Soil biodiversity is declining globally due to human activities and climate change, but the consequences for soil carbon cycling and carbon dioxide (CO2) emissions remain poorly understood. Here, we investigated the relationship between microbial diversity and soil CO2 flux using a microbial dilution-to-extinction approach across three land-use types (forest, grassland and cropland). We find that soil CO2 fluxes respond nonlinearly to diversity loss, increasing initially at moderate diversity loss, then declining sharply at severe loss. Several key microbial physiological properties, including microbial carbon use efficiency (CUE), nitrogen use efficiency (NUE), and turnover rate, exhibit similar hump-shaped responses to declining diversity. Linear mixed-effects models show that microbial turnover and NUE are positively correlated with soil CO2 fluxes, whereas microbial CUE and the interaction between turnover and NUE are negatively correlated with them. Structural equation modeling approaches further demonstrate that indirect effects mediated by microbial physiological properties, especially turnover rate, exert a stronger influence on soil CO2 fluxes than the direct effects of diversity loss. Together, these findings highlight the complexity of biodiversity-function relationships in soils and emphasize the need to incorporate microbial physiological properties into soil carbon cycle models in the context of global biodiversity change.

Understanding the temperature sensitivity (Q10) of soil organic matter (SOM) decomposition is essential for predicting soil carbon (C) emissions under global warming. However, how land-use change alters Q10 and the underlying microbial and biochemical mechanisms remains uncertain, largely due to complex interactions among SOM quality, microbial traits, and Q10. In this study, a total of 144 topsoil and subsoil samples were collected from paired croplands and adjacent afforested lands across eight sites in China. Continuous warming incubation experiments were conducted to determine the Q10 of SOM decomposition. Results showed that warming-induced carbon (C) mineralization was substantially higher in C-poor soils than in C-rich soils, indicating greater vulnerability of C-poor soils to C loss. In topsoil, C mineralization efficiency in croplands and afforested lands of C-poor soils was 437% and 254% higher, respectively, than that in C-rich soils. Importantly, afforestation reduced Q10 in C-poor soils by 13.2% in topsoil and 15.0% in subsoil compared with croplands. This reduction in Q10 was primarily driven by decreased soil organic carbon (SOC) recalcitrance and a lower abundance ratio of recalcitrant- to labile-C degradation genes. Climate exerted a strong influence on Q10 in topsoil but had weaker effects in subsoil. These results indicate that initial soil C status fundamentally mediates microbial and biochemical regulation of SOM decomposition under warming. Therefore, incorporating initial SOC status into ecosystem models can improve predictions of soil C dynamics under climate warming, while prioritizing afforestation in C-poor soils may help mitigate warming-induced SOC loss and support soil conservation.

Loss of soil microbial diversity is accelerating worldwide, yet how this loss alters soil greenhouse gas fluxes remains poorly understood. Here, we provide experimental evidence that diversity loss affects nitrous oxide (N2O) and methane (CH4) fluxes through nonlinear, trait-mediated pathways. Using soil microcosm dilution gradients established across three land-use types (forest, grassland, and cropland), we linked shifts in community diversity with key physiological traits: carbon use efficiency (CUE), nitrogen use efficiency (NUE), and turnover rate. Over a 118-day incubation, soil N2O flux exhibited a pronounced hump-shaped response: moderate diversity loss stimulated emissions, whereas severe loss suppressed them through the breakdown of functional redundancy. Strikingly, even moderate diversity loss reversed soils from CH4 sinks to net sources. Microbial turnover consistently emerged as the core driver of both N2O and CH4 fluxes, with additional contributions from the turnover interactions with CUE and NUE. Variance partitioning further showed that microbial physiological traits explained 62

