Earthworms Enhance Global Soil Carbon Storage Through Microbial-Mineral Stabilization.

The integration of manure and straw substantially affects soil organic carbon (SOC) dynamics, transformation, and long-term stabilization in agricultural systems. Dissolved organic carbon (DOC), particulate organic carbon (POC), and mineral-associated organic carbon (MOC) are the three main components of the SOC pool, each influencing soil carbon dynamics and nutrient cycling. Current research gaps remain regarding how combined fertilization practices affect the inputs of plant-originated and microbe-derived carbon into SOC pools and stability mechanisms. Our investigation measured SOC fractions (DOC, POC, MOC), SOC mineralization rate (SCMR), microbial necromass carbon, lignin phenols, enzyme activities, and microbial phospholipid fatty acids (PLFAs) over a long-term (17 years) field experiment with four treatments: mineral fertilization alone (CF), manure-mineral combination (CM), straw-mineral application (CS), and integrated manure-straw-mineral treatment (CMS). The CMS treatment exhibited notably elevated levels of POC (7.42 g kg−1), MOC (10.7 g kg−1), and DOC (0.108 g kg−1), as well as a lower SCMR value (1.85%), compared with other fertilization treatments. Additionally, the CMS treatment stimulated the buildup of both bacterial and fungal necromass while enhancing the concentrations of ligneous biomarkers (vanillin, syringyl, and cinnamic derivatives), which correlated strongly with the elevated contents of fungal and bacterial PLFAs and heightened activity of carbon-processing enzymes (α-glucosidase, polyphenol oxidase, cellobiohydrolase, peroxidase, N-acetyl-β-D-glucosidase). Furthermore, fungal and bacterial microbial necromass carbon, together with lignin phenols, significantly contributed to shaping the composition of SOC. Through random forest analysis, we identified that the contents of bacterial and fungal necromass carbon were the key factors influencing DOC and MOC. The concentrations of syringyl phenol and cinnamyl phenols, and the syringyl-to-cinnamyl phenols ratio were the primary determinants for POC, while the fungal-to-bacterial necromass carbon ratio, as well as the concentrations of vanillyl, syringyl, and cinnamyl phenols, played a critical role in SCMR. In conclusion, the manure combined with straw incorporation not only promoted microbial growth and enzyme activity but also enhanced plant- and microbial-derived carbon inputs. Consequently, this led to an increase in the contents and stability of SOC fractions (DOC, POC, and MOC). These results suggest that manure combined with straw is a viable strategy for soil fertility due to its improvement in SOC sequestration and stability.

Background and aimsSoil represents a larger reservoir of soil organic carbon (SOC) than terrestrial vegetation, offering a great potential for reducing the widespread adverse consequences of climate change. In forests and tree plantations, fine roots significantly impact SOC stabilization through their functional traits. However, it is not obvious which fine root traits between those related to chemistry (easily decomposable or recalcitrant), architecture or morphology are the most conducive to SOC stabilization in phylogenetically related fast-growing trees. We assessed the effects of root functional traits on SOC storage and stabilization.MethodsWe studied five hybrid poplar clones with different root traits located in New Liskeard, ON, Canada. We collected soil cores at depths of 0–20 and 20–40 cm, and determined bulk soil organic carbon, particulate organic carbon (> 53 μm, POC) and mineral-associated organic carbon (< 53 μm, MAOC) fractions and fine root (< 2 mm diameter) traits.ResultsWe found that root trait categories influencing SOC storage and stabilization varied by soil depth. At the 0–20 cm depth, root tissue density and dry mass content were negatively correlated with SOC stocks. Higher root length and mass densities were linked to greater SOC stocks and MAOC at the 20–40 cm depth. Root traits indicative of low chemical recalcitrance, such as high soluble compounds concentration and small diameter, also promoted MAOC formation.ConclusionRoot traits that increase the soil volume explored by fine roots and are associated with easily decomposed organic compounds play a key role in SOC persistence.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11104-025-07681-3.

