Mineral-associated Soil Organic Carbon Research Articles

Microbial and mineral interactions decouple litter quality from soil organic matter formation

Current understanding of soil carbon dynamics suggests that plant litter quality and soil mineralogy control the formation of mineral-associated soil organic carbon (SOC). Due to more efficient microbial anabolism, high-quality litter may produce more microbial residues for stabilisation on mineral surfaces. To test these fundamental concepts, we manipulate soil mineralogy using pristine minerals, characterise microbial communities and use stable isotopes to measure decomposition of low- and high-quality litter and mineral stabilisation of litter-C. We find that high-quality litter leads to less (not more) efficient formation of mineral-associated SOC due to soil microbial community shifts which lower carbon use efficiency. Low-quality litter enhances loss of pre-existing SOC resulting in no effect of litter quality on total mineral-associated SOC. However, mineral-associated SOC formation is primarily controlled by soil mineralogy. These findings refute the hypothesis that high-quality plant litters form mineral-associated SOC most efficiently and advance our understanding of how mineralogy and litter-microbial interactions regulate SOC formation.

Does the presence of mineral nutrient enhance the carbon sequestration, soil aggregation, and microbial biomass in the subtropical clay soil?

Crop residue incorporation is considered as a promising approach for improving carbon sequestration coupling with soil aggregation (MWD). However, better understanding is needed regarding to the mechanism of carbon sequestration and aggregation following crop residue return in subtropical clay soil. A short-term field experiment (1.5 years) was conducted to explore the underlaying potential mechanism of carbon sequestration after crop residues return in the subtropical clay soil. Four treatments were applied as follows: (1) Control, (2) Inorganic fertilization (NPK), (3) Crop residue at 8-ton ha−1, (6) NPK + Crop residue at 8-ton ha−1. MWD, soil organic carbon (SOC), microbial biomass carbon (MBC), glomalin protein (GRSP), mineral and aggregate-associated SOC, and Fe oxides were determined. Results exhibited that the mineral nutrients with or without crop residue enhanced the SOC significantly in comparison to the only straw and control treatments (P < 0.05). The short-term residue return enhanced the MWD compared to the baseline soil, which was less than control due to the greater concentration of amorphous Fe oxides. Moreover, GRSP and MBC were significantly increased upon crop residue return in presence of mineral nutrients treatment (P < 0.01). SOC was reduced in the order of NPK > NPK + Crop residue > Crop residue > control, while MBC and GRSP were increased in the order of control < Crop residue < NPK < Crop residue + NPK. Aggregate-associated and mineral-associated SOC of aggregates were enhanced in presence of mineral nutrients (P < 0.001). Our results suggest that the nutrients addition in presence or absence of crop residue return under short-term field experiment is considered as a promising and sustainable clay soil management approach for enhancing carbon sequestration to mitigate climate change.

Vulnerability of mineral-associated soil organic carbon to climate across global drylands

Vulnerability of mineral-associated soil organic carbon to climate across global drylands

Global pattern of organic carbon pools in forest soils.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Contrasting stocks and origins of particulate and mineral-associated soil organic carbon in a mangrove-salt marsh ecotone