Afforestation can substantially enhance soil organic carbon (SOC) storage, but the mechanisms governing its long-term stabilization remain poorly understood. Here, using a chronosequence (7, 20, 69 and 88 years-old) of Pinus koraiensis plantations in Northeast China, we investigated SOC dynamics by integrating soil physical fractionation, microbial biomarkers, and amplicon sequencing. SOC stock exhibited nonlinear changes, peaking at 69-year-old stands (78.49 g C kg-1 soil). Mineral-associated organic carbon (MAOC) in the organic horizon plateaued after 20 years, while MAOC in the 0-20 cm mineral soil peaked at 69 years. Between 20 and 69 years, SOC accumulation was mainly driven by particulate organic carbon (POC) inputs resulting from increased forest productivity. Microbial necromass contributed substantially to SOC, dominated by fungal necromass (78-97 %), but its relative contribution declined after 20 years as bacterial necromass and the abundance of K-strategists bacteria increased. Path analysis further showed that soil properties and microbial traits were positively related to both POC and MAOC, whereas necromass metrics were negatively related to MAOC, consistent with MAOC accumulation approaching a physicochemical capacity limit. Overall, our findings indicate that SOC peaks in 69 years emerge from a transient balance between high particulate inputs and limited additional stabilization on mineral surfaces. This temporal pattern provides critical insights for forest carbon modeling and suggests that harvesting stands before maturity may help sustain SOC stocks in temperate plantations.

Abstract The persistence of soil organic matter (SOM) is critical for predicting carbon-climate feedbacks, yet how increasing soil organic carbon (SOC) relates to long-term SOM persistence across environmental gradients remains unclear. Soil radiocarbon, SOM physicochemical composition, and microbial carbon use efficiency (CUE) are analyzed along a precipitation-driven gradient integrated with global datasets. Here we show, as SOC content increases, its persistence as reflected by radiocarbon signatures declines at both the transect and global scales. This pattern indicates that higher SOC soils are increasingly dominated by younger, faster-cycling carbon rather than older, stabilized pools. Plant carbon inputs strongly predict SOM persistence and are associated with younger, less persistent SOC. While microbial CUE increases with SOC, higher CUE does not necessarily enhance long-term stabilization. Our findings suggest that SOC content alone provides limited insight into long-term soil carbon persistence and highlight the importance of explicitly representing plant carbon inputs and microbially mediated persistence in Earth system models.

Anthropogenic nitrogen (N) deposition comprising reduced (NHx) and oxidized (NOy) forms, differentially influences ecosystem processes. However, their long-term fates and collective impacts on ecosystem nitrogen use efficiency (NUE) and carbon (C) sequestration remain unresolved. Here, using a decade-long paired labeling (15NH4+ and 15NO3-) experiment in a temperate forest, we demonstrate that initially distinct fates of different N forms converged within 2 y due to low initial loss rates prior to organic incorporation. After 10 y, overall ecosystem retention was remarkably similar (55.3 to 65.6%) for both N forms. A process-based N-cycling model was built and parameterized by our 15N recovery data and the model predicted declining ecosystem NUE with increasing N deposition level. We further identified a deposition-induced C sequestration potential of 41.41 kg C kg-1 N under an incremental deposition of 10 kg N ha-1 y-1, exceeding prior estimates at comparable loads. This study provides mechanistic, high-resolution temporal data critical for refining N-cycle models and reveals an optimal N deposition range (20 to 30 kg N ha-1 y-1) that maximizes C sequestration while minimizing environmental risks (e.g., nutrient imbalance, eutrophication) in temperate forests.

Changes in snow cover significantly influence biogeochemical carbon and nitrogen processes in seasonal snowpack ecosystems. However, a comprehensive understanding of the response patterns of soil and microbial carbon and nitrogen dynamics to changes in snowpack remains limited. In this study, we conducted a meta-analysis of 4324 observations from 128 snow manipulation experiments, covering 21 variables. Our results revealed that snow removal significantly decreased microbial respiration (MR), as well as CH4 and CO2 emissions, while significantly increasing soil dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and N2O emissions. Conversely, snow addition positively impacted microbial biomass carbon (MBC) and nitrogen (MBN), MR, CO2 emissions and soil dissolved inorganic nitrogen (DIN), but negatively influenced N2O emissions. The magnitude and direction of these responses were found to be significantly modulated by treatment duration and intensity, ecosystem type, edaphic and climatic conditions. This study provides crucial insights into the mechanisms underlying the effects of snowpack changes on the biogeochemical cycling of carbon and nitrogen in terrestrial ecosystems, particularly in the context of climate change.

