between microbial decomposition and association of organic matter with the soil mineral matrix has been identified as a focal point for understanding the formation of stable SOM. Using an improved version of the vertically resolved SOM model COMISSION (Ahrens et al., 2015), this paper investigates the effect of a maximum sorption capacity (Qmax) for mineral-associated organic matter (MAOM) formation and its interaction with microbial processes, such as microbial decomposition and microbial necromass production.We define and estimate the maximum sorption capacity Qmax with quantile regressions between mineral-associated organic carbon (MAOC) and the clay plus silt (<20 μm) content. In the COMISSION v2.0 model, plant- and microbial-derived dissolved organic matter (DOM) and dead microbial cell walls can sorb to mineral surfaces up to Qmax. MAOC can only be decomposed by microorganisms after desorption.We calibrated the COMISSION v2.0 model with data from ten different sites with widely varying textures and Qmax values. COMISSION v2.0 was able to fit the MAOC and SOC depth profiles, as well as the respective 14C gradients with soil depth across these sites. Using the generic set of parameters retrieved in the multi-site calibration, we conducted model experiments to isolate the effects of varying Qmax, point-of-entry of litter inputs, and soil temperature. Across the ten sites, the combination of depolymerization limitation of microorganisms due to substrate scarcity in the subsoil and the size of Qmax explain 14C depth gradients in OC.

Relevance of aboveground litter for soil organic matter formation – a soil profile perspective

Abstract. In contrast to mineral topsoils, in subsoils the origin and processes leading to the formation and stabilization of organic matter (OM) are still not well known. This study addresses the fate of litter-derived carbon (C) in whole soil profiles with regard to the conceptual cascade model, which proposes that OM formation in subsoils is linked to sorption–microbial processing–remobilization cycles during the downward migration of dissolved organic carbon (DOC). Our main objectives were to quantify the contribution of recent litter to subsoil C stocks via DOC translocation and to evaluate the stability of litter-derived OM in different functional OM fractions. A plot-scale stable isotope-labeling experiment was conducted in a temperate beech forest by replacing the natural litter layer with 13C enriched litter on an area of 20 m2 above a Dystric Cambisol. After 22 months of field exposure, the labeled litter was replaced again by natural litter and soil cores were drilled down to 180 cm soil depth. Water extraction and density fractionation were combined with stable isotope measurements in order to link the fluxes of recent litter-derived C to its allocation into different functional OM fractions. A second sampling was conducted 18 months later to further account for the stability of translocated young litter-derived C. Almost no litter-derived particulate OM (POM) entered the subsoil, suggesting root biomass as the major source of subsoil POM. The contribution of aboveground litter to the formation of mineral-associated OM (MAOM) in topsoils (0–10 cm) was 1.88±0.83 g C m−2 and decreased to 0.69±0.19 g C m−2 in the upper subsoil (10–50 cm) and 0.01±0.02 g C m−2 in the deep subsoil &gt;100 cm soil depth during the 22 months. This finding suggests a subordinate importance of recent litter layer inputs via DOC translocation to subsoil C stocks, and implies that most of the OM in the subsoil is of older age. Smaller losses of litter-derived C within MAOM of about 66 % compared to POM (77 %–89 %) over 18 months indicate that recent carbon can be stabilized by interaction with mineral surfaces; although the overall stabilization in the sandy study soils is limited. Our isotope-labeling approach supports the concept of OM undergoing a sequence of cycles of sorption, microbial processing, and desorption while migrating down a soil profile, which needs to be considered in models of soil OM formation and subsoil C cycling.

Increasing the organic carbon stocks in mineral soils sequesters large amounts of phosphorus.

Despite the fact that phosphorus (P) is critical for plant biomass production in many ecosystems, the implications of soil organic carbon (OC) sequestration for the P cycle have hardly been discussed yet. Thus, the aims of this study are, first, to synthesize results about the relationship between C and P in soil organic matter (SOM) and organic matter inputs to soils, second, to review processes that affect the C:P ratio of SOM, and third, to discuss implications of OC storage in terrestrial ecosystems for P sequestration. The study shows that the storage of OC in mineral soils leads to the sequestration of large amounts of organic phosphorus (OP) since SOM in mineral soils is very rich in P. The reasons for the strong enrichment of OP with respect to OC in soils are the mineralization of OC and the formation of microbial necromass that is P-rich as well as the strong sorption of OP to mineral surfaces that prevents OP mineralization. In particular, the formation of mineral-associated SOM that is favorable for storing OC in soil over decadal to centennial timescales sequesters large amounts of OP. Storage of 1,000kg C in the clay size fraction in the topsoils of croplands sequesters 13.1kg P. In contrast, the OC:OP ratios of wood and of peatlands are much larger than the ones in cropland soils. Thus, storage of C in wood in peatlands sequesters much less P than the storage of OC in mineral soils. In order to increase the C stocks in terrestrial ecosystems and to lock up as little P as possible, it would be more reasonable to protect and restore peatlands and to produce and preserve wood than to store OC in mineral soils.