The global warming-driven poleward expansion of mangrove habitats (e.g., Avicennia germinans and Rhizophora mangle) into temperate salt marshes (e.g., Spartina alterniflora and Juncus roemerianus) has been shown to alter coastal soil organic carbon (SOC) storage. However, the taxa-specific consequences of this vegetation shift on the origin and size of SOC sub-fractions (particulate OC (POC); mineral-associated OC (MAOC); and reactive iron-associated OC (FeR-MAOC)) remain largely unexplored. In this study, we used a particle size-based SOC fractionation method to compare quantity and δ13C composition of bulk and each SOC sub-fractions in soil cores collected from Apalachicola Bay barrier islands in Florida, USA, the highest latitude where monospecific communities of all four aforementioned plants co-occur. Depth-dependent variation of bulk soil δ13C clearly showed the global warming-driven replacement of S. alterniflora by mangroves, as well as reciprocal substitutions of S. alterniflora and J. roemerianus, probably driven by changes in wetland elevation. Higher OC burial rates in mangrove habitats suggested that mangrove soils were principally developed by particle deposition. In contrast, comparatively lower OC burial rates but higher OC stocks in salt marsh habitats illustrated subsurface OC input from salt marsh roots. POC was primarily derived from contemporary plant detritus; its concentration was higher in salt marsh habitats (58.8 ± 9.0 % of SOC) relative to mangroves (38.4 ± 6.0 % of SOC). In contrast, MAOC content did not vary across plant habitats (53.5 ± 10.9 % of SOC), and principally originated from microbially-transformed OC and pre-existing plants. FeR-MAOC was essentially absent in R. mangle soils (2.9 ± 3.6 % of SOC) while representing a minor fraction of MAOC in three other plant habitats (7.8 ± 7.0 % of SOC). The δ13C of FeR-MAOC was more like the present-day surface plants, highlighting the in situ FeR-MAOC formation in their active oxidizing rhizospheres.

The path from root input to mineral-associated soil carbon is dictated by habitat-specific microbial traits and soil moisture

Soil microorganisms help transform plant inputs into mineral-associated soil organic carbon (SOC) – the largest and slowest-cycling pool of organic carbon on land. However, the microbial traits that influence this process are widely debated. While current theory and biogeochemical models have settled on carbon-use efficiency (CUE) and growth rate as positive predictors of mineral-associated SOC, empirical tests are sparse, with contradictory observations. Using 13C-labeling of an annual grass (Avena barbata) under two moisture regimes, we found that microbial traits associated with formation of 13C-mineral-associated SOC varied by soil habitat, as did active microbial taxa and SOC chemical composition. In the rhizosphere, bacterial-dominated communities with fast growth, high biomass, and high extracellular polymeric substance (EPS) production were positively associated with 13C-mineral-associated SOC. In contrast, the detritusphere held communities dominated by fungi and more filamentous bacteria, and with greater exoenzyme activity; there, 13C-mineral-associated SOC was associated with slower microbial growth and lower microbial biomass. CUE was a negative predictor of 13C-mineral-associated SOC in both habitats. Using 13C-quantitative stable isotope probing, we found that the majority of 13C assimilation in the rhizosphere and detritusphere at week 12 of the experiment was performed by very few bacterial and fungal taxa (3–5% of the total taxa that assimilated 13C). Several complementary chemical analyses (13C-NMR, FTICR-MS, and STXM-NEXAFS) suggested that SOC in the rhizosphere had a more oxidized chemical signature, while SOC in the detritusphere had a less oxidized, more lignin-like chemical signature. Our findings challenge current theory by demonstrating that microbial traits linked with mineral-associated SOC are not universal, but vary with soil habitat and moisture conditions, and are shaped by a small number of active taxa. Emerging SOC models that explicitly reflect these interactions may better predict SOC storage, since climate change causes shifts in soil moisture regimes and the ratio of living versus decaying roots.

Straw return plus zinc fertilization increased the accumulations and changed the chemical compositions of mineral-associated soil organic carbon

Agricultural zinc (Zn) fertilization can markedly promote organo-Zn complex formation, especially when combined with straw. However, how soil organic carbon (SOC) conversion responds to Zn and/or straw plus Zn has received little attention. Here, we explored the SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), as well as the OC chemical compositions’ responses in a field experiment site created in 2016. The following treatments were control (CK, no straw return and no Zn fertilization), straw return (St), Zn fertilization (Zn), and straw return combined with Zn fertilization (St+Zn). The results of our three-year study indicated that MAOC content under St+Zn increased by 4.4–6.4%, which led to a slight SOC accumulation (3.7–7.4%) compared to that of St alone. After including straw return with Zn fertilization, the Zn content of particulate organic matter and that of mineral-associated organic matter (ZnMAOM) both increased and thus helped MAOC formation. The chemical compositions of POC and MAOC were affected mainly by straw return and Zn fertilization, respectively. Both the Zn and St+Zn treatments had increased relative hydroxyl C contents and reduced O-alkyl C contents in MAOC, compared to that of CK and St, and the ZnMAOM content best explained the changes. In both the POC and MAOC fractions, the aromatic C to aliphatic C ratio decreased in Zn treatments compared to those in CK. Altogether, Zn’s effects on SOC and its fractions were closely linked to straw return, and Zn aids in MAOC and SOC accumulations when applied with straw.

