Warmer soil temperatures influence belowground plant allocation, root turnover, and soil organic carbon decomposition. However, the fate of root-derived carbon with depth and temperature remains uncertain. This study traced 13C-labeled Avena fatua root litter incubated at three depths (10-14 cm, 45-49 cm, 85-89 cm) in a temperate conifer forest, where plots have been warmed by 4 degrees C to 100 cm depth since 2014. We recovered the added root carbon in soil organic matter (SOM) density fractions and microbial phospholipid fatty acids (PLFAs) after 1 and 3 years of incubation in situ. Root carbon recovery was similar across all soil depths and warming treatment, and was increasingly recovered in the soil over time, driven by fivefold increase in the occluded SOM fraction. Root addition increased fungal abundance by 3.7 percentage points across all depths, with the highest increase in fungal and Gram-negative bacterial relative abundance at 85-89 cm. By alleviating subsoil substrate limitation, root litter inputs promoted microbial groups that are typically constrained at depth. Despite this, soil warming reduced total microbial PLFAs at 45-49 cm by 5.4 percentage points and fungal relative abundance by 0.7 percentage points across all depths, likely due to a 32-18% decrease in soil moisture with warming, which can limit microbial activity. These results indicate that added root carbon is highly decomposable in the subsoil when substrate and moisture availability are not limiting, and highlight the essential role that fungi play in root litter decomposition throughout the soil profile.

Abstract. Field-warming experiments offer insight into the response of ecosystems to rising temperatures, but cross-site comparison is needed to determine both the general tendencies of warming responses and the context dependencies of deviations from those norms. These responses are not limited to the direct effects of temperature but also their indirect effects on soil moisture, a critical factor controlling ecosystem productivity and carbon fluxes. Here we introduce SWEDDIE: the first database to characterize the whole soil profile warming response across 26 distinct soil warming experiments, encompassing forest, grassland, cropland, tundra, and wetland ecosystems. SWEDDIE is needed because prior databases and syntheses of warming effects on ecosystems were dominated by aboveground warming studies, many of which warmed soil modestly or negligibly during much of the growing season and reported only growing season averages. We demonstrate the potential of the SWEDDIE database by quantifying soil temperature and moisture changes for each experiment as a function of depth, warming methodology, ambient climate conditions, and ecosystem, as well as the relationship between soil moisture and imposed warming. Warming attenuated with depth at sites with aboveground warming only but increased with depth at sites with belowground warming only, as hypothesized. Warming led to soil drying at most sites, and drying was positively correlated with the magnitude of warming. However, the relationship between soil warming and soil drying varied by ecosystem: forest soils dried the most, while tundra soils became wetter with warming. Ambient climatic conditions also significantly influenced the relationship between experimental warming and drying, with more drying per degree of warming observed in soils with higher ambient moisture. The inconsistency of soil moisture changes with warming across ecosystems and warming methodologies demonstrates the importance of quantifying shifts in temperature and moisture in both space and time in order to overcome site-specific bias in ecosystem warming responses. The high temporal resolution and depth-resolved observations of the fundamental ecosystem properties of soil temperature and moisture in SWEDDIE v1.0.0 serve as a foundation for future experimental soil warming synthesis efforts and demonstrate the power of this actively growing community resource.

Global warming could potentially increase the decomposition rate of soil organic matter (SOM), not only in the topsoil (<20 cm) but also in the subsoil (>20 cm). Despite its low carbon content, subsoil holds on average nearly as much SOM as topsoil across various ecosystems. However, significant uncertainties remain regarding the impact of warming on SOM decomposition in subsoil, particularly root-derived carbon, which serves as the primary organic input at these horizons. In a whole-soil field warming experiment at Blodgett Forest Research Station (California, USA), we investigated whether warming accelerates the decomposition of root-derived hydrolysable lipids in the top- (10–14 cm) and subsoil (45–49, 85–89 cm) by using molecular markers and in-situ incubation of 13C-labeled root litter at each depth. Our results reveal that at compound-class level, hydrolysable lipids presented compound-dependent responses. Warming consistently reduced fatty acid mass change across soil depths, particularly at 85–89 cm. In subsoil, there was accumulation of fatty acids, which primarily originated from microbial-derived mid-chain fatty acids such as octadecanoic acid (C18:0 fatty acids), octadecenoic acid (C18:1 fatty acids), and hexadecanoic acid (C16:0 fatty acids). Higher temperature attenuated this accumulation, indicating less microbial transformation of root-derived carbon under warming. At monomer level, ω-hydroxy acids and diacids as suberin markers were more resistant to decomposition than bulk root-derived carbon and their resistance increased with chain-length. Moreover, warming accelerated decomposition of individual suberin monomers in the topsoil but suppressed it in the subsoil. The slower decomposition in the subsoil was likely due to lower microbial abundance and lower soil moisture induced by warming. Our study demonstrates that the impact of warming on the decomposition of root-derived hydrolysable lipids in a temperate forest is compound class- and depth-dependent. The persistence of long-chain ω-hydroxy acids and diacids may provide a potential way for long-term carbon stabilization in subsoil under climate change. Nevertheless, due to the substantial heterogeneity of subsoil environment, further studies are required to confirm and generalize this finding.