Species choice and richness in intensively managed grassland mixtures regulate soil carbon (C) input via rhizodeposition, with potential consequences for long-term soil organic carbon storage. Based on a field trial with different grass-legume-forb mixtures, we removed roots from the soil, which was then subjected to particle-size fractionation to trace fresh organic carbon (net C rhizodeposition) into particulate organic matter (POM) and mineral-associated organic matter (MAOM). We related these C input fractions to root traits. Using multiple-pulse 13C-CO2-labeling, we captured the net formation of mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) at the end of the growing season. Pure stand perennial ryegrass (Lolium perenne) had higher quantities of rhizodeposited C allocated to MAOC and POC (0.21 and 0.13 g C kg-1 dry soil, respectively) compared to grass-legume-forb mixtures (ranging from 0.10 to 0.12 for MAOC and 0.05 to 0.06 g C kg-1 dry soil for POC). However, the proportion of MAOC (%MAOC of net C rhizodeposition) in relation to that of POC was higher in mixtures with legumes. Species richness did not affect the quantity of MAOC or POC, nor %MAOC. The quantities of MAOC and POC were positively associated with root length. In contrast, %MAOC was positively associated with root diameter and a lower root C:N ratio. Despite higher %MAOC in mixtures with legumes, the main driver of MAOC and POC quantities was the total amount of C rhizodeposition. These results highlight the importance of legumes in the formation of MAOC from rhizodeposition and of high root length for increasing both MAOC and POC quantities. Our study shows how plant community design can be used to increase MAOC and/or POC and facilitate soil C storage. By revealing the traits behind the relationships between plant communities and MAOC and POC formation, we provide a guide for species selection in intensively managed grasslands to mitigate climate change.

Soil organic carbon (SOC) is the largest terrestrial carbon pool. SOC is composed of a continuum set of compounds with different chemical composition, origin and susceptibilities to decomposition, that are commonly separated into pools characterised by different responses to anthropogenic and environmental disturbance. Here we map the contribution of three SOC fractions to the total SOC content of Australia’s soils. The three SOC fractions: mineral-associated organic carbon (MAOC), particulate organic carbon (POC) and pyrogenic organic carbon (PyOC), represent SOC composition with distinct turnover rates, chemistry, and pathway formation. Data for MAOC, POC, and PyOC were obtained with near- and mid-infrared spectral models calibrated with measured SOC fractions. We transformed the data using an isometric log-ratio transformation (ilr) to account for the closed compositional nature of SOC fractions. The resulting, back-transformed ilr components were mapped across Australia. SOC fraction stocks for the 0–30 cm were derived with maps of total organic carbon concentration, bulk density, coarse fragments and soil thickness. Mapping was done by quantile regression forest fitted with the ilr transformed data and a large set of environmental variables as predictors. The resulting maps along with the quantified uncertainty show the unique spatial pattern of SOC fractions in Australia. MAOC dominated the total SOC with an average of 59 % ± 17.5 %, whereas 28 % ± 17.5 % was PyOC and 13 % ± 11.1 % was POC. The allocation of TOC into the MAOC fractions increased with depth. SOC vulnerability (i.e., POC/[MAOC + PyOC]) was greater in areas with Mediterranean and temperate climate. TOC and the distribution among fractions were the most influential variables on SOC fraction uncertainty. Further, the diversity of climatic and pedological conditions suggests that different mechanisms will control SOC stabilisation and dynamics across the continent, as shown by the model covariates importance metric. We estimated the total SOC stocks (0–30 cm) to be 12.7 Pg MAOC, 2 Pg POC and 5.1 Pg PyOC, which is consistent with previous estimates. The maps of SOC fractions and their stocks can be used for modelling SOC dynamics and forecasting changes in SOC stocks as response to land use change, management, and climate change.