Phosphorus Fertilization Boosts Mineral-Associated Soil Organic Carbon Formation Associated with Phagotrophic Protists.

Long-term fertilization affects soil organic C accumulation. A growing body of research has revealed critical roles of bacteria in soil organic C accumulation, particularly through mineral-associated organic C (MAOC) formation. Protists are essential components of soil microbiome, but the relationships between MAOC formation and protists under long-term fertilization remain unclear. Here, we used cropland soil from a long-term fertilization field trial and conducted two microcosm experiments with 13C-glucose addition to investigate the effects of N and P fertilizations on MAOC formation and the relationships with protists. The results showed that long-term fertilization (especially P fertilization) significantly (P < 0.05) increased 13C-MAOC content. Compared with P-deficient treatment, P replenishment enriched the number of protists (mainly Amoebozoa and Cercozoa) and bacteria (mainly Acidobacteriota, Bacteroidota, and Gammaproteobacteria), and significantly (P < 0.001) promoted the abundances of bacterial functional genes controlling C, N, P, and S metabolisms. The community composition of phagotrophic protists prominently (P < 0.001) correlated with the bacterial community composition, bacterial functional gene abundance, and 13C-MAOC content. Co-occurrence networks of phagotrophic protists and bacteria were more connected in soil with the N inoculum added than in soil with the NP inoculum added. P replenishment strengthened bacterial 13C assimilation (i.e., 13C-phospholipid fatty acid content), which negatively (P < 0.05) correlated with the number and relative abundance of phagotrophic Cercozoa. Together, these results suggested that P fertilization boosts MAOC formation associated with phagotrophic protists. Our study paves the way for future research to harness the potential of protists to promote belowground C accrual in agroecosystems.

The Assessment of Soil Organic Carbon Pools in Different Soils Using Four Fractionation Methods

ABSTRACT The sensitivity of SOC (soil organic carbon) fractions can provide information on soil organic matter proxies and kinetically defined components related to their chemical composition and oxidability. The purpose of this study was to assess the stability of SOC on five soil types (Arenosols, Chernozems, Fluvisols, Vertisols and Solonetz) and three land uses (cropland, meadow and forest). To obtain SOC functional fractions chemical, physical and density fractionating methods were used as follows: SOC susceptible to oxidation with 333 mM KMnO4 (SOCKMnO4S), SOC with varying degrees of oxidation resistance by the modified Walkley-Black method (SOCwb1, SOCwb2, SOCwb3, SOCwb4), particulate SOC (SOCpar); mineral-associated SOC (SOCmas), aggregate-associated size classes of the SOC (SOC >2000, SOC2000–250, SOC250–53, SOC <53). The relative contribution of fractions in SOC was as follows: SOCKMnO4S (7.9–42.5%), SOCwb1 (35.6%) > SOCwb2 (27.5%) > SOCwb4 (22.7%) > SOCwb3 (14.2%), SOCmas (56.6–76.4%) > SOCpar (23.6–43.4%), SOC >2000 (8.2–44.8), SOC 2000–250 (27.4–34.9), SOC 250–53 (13.4–27.4%), SOC <53 (5.4–28.8%). Fractions SOCpar, SOCwb4, SOC >2000 and SOC <53 showed sensitivity to land use, whereas SOCmas, SOC KMnO4S, SOCwb3 showed variations among soil types. The fraction SOCwb1 indicated sensitivity to both land use and soil type. Considering obtained functional fractions of SOC it was possible to determine discriminators between particular soil types or indicators of land use changes due the anthropogenization.