Microbial carbon and nitrogen use efficiencies (CUE and NUE) are critical regulators of soil carbon and nitrogen cycling, with their temperature sensitivities playing a pivotal role in mediating biogeochemical feedbacks under global warming. However, how the temperature sensitivity (Q10) of CUE and NUE varies at different temperature ranges and whether their thermal responses are coordinated remains poorly understood. Here, we quantified the Q10 of CUE and NUE in 55 soil samples collected from a ~4000 km latitudinal forest transect in eastern China. We further identified key drivers that shaped Q10 variability from climatic, edaphic, and microbial factors. On average, Q10 was 1.22 ± 0.08 for CUE and 1.46 ± 0.13 for NUE. However, both efficiencies exhibited clear temperature-interval dependence: the mean Q10 of CUE declined from 1.47 ± 0.14 at 12°C-20°C to 0.97 ± 0.08 at 20°C-28°C, while the mean Q10 of NUE decreased from 2.00 ± 0.23 to 0.93 ± 0.09. The Q10 values of CUE and NUE were strongly correlated across temperature ranges and positively associated with the Q10 of microbial growth, indicating a coordinated thermal response governed primarily by growth-based processes. At lower incubation temperature interval (12°C-20°C), variation in the Q10 of CUE was primarily explained by soil stoichiometry and microbial community attributes, whereas under warmer conditions (20°C-28°C), climatic and edaphic constraints, particularly precipitation and soil N/P ratio, became dominant. Although microbial community attributes consistently explained most of the variance in the Q10 of NUE, their influence weakened at higher incubation temperatures, paralleling the pattern observed for CUE and indicating a shift from biotic to abiotic control. Overall, these findings highlight that the temperature sensitivities of microbial CUE and NUE are tightly coupled, growth-mediated, and strongly temperature-context dependent, providing novel insights for improving predictions of soil carbon-nitrogen turnover under climate warming.

Microbial necromass is increasingly recognized as a major source of stable soil organic matter (SOM), and its persistence is often attributed to interactions with clay-sized minerals. However, the mechanisms underlying this mineral-mediated stabilization remain poorly understood. Here, we conducted an in situ dual-labeled (13C and 15N) microbial necromass experiment across a clay gradient to quantify how clay content and necromass origin (bacterial vs. fungal) regulate necromass persistence. We find that higher clay content markedly enhances necromass retention by strengthening mineral protection, suppressing microbial activity and diversity, and limiting leaching losses. NanoSIMS imaging shows that new necromass preferentially associates with organic matter coatings on the rough mineral surfaces, highlighting organo-organic interfaces as important stabilization pathways. Necromass origin exerts little effect on retention despite marked differences in C:N ratios and bulk chemical composition, indicating that finer-scale molecular features, rather than broad compositional differences, govern necromass stabilization in soils.

Stable isotope probing (SIP) using O-18-enriched water (O-18-H2O) is widely used to quantify microbial growth and activity, yet this approach relies on the assumption that O-18 enrichment is biologically inert. If enrichment itself alters microbial community composition or functions, SIP-derived estimates may be biased. Here, we exposed forest soil to a continuous O-18-H2O enrichment gradient (0.2-80 atom% O-18) to determine whether isotope enrichment altered microbial community composition, functional gene profiles, respiration, and growth, and to identify enrichment levels suitable for SIP applications. Approximately 6% of bacterial taxa, accounting for similar to 50% of community abundance, responded markedly to the enrichment gradient. These responsive taxa exhibited distinct breakpoints, with negatively and positively responding groups shifting at similar to 25 and similar to 40 atom% O-18, respectively. Functional gene profiles showed similar but slightly higher thresholds, with negative and positive responses emerging at similar to 40 and similar to 45 atom% O-18, respectively. Microbial respiration displayed a threshold at similar to 22 atom% O-18, whereas microbial growth increased modestly but showed no clear threshold response across the gradient. Together, these results reveal coordinated but process-specific threshold responses of microbial community composition, functional genes profiles, and physiological processes to O-18-H2O enrichment. Although no single enrichment level is universally suitable, enrichment above 40 atom% O-18 is likely to induce substantial ecological perturbations and should therefore be used with caution in SIP experiments.