Responses of particulate and mineral-associated organic carbon to temperature changes and their mineral protection mechanisms: A soil translocation experiment

Carbon use efficiency has recently been proposed as a central parameter that promotes soil organic carbon storage based on data assimilation with a global soil organic carbon database and a vertical, microbial explicit soil organic carbon model (Tao et al., 2023). In this research, we present a sensitivity study with a vertical soil organic carbon model, COMISSION v2.0 (Ahrens et al., 2020), that not only models microbial interactions explicitly but also represents organo-mineral interactions with a maximum capacity, Qmax, to form mineral-associated organic carbon (MAOC).The COMISSION model represents the formation of MAOC from microbial necromass and dissolved organic carbon analogous to Langmuir sorption. Empirical studies have provided Qmax parameterizations derived from quantile or boundary line regressions with clay and silt content. For the sensitivity study, we vary Qmax along the full range of observed Qmax values while simultaneously varying carbon use efficiency (CUE). Our results highlight that CUE and Qmax promote soil organic carbon storage to similar degrees along their respective observed ranges. The remaining parameters of the COMISSION model were kept at their calibrated values from a multi-site calibration with soil organic carbon, mineral-associated organic carbon, and radiocarbon profiles (Ahrens et al., 2020). While Qmax and CUE are of similar importance for promoting soil organic carbon storage, they also interact in promoting SOC storage. Higher Qmax values strengthen the promotion of soil organic carbon storage with higher CUE. This positive interaction results from higher microbial necromass with higher CUE and the subsequent association of microbial necromass on mineral surfaces mediated through Qmax. The sensitivity study revealed that CUE is the dominant driver for microbial biomass levels. Qmax affects microbial biomass only to a small degree through 'competition' between mineral surfaces and microbial biomass for dissolved organic carbon. While the effect of Qmax on microbial biomass is small, the relationship between Qmax and microbial biomass is generally negative. At the lower end of the tested range of carbon use efficiencies (CUE &lt; 0.15), further model experiments reveal that imposing a stronger microbial limitation of depolymerization can lead to a negative relationship between CUE and soil organic carbon storage.Overall, our results highlight that in soil organic carbon models with microbial interactions and a limited capacity to form organo-mineral associations, both processes can be of similar importance in promoting soil organic carbon storage. The current debate in the observational realm, whether there is indeed an upper limit for mineral-associated organic carbon formation, can spark a similar debate in the modeling realm on how to represent mineral-associated organic carbon formation in models mechanistically.&amp;#160;ReferencesAhrens B, Guggenberger G, Rethemeyer J et al. (2020) Combination of energy limitation and sorption capacity explains 14C depth gradients. Soil Biology and Biochemistry, 148, 107912.Tao F, Huang Y, Hungate BA et al. (2023) Microbial carbon use efficiency promotes global soil carbon storage. Nature, 618, 981-985.Funding acknowledgment: Bernhard Ahrens has received funding through the AI4SoilHealth project. The AI4SoilHealth project has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No. 101086179.

Soil organic carbon (SOC) is operationally partitioned into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to infer soil carbon persistence and sensitivity to disturbance or management. Yet the combined roles of climate, mineralogy, and land use in shaping these fractions remain unresolved across continental gradients. Here, we analysed 249 Australian topsoils from 65 paired sites along a 550 mm rainfall isohyet. Samples were grouped into four climate–texture clusters (Subtropical Coarse, Subtropical Fine, Temperate Coarse, Temperate Fine) to disentangle the effects of thermal and hydrological regimes, soil properties, and land use on SOC partitioning and stabilisation. Subtropical soils consistently exhibited a high proportion of MAOC (fMAOC ≈ 0.8) despite low SOC stocks, reflecting preferential retention of mineral–organic interactions under carbon-limited and water-stressed conditions. In contrast, temperate soils stored greater SOC and POC, indicating higher carbon inputs and slower decomposition. In subtropical fine-textured soils, agriculture elevated fMAOC through microbial activity and nutrient inputs, yet this occurred alongside depleted SOC and POC, highlighting a trade-off between stabilisation efficiency and carbon stock depletion. Across all clusters, land use effects were detectable but secondary to climate and mineral properties. These findings show that temperate and subtropical soils follow contrasting carbon stabilisation pathways: temperate systems store more carbon overall, while subtropical systems allocate a larger share of their carbon to mineral-associated carbon. Our climate–texture framework highlights region-specific management priorities: enhancing mineral–organic interactions through increased root inputs and organic or mineral amendments in subtropical soils, and protecting vulnerable carbon stocks through reduced disturbance and residue retention in temperate systems. • SOC stability is context dependent, not universal. • Temperature and clay jointly control particulate and mineral-associated pools. • Agriculture reduces both particulate and mineral-associated carbon stocks. • Soil carbon responses to land use depend on climate and texture. • The climate–texture framework identifies region-specific pathways.