Carbon sequestration in paddy soils: Contribution and mechanisms of mineral-associated SOC formation

In this work, comparative study of paddy and upland soils were carried out to unravel mechanisms of enhanced soil organic carbon (SOC) sequestration in paddy soils using fractionation methods, 13C NMR and Nano-SIMS analysis, as well as organic layer thickness calculations (Core-Shell model). The results showed that although there is a strong increase in particulate SOC in paddy soils compared to that in the upland soils, the increase in mineral-associated SOC is more important, explaining 60–75% of SOC increase in the paddy soils. In the wet and dry alternate cycles of paddy soil, iron (hydr)oxides adsorb relatively small and soluble organic molecules (fulvic acid-like), promote catalytic oxidation and polymerization, thus accelerating formation of larger organic molecules. Upon reductive iron dissolution, these molecules are released and incorporated into existing less soluble organic compounds (humic acid or humin-like), which are coagulated and associated with clay minerals, becoming part of the mineral-associated SOC. The functioning of this “iron wheel” process stimulates accumulation of relatively young SOC into mineral-associated organic carbon pool, and reduces the difference in chemical structure between oxides-bound and clay-bound SOC. Further, the faster turnover of oxides and soil aggregates in paddy soil also facilities interaction between SOC and minerals. The formation of mineral-associated SOC may delay degradation of organic matter during both wet and dry period in the paddy field, therefore enhancing carbon sequestration in paddy soils.

Subarctic soil carbon losses after deforestation for agriculture depend on permafrost abundance

The northern circumpolar permafrost region is experiencing considerable warming due to climate change, which is allowing agricultural production to expand into regions of discontinuous and continuous permafrost. The conversion of forests to arable land might further enhance permafrost thaw and affect soil organic carbon (SOC) that had previously been protected by frozen ground. The interactive effect of permafrost abundance and deforestation on SOC stocks has hardly been studied. In this study, soils were sampled on 18 farms across the Yukon on permafrost and non-permafrost soils to quantify the impact of land-use change from forest to cropland and grassland on SOC stocks. Furthermore, the soils were physically and chemically fractionated to assess the impact of land-use change on different functional pools of SOC. On average, permafrost-affected forest soils lost 15.6 ± 21.3% of SOC when converted to cropland and 23.0 ± 13.0% when converted to grassland. No permafrost was detected in the deforested soils, indicating that land-use change strongly enhanced warming and subsequent thawing. In contrast, the change in SOC at sites without permafrost was not significant but had a slight tendency to be positive. SOC stocks were generally lower at sites without permafrost under forest. Furthermore, land-use change increased mineral-associated SOC, while the fate of particulate organic matter (POM) after land-use change depended on permafrost occurrence. Permafrost soils showed significant POM losses after land-use change, while grassland sites without permafrost gained POM in the topsoil. The results showed that the fate of SOC after land-use change greatly depended on the abundance of permafrost in the pristine forest, which was driven by climatic conditions more than by soil properties. It can be concluded that in regions of discontinuous permafrost in particular, initial conditions in forest soils should be considered before deforestation to minimize its climate impact.

Global stocks and capacity of mineral-associated soil organic carbon

Soil is the largest terrestrial reservoir of organic carbon and is central for climate change mitigation and carbon-climate feedbacks. Chemical and physical associations of soil carbon with minerals play a critical role in carbon storage, but the amount and global capacity for storage in this form remain unquantified. Here, we produce spatially-resolved global estimates of mineral-associated organic carbon stocks and carbon-storage capacity by analyzing 1144 globally-distributed soil profiles. We show that current stocks total 899 Pg C to a depth of 1 m in non-permafrost mineral soils. Although this constitutes 66% and 70% of soil carbon in surface and deeper layers, respectively, it is only 42% and 21% of the mineralogical capacity. Regions under agricultural management and deeper soil layers show the largest undersaturation of mineral-associated carbon. Critically, the degree of undersaturation indicates sequestration efficiency over years to decades. We show that, across 103 carbon-accrual measurements spanning management interventions globally, soils furthest from their mineralogical capacity are more effective at accruing carbon; sequestration rates average 3-times higher in soils at one tenth of their capacity compared to soils at one half of their capacity. Our findings provide insights into the world’s soils, their capacity to store carbon, and priority regions and actions for soil carbon management.