ABSTRACTIntroductionImproving soil health while maintaining crop yield is a key challenge for farmers. So far, only a few studies assessed the effects of compost and solid digestate application on soil health and plant yield under practical on‐farm conditions across both organic and conventional cropping systems.Materials and MethodsThis study examined 56 arable fields in Switzerland, managed either conventionally (n = 39) or organically (n = 17) by individual farmers. Fields were categorised based on their fertilisation history: standard fertilisation (n = 21), including livestock manure, slurry, and mineral fertilisers (reference), or with additional compost (n = 26) or solid digestate (n = 9) amendments. Soil health was assessed based on eight chemical, biological, and physical soil health indicators.ResultsCompost use, but not solid digestate use, was associated with enhanced average soil health ( + 31% over reference fields), driven by increases in basal respiration ( + 45%), cation exchange capacity ( + 42%), fungal richness ( + 18%), and marginally higher soil organic carbon stocks ( + 28%). These differences were consistent across management systems, despite site variability. Clay content and extended periods of crop cover also positively influenced soil health. Wheat yields were 21% lower under organic management but unaffected by compost or digestate use.ConclusionThese findings suggest that using compost alongside practices like extended periods of crop cover can effectively promote soil health while maintaining yields in practical farming scenarios, offering a means to balance multiple sustainability goals simultaneously.

Global warming and increasing air temperatures also result in rising soil temperatures. Although acceleration of soil organic carbon cycling can be expected, the order of magnitude and speed of adaptation of carbon cycling to warming still remains largely unknown. This is especially crucial in boreal peatlands, where large reserves of terrestrial carbon are stored and these systems are known for their vulnerability to environmental changes. We investigated the organic matter composition in the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment, where a boreal peatland was exposed to temperatures of up to +9°C and increased CO2 concentration compared to control conditions in open top chambers. A broad set of molecular markers (e.g., free extractable and bound lipids, lignin, benzene polycarboxylic acids) was used to trace incorporation and cycling of organic matter in the peat profile down to three meters depth four years after the start of the experiment. A strong response to increasing temperature was observed in the plant, microbial and peat chemical composition, the latter mainly in the acrotelm (0-30 cm) and partially also in the mesotelm (30-70cm). The response of the plant chemical composition was species-specific with the exception of nitrogen concentrations that increased for all plants. This is related to the stronger degradation of peat organic matter and thus increasing availability of nitrogen with rising temperature. All investigated molecular markers indicated a very fast response of carbon cycling in the whole acrotelm of the peat profile. This resulted from a dropping water table and thus more oxic conditions in the peat, which further enabled increasing shrub and tree root growth and increasing microbial abundance and activity. As a consequence of the more aerobic conditions, not only the comparatively easily degradable free extractable lipids, but also slow cycling polymeric substances such as suberin/cutin, lignin, and benzene polycarboxylic acids rapidly degraded and reflect an unexpectedly fast cycling of organic matter in the boreal peatland with increasing temperature. The acceleration of carbon cycling within the peatland with rising temperature is also reflected by the partial uptake of respired CO2 by the plants as indicated by the bulk and compound-specific d13C composition of the plants. Overall, our results illustrate the fast alteration of organic matter cycling in a boreal peatland when exposed to increasing temperature.

Calcium (Ca) may contribute to the preservation of soil organic carbon (SOC) in more ecosystems than previously thought. Here, we provide evidence that Ca is co-located with SOC compounds that are enriched in aromatic and phenolic groups, across different acidic soil types and locations with different ecosystem properties, differing in terms of climate, parent material, soil type, and vegetation. In turn, this co-localised fraction of Ca–SOC is removed through cation exchange, and the association is then only re-established during decomposition in the presence of Ca (Ca addition incubation). Thus, this highlights a causative link between decomposition and the co-location of Ca with a characteristic fraction of SOC. Decomposition increases the relative proportion of negatively charged functional groups, which can increase the propensity for the association between SOC and Ca; in turn, this association can inhibit dissolved organic carbon export or further decomposition. We propose that this mechanism could be driven by Ca hotspots at the microscale shifting local decomposition processes and thereby explaining the co-location of Ca with SOC of a specific composition across different acidic soil environments. Incorporating this biogeochemical process into Earth system models could improve our understanding, predictions, and management of carbon dynamics in soils, as well as accounting for their response to Ca-rich amendments.