Litter decomposition plays a critical role in the formation and turnover of soil organic carbon (SOC) and its fractions. However, the effects of litter on SOC dynamics across grassland degradation remain poorly understood. The objectives of this study were to investigate the responses of SOC and its fractions, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), to litter decomposition in lightly, moderately, and highly degraded grasslands. A 240-day incubation experiment using Leymus chinensis litter incubated on day 0, 23, 60, and 240 was conducted to investigate the biotic and abiotic factors regulating SOC stability. Grassland degradation significantly reduced POC and MAOC concentrations; moreover, litter addition in degraded grasslands further reduced SOC. In the lightly and moderately degraded grasslands, litter addition modulated POC and MAOC via both microbial and physicochemical pathways. In the highly degraded grasslands, litter addition influenced POC and MAOC not only indirectly through microbial and physicochemical pathways but also directly by promoting MAOC formation. Overall, although litter decomposition altered SOC and its fractions, its effects were constrained by the degree of grassland degradation. These findings indicate that restoration strategies should prioritize enhancing microbial biomass and activity in lightly and moderately degraded grasslands while increasing litter inputs in highly degraded grasslands to improve soil carbon sequestration.

Soil organic carbon (SOC) sequestration is vital for maintaining the function of grassland ecosystems under grazing conditions, yet the relationship between different carbon fractions and carbon sequestration remains unclear. This study investigated SOC dynamics under a 5‐year controlled grazing gradient (ranging from grazing exclusion to extremely heavy grazing) in the Qilian Mountains, China. It aimed to elucidate the variations in SOC and its fractions and further discuss the mechanisms of SOC sequestration under grazing. The results showed that with increasing grazing intensity, SOC and all carbon fractions exhibited a significant declining trend, except for dissolved organic carbon (DOC). The decline in SOC was directly influenced by aboveground biomass and available phosphorus. Furthermore, organic carbon dynamics were regulated by the mineral protection mechanism represented by particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC), whereas other carbon fractions did not exhibit significant effects. However, Partial Least Squares Structural Equation Modeling (PLS‐SEM) revealed distinct regulatory pathways for the two; specifically, the reduction in POC induced by changes in vegetation characteristics was a significant cause of the SOC decline. In contrast, MAOC, serving as the primary reservoir for carbon sequestration, was mainly regulated by soil factors. Overall, the insensitivity of MAOC to grazing pressure rendered it the most stable carbon fraction. Meanwhile, the contribution of MAOC to long‐term carbon preservation exceeded that of POC, highlighting the critical significance of stable organic carbon for the long‐term retention of SOC. These findings suggest that maintaining grazing at moderate levels can preserve the labile organic carbon inputs required for nutrient cycling, while the stable carbon fraction (MAOC) ensures the long‐term carbon sink function of alpine meadows.