Fast-decaying plant litter enhances soil carbon in temperate forests but not through microbial physiological traits

Conceptual and empirical advances in soil biogeochemistry have challenged long-held assumptions about the role of soil micro-organisms in soil organic carbon (SOC) dynamics; yet, rigorous tests of emerging concepts remain sparse. Recent hypotheses suggest that microbial necromass production links plant inputs to SOC accumulation, with high-quality (i.e., rapidly decomposing) plant litter promoting microbial carbon use efficiency, growth, and turnover leading to more mineral stabilization of necromass. We test this hypothesis experimentally and with observations across six eastern US forests, using stable isotopes to measure microbial traits and SOC dynamics. Here we show, in both studies, that microbial growth, efficiency, and turnover are negatively (not positively) related to mineral-associated SOC. In the experiment, stimulation of microbial growth by high-quality litter enhances SOC decomposition, offsetting the positive effect of litter quality on SOC stabilization. We suggest that microbial necromass production is not the primary driver of SOC persistence in temperate forests. Factors such as microbial necromass origin, alternative SOC formation pathways, priming effects, and soil abiotic properties can strongly decouple microbial growth, efficiency, and turnover from mineral-associated SOC.

Soil Carbon Stabilization Under Coniferous, Deciduous and Grass Vegetation in Post-mining Reclaimed Ecosystems

Vegetation plays an important role in determining soil organic carbon (SOC) stocks, and influences the mechanisms through which SOC is stabilized within the soil. The type of vegetation selected for use in reclamation may therefore influence the accumulation rate and residence time of SOC in these ecosystems. Earlier studies at reclaimed sites in the Alberta Oil Sands demonstrated that reclaimed ecosystems planted with deciduous trees accumulated the most soil organic matter in the top 10 cm of reclamation material, followed by grass sites, while coniferous sites accumulated the least SOM. The objective of this study was to assess differences in SOC stabilization in the upper 10 cm of soil among revegetated deciduous, coniferous and grass ecosystems 20–40 years following reclamation. We compared soil C in unprotected, physically protected, and chemically protected forms among the three reclamation treatments using density flotation to isolate free particulate (unprotected) SOC from the soil sample, and size fractionation to separate the remaining sample into heavy particulate (physically protected) SOC and mineral-associated (chemically protected) SOC. In addition to this analysis, we used NaOCl oxidation to distinguish chemically resistant and chemically oxidizable C stocks. Chemically resistant C was consistent across all vegetation treatments at approximately 25% of total soil C, while the remaining 75% was chemically oxidizable. Total SOC stocks were also not significantly different among vegetation treatments. Deciduous sites had 57.8 Mg ha–1 SOC, grass sites had 52.7 Mg ha–1 SOC, and coniferous sites had 43.7 Mg ha–1 SOC. Two-thirds of total SOC at grass sites was in protected forms, compared to half of total SOC at coniferous sites and one-third of total SOC at deciduous sites (33.6, 22.6, and 15.6 Mg ha–1, respectively). Grass sites had significantly more physically protected SOC than deciduous sites while deciduous sites had more unprotected SOC than grass sites. Our findings indicate that the type of vegetation selected for reclaimed areas has important implications for soil carbon in persistent versus unprotected pools.

Effects of microplastics on soil organic carbon and greenhouse gas emissions in the context of straw incorporation: A comparison with different types of soil