Understanding factors influencing carbon effluxes from soils to the atmosphere is important in a world experiencing climatic change. Two important uncertainties related to soil organic carbon (SOC) stock responses to a changing climate are (a) whether soil microbial communities acclimate or adapt to changes in soil temperature and (b) how to represent this process in SOC models. To further explore these issues, we included thermal adaptation of enzyme-mediated processes in a mechanistic SOC model (ReSOM) using the macromolecular rate theory. Thermal adaptation is defined here to encompass all potential responses of soil microbes and microbial communities following a change in temperature. To assess the effects of thermal adaptation of enzyme-mediated processes on simulated SOC losses, ReSOM was applied to data collected from a 13-year soil warming experiment. Results show that a model omitting thermal adaptation of enzyme-mediated processes substantially overestimates observed CO2 effluxes during the initial years of soil warming. The bias against observed CO2 effluxes was lower for models including thermal adaptation of enzyme-mediated processes. In addition, for a simulated linear 3 degrees C soil warming over 100 years, models including thermal adaptation of enzyme-mediated processes simulated SOC losses of a factor of three smaller than models omitting this process. As thermal adaptation of microbial community characteristics is generally not included in models simulating feedback between the soil, biosphere and atmosphere, we encourage future studies to assess the potential impact that microbial adaptation has on soil carbon - climate feedback representations in models.

The spread of antimicrobial resistance (AMR) in agricultural systems via irrigation water is a serious public health issue as it can be transmitted to humans through the food chain. Therefore, understanding the dissemination routes of antibiotic resistance genes (ARGs) in agricultural systems is crucial for the assessment of health risks associated with eating fresh vegetables such as spinach and radish irrigated with treated municipal wastewater (TMW). In this study, we investigated the bacterial community structure and resistome in the soil -plant -earthworm continuum after irrigation of spinach and radish with TMW containing the antibiotics trimethoprim (TMP), sulfamethoxazole (SMZ), and sulfapyridine (SPD) using 16S rRNA gene sequencing and high throughput quantitative PCR (HT-qPCR). The study was conducted in two phases: Phase I involved eight weeks of spinach and radish production using TMW for irrigation, whereas Phase II entailed three weeks of earthworm exposure to contaminated plant material obtained in Phase I. The 16S data indicated that the rhizosphere bacterial community composition and structure were more resilient to antibiotic residuals in the irrigated water, with radish showing less susceptibility than spinach than those of bulk soils. The HT-qPCR analysis revealed that a total of 271 ARGs (out of 285) and 9 mobile genetic elements (MGEs) (out of 10) were detected in all samples. Higher diversity and abundance of ARGs were observed for samples irrigated with higher concentrations of antibiotics in both spinach and radish treatments. However, compared to spinach, radish ARG dynamics in the soil biome were more stable due to the change of antibiotic introduction to the soil. At the class level, multi -drug resistance (MDR) class was altered significantly by the presence of antibiotics in irrigation water. Compared to earthworm fecal samples, their corresponding soil environments showed a higher number of detected ARGs, suggesting that earthworms could play a role in reducing ARG dissemination in the soil environments. These findings will not only provide insight into the dissemination of ARGs in agricultural environments due to antibiotic residuals in irrigated water but could help understand the potential human health risks associated with ARGs.