The extent to which microbial processes control soil organic carbon (SOC) dynamics remains uncertain. Carbon use efficiency (CUE), that is, the fraction of assimilated carbon allocated to growth, has been used as a key parameter but its relationship with SOC reflects carbon partitioning rather than the absolute magnitude of microbial fluxes. The microbial growth rate could provide a more mechanistic link to SOC accumulation because it quantifies biomass production and reflects necromass formation. Here we combine a global ¹⁸O-H2O dataset (n = 268 paired observations) with outputs from four land surface models to test whether growth rate predicts SOC more strongly than CUE. In the incubation experiments, growth rates are more closely associated with SOC than CUE, although soil properties and climate explain equal or greater variance. Models reproduce the stronger role of growth rate over CUE but tend to underestimate the abiotic controls. The models also emphasize CUE as the main predictor of the SOC-to-net primary production ratio, in contrast to observations, which indicates the soil's capacity to retain plant carbon inputs. Together, these findings identify the microbial growth rate as a diagnostic that can help bridge models with empirical data and guide a more balanced representation of microbial and mineral controls in SOC projections.

Soil organic carbon (SOC) sequestration through vegetation restoration may be hindered by increased soil carbon emissions under global warming. Soil microbial carbon use efficiency (CUE) and priming effects (PEs) are crucial to SOC dynamics. Here we selected croplands and forests restored from croplands 40 years ago to evaluate the simultaneous effects of temperature and vegetation restoration on microbial CUE and PEs. Microbial CUE was measured using O-18-labeled water, C-13-labeled glucose, C-13-labeled litter, and stoichiometric modeling methods, respectively. Glucose and litter were used as labile and complex substrates, respectively, to study how substrate quality affects CUE and PEs. Our results showed that microbial CUE estimated by the four methods ranged from 0.10 to 0.84, with the lowest and the highest CUE being estimated by the O-18-method (0.10 similar to 0.21) and C-13-method (0.50 similar to 0.84), respectively. All methods showed restoration reduced the ratio of soil dissolved organic carbon to dissolved nitrogen, implying increased substrate quality and thus microbial CUE. Microbial CUE declined with rising temperature, with greater declines in forests (-31 %) than that in croplands (-19 %) when estimated by the O-18-method. Positive PEs were observed and dominated by the nitrogen-mining mechanism. Vegetation restoration alleviated microbial nitrogen limitation and reduced relative PEs. Glucose-induced absolute PEs increased but litter-induced absolute PEs decreased with rising temperature due to higher nitrogen limitation with glucose addition compared to litter addition. The relative PEs declined with rising temperature under either glucose or litter addition, with greater declines in croplands (-45 %similar to-81 %) than in forests (-40 %similar to-70 %). Overall, vegetation restoration contributes to SOC accumulation by increasing microbial CUE and decreasing PEs, but warming would counteract the positive effects of forests and be detrimental to increasing SOC sequestration through vegetation restoration. Understanding the ecosystem-specific response of microbial CUE and PEs to climate change is urgently needed to identify ecosystem management that benefits soil carbon sequestration.

Microbial nitrogen (N) use efficiency (NUE) is crucial for retaining N in soils and supplying N to plants. However, how soil microbial NUE in N-limited dryland responds to aridity remains poorly understood. Here we used 18O and 15N isotope labeling techniques to investigate the effects of climatic, edaphic, and biotic factors on microbial N metabolism along a 2200 km aridity gradient on the Tibetan Plateau. We found soil microbes could enhance their NUE to cope with N limitation, but this ability was hindered when aridity index (AI) < 0.12 (extremely dry conditions) where water limitation directly inhibited microbial growth. As water limitation weakened (AI > 0.12), microbes increased their NUE with decreasing aridity because they got more limited by N and temperature. The increase of microbial NUE increased microbial necromass N, contributing to the increase of soil total N. Our findings provide new insights into microbial N use strategies under water- and N-limited conditions and their vital role in N retention in ecosystems. This helps to deploy microbial potential in conserving N in soils for higher productivity and to better predict soil N processes under global changes.