Microbial assimilation dynamics differs but total mineralization from added root and shoot residues is similar in agricultural Alfisols

Microbial transformation of crop residue is the key process of soil organic matter (SOM) formation and mineralization, which determines soil fertility and affects global climate change. However, utilization dynamics of residue-derived carbon (residue C) by various microbial communities is still not well understood, especially under different residue quality and soil fertility conditions over a long-term scale (i.e., >1 year). In this study, a 500-day in-situ field experiment was conducted using 13C-labeled maize (Zea mays L.) root and shoot (composed of both stem and leaf) to examine the role of microbial community composition on the C processing. Specifically, the mineralization of residue C and incorporation of residue C into microbial biomass in low fertility (LF) and high fertility (HF) soils were investigated. The abundance of 13C in soil samples and microbial phospholipid fatty acids (PLFAs) were measured after 60, 90, 150 and 500 days since the residues added. The results showed that the mineralization rate of residue C was significantly higher in the LF than that in the HF soil for the first 150 days, and the shoot-derived C was more susceptible to degradation than root-derived C, but the final mineralization rates (~78%) were not significantly different among treatments on the day 500. Soil fertility significantly affected the relative composition of different microbial groups and distribution of residue C in microbial communities, but residue type did not do so. Furthermore, residue C contributed more to PLFA-C pool in the LF than HF treatments, and the proportion of root C in PLFA-C pool was higher than that of shoot C, indicating easier immobilization of root C by soil microbial anabolism. Accordingly, soil fertility and residue quality could both regulate the kinetics of the microbial immobilization of crop residue C, but overall the available residual quantity of applied (plant-derived) C to enhance or maintain soil C pool did not depend on them in a long term in the agricultural Alfisols.

Management practices differently affect particulate and mineral-associated organic matter and their precursors in arable soils

Our understanding of soil organic matter (SOM) formation and stabilization mechanisms has evolved recently. The SOM is considered as a continuum from coarse fresh and decomposing plant residues (particulate organic matter, >53 μm; POM) to fine and presumably more stable mineral-associated organic matter (fine organic matter, <53 μm; FOM), which represents the major portion of total SOM. Current theoretical models call for a prominent role of microbial by-products in FOM formation. However, there is still relatively little field-based evidence to support these theoretical models, especially in arable soils. We examined surface (0–10 cm) soil quality indicators over 7 years in two Brunisols with contrasting textures (silty clay and sandy loam) under twenty different management systems, combining two tillage regimes (minimum tillage vs inversion tillage), two crop residue management (residue retained vs harvested) and five different fertilizer sources (no-N control, complete mineral fertilization, liquid dairy manure, liquid swine manure, solid poultry manure). Surface soil quality indicators included soil organic carbon (SOC), POM-C, FOM-C, microbial biomass-C (MBC), plant- and microbe-derived carbohydrates, and mean-weight diameter of water-stable aggregates (MWD). An univariate statistical analysis of these indicators was combined with a principal component analysis (PCA) to explore the relationships among soil quality indicators and SOC dynamics. In both soil types, POM-C was closely related to MWD and plant-derived carbohydrates, whereas FOM-C was closely related to microbial amino-sugars. These results support the recent conceptual models mainly attributing the formation of FOM-C to microbial by-products. We found that management practices that favored POM-C formation, such as reduced tillage and residue retention, were particularly efficient to increase surface SOC content, irrespective of soil texture. By contrast, manuring had a pronounced effect on microbial biomass, which resulted in the accumulation of microbial amino sugar and SOC, in the silty clay soil only. Use of N-rich organic amendments with low lignin content, such as manures, could therefore be an efficient way to foster the formation of stable SOC through the accumulation of microbe-derived FOM precursors in arable soils where clay content is sufficient to allow for the stabilization of microbial by-products.

Temperature sensitivity of decomposition: Discrepancy between field and laboratory estimates is not due to sieving the soil

Is persistent soil organic matter (SOM), characterised by an old age and long-turnover time, more or less sensitive to changes in temperature than fast-cycling, recent SOM? Largely due to our limited understanding of the mechanisms of SOM formation, this question remains controversial. Laboratory incubation studies, through sieving the soil, may create conditions in which substrate accessibility is modified. The recent recognition of SOM accessibility as a defining factor of SOM persistency calls into question conclusions from these studies. Previously, in a study using root exclusion plots of increasing age, we showed in the field that the temperature sensitivity of SOM decomposition decreased with increasing persistence of SOM (Moinet et al., 2020), in opposition to many laboratory incubation studies. Here we sampled soils from the same root exclusion plots and conducted a laboratory incubation experiment to test the hypotheses that (i) the relationship between temperature sensitivity and SOM persistence is inverted as compared to the field, and (ii) the discrepancy is due to sieving the soil. We showed that, in the laboratory, the relationship was indeed inverted, with the temperature sensitivity being higher for the old root exclusion plots. However, sieving the soil at 2 mm did not affect estimates of the temperature sensitivity of SOM decomposition, suggesting that discrepancies between field and laboratory estimates are unlikely to stem from artificially modified substrate accessibility due to sieving.