Plastic mulching and straw incorporation are common agricultural practices in China. Plastic mulching is suspected to be a significant source of microplastics in terrestrial environments. Straw incorporation has many effects on the storage of soil organic carbon (SOC) and greenhouse gas emissions, but these effects have not been studied in the presence of microplastic pollution. In this study, 365-day soil incubation experiments were conducted to assess the effects of maize straw and polyethylene microplastics on SOC fractions and carbon dioxide (CO2) and nitrous oxide (N2O) emissions in two different soils (fluvo-aquic and latosol). Against the background of straw incorporation, microplastics reduced the mineralization and decomposition of SOC, resulting in a microbially available SOC content decrease by 18.9%. In addition, microplastics were carbon-rich, but relatively stable and difficult to be used by microorganisms, thus increasing the mineral-associated SOC content by 52.5%. This indicated that microplastics had adverse effects on microbially available SOC and positive effects on mineral-associated SOC. Microplastics also decreased coarse particulate SOC (>250 μm), and increased non-aggregated silt and clay aggregated SOC (<53 μm). Furthermore, microplastics changed microbial community compositions, thereby reducing the CO2 and N2O emissions of straw incorporation by 26.5%–33.9% and 35.4%–39.7%, respectively. These results showed that microplastics partially offset the increase of CO2 and N2O emissions induced by straw incorporation. Additionally, the inhibitory effect of microplastics on CO2 emissions in fluvo-aquic soil was lower than that in latosol soil, whereas the inhibitory effect on N2O emissions had the opposite trend.

Different climate sensitivity of particulate and mineral-associated soil organic matter

Soil carbon sequestration is seen as an effective means to draw down atmospheric CO2, but at the same time warming may accelerate the loss of extant soil carbon, so an accurate estimation of soil carbon stocks and their vulnerability to climate change is required. Here we demonstrate how separating soil carbon into particulate and mineral-associated organic matter (POM and MAOM, respectively) aids in the understanding of its vulnerability to climate change and identification of carbon sequestration strategies. By coupling European-wide databases with soil organic matter physical fractionation, we assessed the current geographical distribution of mineral topsoil carbon in POM and MAOM by land cover using a machine-learning approach. Further, using observed climate relationships, we projected the vulnerability of carbon in POM and MAOM to future climate change. Arable and coniferous forest soils contain the largest and most vulnerable carbon stocks when cumulated at the European scale. Although we show a lower carbon loss from mineral topsoils with climate change (2.5 ± 1.2 PgC by 2080) than those in some previous predictions, we urge the implementation of coniferous forest management practices that increase plant inputs to soils to offset POM losses, and the adoption of best management practices to avert the loss of and to build up both POM and MAOM in arable soils. Particulate and mineral-associated soil organic carbon have different climate sensitivity and distributions in Europe, according to analyses of measurements of soil carbon fractions from 352 topsoils.

Feasibility of the 4 per 1000 aspirational target for soil carbon: A case study for France.

Increasing soil organic carbon (SOC) stocks is a promising way to mitigate the increase in atmospheric CO2 concentration. Based on a simple ratio between CO2 anthropogenic emissions and SOC stocks worldwide, it has been suggested that a 0.4% (4 per 1000) yearly increase in SOC stocks could compensate for current anthropogenic CO2 emissions. Here, we used a reverse RothC modelling approach to estimate the amount of C inputs to soils required to sustain current SOC stocks and to increase them by 4‰ per year over a period of 30 years. We assessed the feasibility of this aspirational target first by comparing the required C input with net primary productivity (NPP) flowing to the soil, and second by considering the SOC saturation concept. Calculations were performed for mainland France, at a 1 km grid cell resolution. Results showed that a 30%–40% increase in C inputs to soil would be needed to obtain a 4‰ increase per year over a 30‐year period. 88.4% of cropland areas were considered unsaturated in terms of mineral‐associated SOC, but characterized by a below target C balance, that is, less NPP available than required to reach the 4‰ aspirational target. Conversely, 90.4% of unimproved grasslands were characterized by an above target C balance, that is, enough NPP to reach the 4‰ objective, but 59.1% were also saturated. The situation of improved grasslands and forests was more evenly distributed among the four categories (saturated vs. unsaturated and above vs below target C balance). Future data from soil monitoring networks should enable to validate these results. Overall, our results suggest that, for mainland France, priorities should be (1) to increase NPP returns in cropland soils that are unsaturated and have a below target carbon balance and (2) to preserve SOC stocks in other land uses.