Soil food webs regulate microbial biomass and necromass production and are therefore critical for carbon sequestration. The mechanisms by which top predators regulate microbial necromass formation across multitrophic levels in the real-world soil food web remain nearly unknown. This study investigates how top-down forces-from omnivorous-predaceous nematodes to microbivorous nematodes and microbes-affect the formation of microbial necromass within tritrophic food webs under contrasting tillage regimes (tillage (till) vs. no-tillage (no-till)) on black soils (Mollisols), using a 1-year 13C-labeled straw in situ tracing experiment integrated with a long-term (> 5 years) tillage trial. The fungal-to-bacterial necromass ratio increased strongly in the no-till soil compared to the till soil, with omnivores-predators being the key factor for these changes. In the no-till soil, abundant and diverse omnivores-predators (46% and 67% higher in abundance and richness than in the till soil) created a typical predator-prey relationship with fungivores. This relationship was characterized by heavy predation on fungivores (51% of omnivore-predator diet) and opposite 1-year dynamics of 13C content between omnivores-predators and fungivores. Such a predator-prey relationship substantially reduced fungivore activity (73% and 90% decrease in 13C content and enrichment rate), while accompanied by increased fungal activity (64% and 50% increase in 13C content and enrichment rate) in the no-till soil compared to the till soil. This predator-driven cascade down the food chain amplified the fungal contribution to the fungal-to-bacterial necromass ratio. Conversely, these interactions, disrupted by continuous tillage, weakened fungal functions by interrupting the trophic cascade. In conclusion, these tiny yet ubiquitous omnivorous-predaceous nematodes exert a disproportionate impact on necromass formation by boosting fungal biomass and activity. Further manipulative experiments targeting multi-trophic interactions are essential to disentangle the mechanisms of microbial necromass formation, given the inherent complexity of soil food webs and the observational nature of this study.

Gaseous nitrogen (N) losses from nitrification and denitrification (NO + N 2 O + N 2 ) pathways contribute a significant fraction of the total N losses from cropland ecosystems. The N mass balance and process‐based models are commonly applied to estimate the NO + N 2 O + N 2 losses but have suffered from systematic error accumulations or model over‐parameterization, leading to a large uncertainty in estimation, hindering effective management of the global N budget. Here, we proposed a novel N isotope model, which considers fertilizer, ammonia volatilization and harvest after testing steady‐state assumption of soil δ 15 N and N pool for croplands, and justified if it could be successfully applied to constrain NO + N 2 O + N 2 losses from cropland ecosystems. We compiled the first bulk‐soil δ 15 N data set of 0–30 cm soils ( n = 738) from croplands and produced a global map of cropland soil δ 15 N, which is crucial input data for an isotope model to quantify NO + N 2 O + N 2 losses. The results show that the cropland soil δ 15 N ranges from 3.5 to 9.0‰, with a mean value of 6.6 ± 0.8‰ (mean ± standard deviation). The estimated NO + N 2 O + N 2 losses accounted for an average of 17 ± 9% of N outputs and were 35.86 ± 24.17 kg N ha −1 yr −1 in China's rice paddies, with an increasing trend from Central China to South or North China. The estimations were comparable with the results from observation‐constrained denitrification‐decomposition modeling (38.9 ± 4.8 kg N ha −1 yr −1 ) and in good agreement with experimental observations at site scale ( R 2 = 0.58). Our results suggest that soil N isotopes, as a quantitative tracer, provide a valuable alternative approach to constrain the NO + N 2 O + N 2 losses in croplands at large geographic scales.