The long‐term impacts of intensive agricultural reclamation on the turnover and stabilization of soil organic carbon (SOC) in desert ecosystems remain poorly understood, particularly throughout deep soil profiles. Using a paired‐site approach in northwest China, we investigated how the conversion of sandy land to cropland following 15 years of reclamation shaped the vertical distribution (0–200 cm) and composition of SOC fractions, focusing on particulate (POC) and mineral‐associated organic carbon (MAOC). Our results show that over 60% of total SOC stock is stored below 60 cm depth, with deep‐soil carbon increasing significantly from 18.4 t ha −1 in natural land to 27.2 t ha −1 following reclamation, underscoring the critical role of subsoil carbon sequestration in arid regions under land‐use change. Reclamation fundamentally shifted SOC composition from POC dominance to MAOC dominance, with subsoil MAOC increasing by up to 133.9%, indicating enhanced stability. A strong correlation between microbial necromass carbon (MNC) and MAOC in the topsoil suggests a predominantly microbial‐mediated pathway for MAOC formation in surface layers. MAOC correlated negatively with aliphatic‐C and positively with polysaccharide‐C, collectively pointing to microbial transformation and subsequent mineral stabilization as key processes in MAOC formation. Key factors including total nitrogen, available nitrogen, clay content, and soil moisture were identified as primary predictors of MAOC accumulation, with depth‐dependent influences. These findings demonstrate that long‐term reclamation markedly promotes MAOC accumulation and carbon sequestration capacity in deep soil, while clarifying associated biological and physicochemical stabilization mechanisms. These insights into SOC persistence under land‐use change are crucial for developing sustainable soil management and climate‐adaptive agriculture in drylands.

In desert ecosystems, deep-rooted plants like Alhagi sparsifolia contribute not only to wind prevention and sand fixation but also to the transport of carbon into deep soil layers through their root systems. However, the sources and stabilization mechanisms of soil organic carbon (SOC) following plant carbon input remain unclear. This study investigated a dominant A. sparsifolia community at the southern edge of the Taklimakan Desert. We analyzed plant traits and the vertical distribution (0-200 cm) of SOC fractions-particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and calcium/iron-bound organic carbon (Ca/Fe-OC)-along with carbon sources (microbial biomass, microbial necromass, and plant residue). As growth advanced, stem and root biomass increased, while leaf and thorn biomass remained stable. SOC and POC decreased by 5.38-29.43% with soil depth, whereas MAOC and Ca/Fe-OC increased by 32.34-48.15%. Plant residue contributed more to SOC (average 30.56%) than microbial necromass (8.28%), and both contributions increased by 9.60-167.68% with soil depth. No significant correlation was found between plant residue and SOC fractions, but a significant correlation with microbial necromass. In conclusion, although plant residues constitute the primary source of SOC in desert ecosystems, microbial necromassa exerts a stronger influence on SOC stability.

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

Soil erosion transports light density and fine particle soil material from hills down to low-lying land areas, which can lead to carbon loss and subsequent sequestration. In the present paper, the profile distribution of soil organic carbon (SOC) and soil 13C natural abundance (δ13C) were analyzed across five geomorphic positions, distributed along a typical rolling farmland in the Black Soil region of Northeast China. The contents of particulate organic carbon (POC) and mineral-associated organic carbon (MOC) at each geomorphic position were measured with physical fraction method. The results showed that soil erosion decreased 5.3–22.4% of SOC and increased 4.0–6.1% of δ13C of surface soils at the eroding sites. At the typical depositional sites, SOC content and δ13C value in the buried surface layer were 1.5 times and 1.1 times as much as those of the current plough layer, respectively. Soil erosion did not change the POC content, but MOC content decreased by 9.3–35.2%. At the eroding sites, the coefficient of determination between soil δ13C and MOC (R 2 = 0.52) was higher than that between soil δ13C and POC (R 2 = 0.37). Our study indicated that soil erosion decreased SOC content and increased δ13CSOC in surface layer mainly through transferring fine sized and 13C-depleted SOC fraction. Deep burial and re-aggregation of eroded materials at depositional sites were in favor of stabilization and sequestration of SOC.