Evidence linking calcium to increased organo-mineral association in soils

Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO3-bearing) or without carbonates (CaCO3-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δ13C values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δ13C values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO3-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δ13C values. Bulk SOC content was twice as high in the CaCO3-bearing profiles, which also had lower bulk δ13C values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δ13C values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δ13C values.

Biological mechanisms may contribute to soil carbon saturation patterns.

Increasing soil organic carbon (SOC) storage is a key strategy to mitigate rising atmospheric CO2 , yet SOC pools often appear to saturate, or increase at a declining rate, as carbon (C) inputs increase. Soil C saturation is commonly hypothesized to result from the finite amount of reactive mineral surface area available for retaining SOC, and is accordingly represented in SOC models as a physicochemically determined SOC upper limit. However, mineral-associated SOC is largely microbially generated. In this perspective, we present the hypothesis that apparent SOC saturation patterns could emerge as a result of ecological constraints on microbial biomass-for example, via competition or predation-leading to reduced C flow through microbes and a reduced rate of mineral-associated SOC formation as soil C inputs increase. Microbially explicit SOC models offer an opportunity to explore this hypothesis, yet most of these models predict linear microbial biomass increases with C inputs and insensitivity of SOC to input rates. Synthesis of 54 C addition studies revealed constraints on microbial biomass as C inputs increase. Different hypotheses limiting microbial density were embedded in a three-pool SOC model without explicit limits on mineral surface area. As inputs increased, the model demonstrated either no change, linear, or apparently saturating increases in mineral-associated and particulate SOC pools. Taken together, our results suggest that microbial constraints are common and could lead to reduced mineral-associated SOC formation as input rates increase. We conclude that SOC responses to altered C inputs-or any environmental change-are influenced by the ecological factors that limit microbial populations, allowing for a wider range of potential SOC responses to stimuli. Understanding how biotic versus abiotic factors contribute to these patterns will better enable us to predict and manage soil C dynamics.

Transformation of litter carbon to stable soil organic matter is facilitated by ungulate trampling

Plant litter is an important source of soil organic carbon (SOC) in terrestrial ecosystem. The formation of SOC from plant litter and SOC mineralization to atmosphere is a critical determinant of long-term net ecosystem C balance. While it is generally understood that grazing plays a role in SOC cycling, what mechanisms are involved in driving SOC dynamic are not clear. Trampling enhances microbial processes by incorporating litter into the soil, likely influencing the fate of litter C and SOC mineralization. We conducted a controlled microcosm experiment to assess the role of trampling on the fate of litter C, litter-derived SOC mineralization (priming effect), and net C balance through incubation of 13C isotopically labelled Stipa krilovii litter placed on the soil surface, or incorporated into soil via simulated trampling, for 6 months. Litter C transferred into the SOC pool was further fractionated into mineral-associated soil organic carbon pool (MASOC, <53 μm) and particulate organic carbon pool (POC, >53 μm). We analyzed soil enzyme activities and litter-derived microbial biomass carbon (MBC) to determine microbial activities. We found trampling increased overall losses in litter mass (+16%) and litter C (+14%), with proportionally more decomposed litter C transferred to SOC pools (MASOC + 47% and POC + 157%), compared to absence of trampling. The litter-derived MBC was positively correlated with SOC formation, which was increased (+40.78%) by trampling, indicting a stronger microbial contribution to SOC formation. The disturbance of trampling did not induce significant positive priming effects, consistent with invariant soil total MBC and soil enzyme activities following trampling. As a result, trampling induced an increase in SOC formation and invariant priming effect, which contributed to positive net soil C balance (-230.74 ± 89.44 vs 100.33 ± 32.65 mg C/kg soil). Our results show trampling incorporates litter into soil and promotes microbial utilization of litter C and physiochemical stabilization of decomposed litter C, suggesting trampling is an important mechanism of SOC storage with litter C efficiently transferred into SOC pool. We demonstrate the importance of trampling in SOC formation and stabilization. Our findings indicate SOC formation efficiency from C input should be included in SOC predictive models in managed ecosystems.