Litter decomposition is an important process of nutrient cycling and is primarily driven by microbes. However, whether the microorganisms in decomposing litter come from the phyllosphere or soil is still unclear. In this study, we collected litter of two dominant species in a temperate forest, Fraxinus mandshurica and Pinus koraiensis (newly shed, and decomposing litter in the early-stage and late-stage) and surrounding soil to approach this question. Our results suggested that in the early-stage of decomposition, phyllosphere bacteria utilized readily available substances, preempting the niche of soil bacteria, while soil fungi were able to invade the litter through hyphae and spores. These activities further modified the ecological niche in the decomposing litter, facilitating the subsequent entry of soil bacteria and fungi. The timing of soil microbial invasion was influenced by litter quality. In the low-quality litter, the resource limitation hindered the entry of soil microorganisms and consequently slowed down the decomposition process. These findings offer crucial insights for better understanding of the litter decomposition process during which substantial carbon is lost from the ecosystem.

Although Vegetation Restoration Programs (VRPs) on the Loess Plateau, China, have significantly improved the region’s ecological condition, their impact on the local economy and agriculture remain unclear. Here we used the difference-in-differences analysis to quantify the effects of the VRPs on population, economic, and agricultural aspects. Results suggest that the implementation of the VRPs increased mean county-based Gross Domestic Product by 148 % and per capita grain production by 30 %, but decreased rural labor resources by 11 %. VRPs promoted the transfer of population to the secondary industry and increased the income of local farmers. We predict that grain production will likely start to decline when the restoration area exceeds approximately 55 % of the total county area in the future. Our study suggests that while VRPs on the Loess Plateau are economically sustainable, their expansion beyond a certain threshold could jeopardize agriculture.

Soil dissolved organic matter (DOM) is a critical reservoir of carbon and nutrients in forest ecosystems, playing a central role in carbon cycling and microbial community dynamics. However, the influence of DOM molecular-level diversity (chemodiversity) on microbial community diversity and spatial distribution remains poorly understood. In this study, we used Fourier transform ion cyclotron resonance mass spectrometry and high-throughput sequencing to analyze soil DOM and microbial diversity along a ~4,000 km forest transect in China. We found that soil DOM chemodiversity varies significantly across sites, initially increasing and then decreasing with latitude. Additionally, we observed that the biogeographic distribution of DOM components has differential effects on bacterial and fungal diversity: lipid-like compounds are strongly associated with bacterial diversity, while aromatic-, carbohydrate-, and lipid-like compounds primarily influence fungal diversity. Linear models and structural equation modeling both reveal that DOM acts as a key intermediary, mediating the effects of temperature and soil properties on microbial spatial distribution. Our findings emphasize the importance of DOM molecular characteristics in shaping microbial community structure and functioning, providing new insights into how environmental factors influence microbial ecosystems and soil carbon cycles in forest ecosystems.

A series of vegetation restoration programs (VRPs) have been implemented on the Loess Plateau, China to increase vegetation cover and soil organic carbon (SOC) content. Here, we used a Coupled Litter-Soil (CLS) model to estimate the spatiotemporal distribution of SOC change (0-100 cm) on the VRPs areas on the Loess Plateau from the vegetation restoration starting to 2100. The vertical transport process of SOC along the profile was considered in the model. Due to the starting time of VRPs varied across different areas on the Loess Plateau, we detected the implementation areas and starting times of VRPs from remote sensing data to constrain the model. Results indicate that between the restoration starting and the year 2022, VRPs areas increased a total of 0.8 Pg C of SOC (0-100 cm), showing an increasing trend of SOC from northwest to southeast on the Loess Plateau, China. Under the business-as-usual scenario (2022), our model predicts that SOC will reach a steady-state after 150 years of VRPs implementation, with a potential SOC storage of 2.9 Pg C (0-100 cm). The differences in SOC density among different future climate scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) were nonsignificant. Our study provides a platform for combining remote sensing techniques and a process-based model to better estimate and predict SOC change after the implementation of an ecological restoration program.