Soil organic carbon (OC) storage across regions is influenced by climate and parent materials, which determine soil properties like clay content and mineralogy. Within homogeneous soil regions, land use and management practices are further important controls for soil OC contents and turnover. Here, we studied the impact of study region, land use (forest, grassland), forest management (spruce and beech forest under age-class management, unmanaged beech forest), and grassland management (meadow, mown pasture, unmown pasture) on stocks and turnover (based on Δ14C values) of soil OC in density frations of topsoil horizons. Samples were taken from 36 plots in the regions Hainich–Dun (HAI) and the Schwabische Alb (ALB) in Germany. They were separated into two light fractions (free light fraction (LF1), occluded light fraction (LF2)) and the mineral-associated organic matter (MOM) fraction using sodium polytungstate with a density of 1.6 g cm−3. Overall most soil OC was stored in the MOM fraction (73%). Soil OC concentrations and stocks in the MOM fraction differed between study regions, probably due to larger amounts of pedogenic Al- and Fe-oxides in the ALB than in the HAI region. Within each region, forest soils stored significantly higher proportions of total OC in the two LF (33±1.9 %) than grassland soils (20±2.3 %). Different management of forests and grasslands affected the C:N ratio of density fractions, but not OC storage. While modelled soil OC turnover in the MOM was longest of all fractions, all fractions had average Δ14C values above atmospheric values, suggesting a significant fast-cycling component in all of them. Different from stocks, turnover of OC in the MOM fraction were not affected by study region or contents of pedogenic oxides. Radiocarbon contents in the LF were higher for forest than for grassland sites, indicating faster turnover of OC at grassland sites. However, some of the observed difference could originate from different average lifetimes of roots in forests and grasslands. Applying different lag-times for OC input for forests and grasslands significantly reduced the differences in modelled turnover times. Lower Δ14C values of mown pastures than pasture soils in both regions suggest a management effect on soil C turnover in grasslands.We conclude that OC storage in the MOM of topsoil layers is more affected by regional differences in soil texture and mineralogy than by land use and management, while its turnover could not be explained with the studied soil properties. Soil OC storage and turnover in the two LFs is influenced by land use (forest or grassland) and management, but ecosystem specific lag-times have to be considered for modelling OC turnover in these fractions.

Reforestation after forest clear‐cutting is an effective measure to increase soil organic carbon (SOC) sequestration; still, the soil C balance under reforestation and the role of microbial communities in that process remain to be determined. Samples of organic (0–2 cm) and mineral (2–10 cm) horizons were collected from the 7‐, 15‐, 20‐, 29‐, and 36‐year‐old forest stands of Chinese fir (Cunninghamia lanceolata) after plantation clear‐cutting in subtropical zone under the condition of phosphorus limitation. Particulate organic carbon (POC), mineral‐associated organic carbon (MAOC), microbial phospholipid fatty acids (PLFAs), and enzymatic activities for C, nitrogen (N), and phosphorus (P) acquisition were analyzed. The lowest contents of POC (10%) and MAOC (13%) in the organic horizon were found in 7‐year‐old stands due to the slow tree regrowth and extensive decomposition of SOC in the first years of forest regrowth. POC (2.0×) and MAOC (0.8×) increases in the organic horizon with forest age were attributed to the stand development and accumulation of above‐ and belowground litter. The organic horizon had a higher POC:MAOC ratio than the mineral (0.7–1.1 vs. 0.2–0.5), indicating lower SOC stability in the first one. The ratio of POC:MAOC increased with the Gram‐positive to Gram‐negative bacteria (G+:G‐) ratio, pointing out that microbial communities developed a specific community structure and substrate utilization strategies of organic matter under plantation restoration. The increase of total PLFAs and the G+:G‐ ratio was closely linked with the microbial C and P limitations, indicating that microorganisms shifted community structure to slow‐growing species and increased their content to cope with the C and P restrictions. In the soils of young plantations, microorganisms were limited by C and P; however, the C limitation was alleviated in the 36‐year‐old plots in the organic horizon due to increased litter input, whereas the P limitation was not. This discrepancy between C and P limitation suppressed the decomposition of litter entering the soil, which was seen in decreased specific activity of C degrading enzymes and led to the accumulation of POC in the organic horizon. Thus, soil C sequestration under reforestation of Chinese fir can be controlled by the amount of litter entering the soil and by metabolic C, N, and P limitations that force microorganisms to shift community structure and change their activity.