How do soils respond to warming temperatures? The importance of soils in the global carbon cycle and as hotspots of biogeochemical processes in terrestrial ecosystems underscores the imperative of understanding this response. Soil warming experiments have proved to be a key tool for probing the mechanisms underlying warming responses. However, climate, mineralogy, flora, fauna, and methodology specific to each experimental site hamper efforts to generalize and upscale these findings. The DeepSoil 2100 project was initiated to synthesize data from soil warming experiments worldwide through the creation of a harmonized database (SWÆDIE, the Soil WArming Experiment Data Integration Effort). SWÆDIE emphasizes experiments in which soils have been warmed ≥ 1 m, and will enable us to explore depth-dependence and coupling between above and belowground processes, assess feedbacks and interactions between C stocks, nutrients, and soil moisture, compare short versus long-term warming responses, and identify global patterns. Collaborative projects such as SWÆDIE require establishing clear guideline for data sharing and attribution of credit, for which we are building on the models provided by Ameriflux and NutNet. We have also drawn from other soil carbon-focused synthesis efforts such as ISRaD, SoDaH, and ISCN to construct a transparent and flexible data model with a user-friendly data access interface. Data are organized hierarchically, with a static site-level table and dynamic subordinate data tables, e.g., time series of fluxes, moisture, and temperature, resolved by depth. We maintain raw data files that are harmonized in a scripted data entry pipeline with the aid of separate metadata files describing variable names and units. Such an approach facilitates new data ingestion while also ensuring reproducibility and transparency. We will present the results from the initial site characterization, including quantification of heating efficacy and the relationship to changes in soil moisture with depth and across sites. This initial site characterization will also allow us to compare data coverage and define the scope of soil, climatic, and vegetation gradients across the database. On the basis of this comparison, we will present plans for coordinated future sampling. Finally, we will present initial work on establishing improved metrics for model benchmarking, i.e., which modellable response variables are both sensitive and robust when measured across sites?

Carbon-rich peat soils have been drained and used extensively for agriculture throughout human history, leading to significant losses of their soil carbon. One solution for rewetting degraded peat is wet crop cultivation. Crops such as rice, which can grow in water-saturated conditions, could enable agricultural production to be maintained whilst reducing CO2 and N2O emissions from peat. However, wet rice cultivation can release considerable methane (CH4). Water table and soil management strategies may enhance rice yield and minimize CH4 emissions, but they also influence plant biomass allocation strategies. It remains unclear how water and soil management influences rice allocation strategies and how changing plant allocation and associated traits, particularly belowground, influence CH4-related processes. We examined belowground biomass (BGB), aboveground biomass (AGB), belowground:aboveground ratio (BGB:ABG), and a range of root traits (root length, root diameter, root volume, root area, and specific root length) under different soil and water treatments; and evaluated plant trait linkages to CH4. Rice (Oryza sativa L.) was grown for six months in field mesocosms under high (saturated) or low water table treatments, and in either degraded peat soil or degraded peat covered with mineral soil. We found that BGB and BGB:AGB were lowest in water saturated conditions where mineral soil had been added to the peat, and highest in low-water table peat soils. Furthermore, CH4 and BGB were positively related, with BGB explaining 60% of the variation in CH4 but only under low water table conditions. Our results suggest that a mix of low water table and mineral soil addition could minimize belowground plant allocation in rice, which could further lower CH4 likely because root-derived carbon is a key substrate for methanogenesis. Minimizing root allocation, in conjunction with water and soil management, could be explored as a strategy for lowering CH4 emissions from wet rice cultivation in degraded peatlands.

The long-standing assumption that aboveground plant litter inputs have a substantial influence on soil organic carbon storage (SOC) and dynamics has been challenged by a new paradigm for SOC formation and persistence. We tested the importance of plant litter chemistry on SOC storage, distribution, composition, and age by comparing two highly contrasting ecosystems: an old-growth coast redwood (Sequoia sempervirens) forest, with highly aromatic litter, and an adjacent coastal prairie, with more easily decomposed litter. We hypothesized that if plant litter chemistry was the primary driver, redwood would store more and older SOC that was less microbially processed than prairie. Total soil carbon stocks to 110 cm depth were higher in prairie (35 kg C m-2) than redwood (28 kg C m-2). Radiocarbon values indicated shorter SOC residence times in redwood than prairie throughout the profile. Higher amounts of pyrogenic carbon and a higher degree of microbial processing of SOC appear to be instrumental for soil carbon storage and persistence in prairie, while differences in fine-root carbon inputs likely contribute to younger SOC in redwood. We conclude that at these sites fire residues, root inputs, and soil properties influence soil carbon dynamics to a greater degree than the properties of aboveground litter.

Understanding the changes in soil organic carbon (SOC) storage and chemical stabilization dynamics is important for accurately predicting ecosystem C sequestration and/or potential C loss, but the relevant information, especially for the intervention of environmental controls on grassland soil is limited in Tibetan plateau regions. Here we used a 9-year two-way factorial experiment involving warming with open top chambers (+1.80 °C in the daytime and +0.77 °C in the nighttime at the soil surface) and multilevel nitrogen (N) enrichment treatments (0, 5, 10, and 15 g m-2 year-1) in the Tibetan plateau to investigate the changes in SOC pool size and chemical structure. 9-year warming treatment significantly decreased SOC stock in the Tibetan grassland. We observed decreasing SOC concentrations which may be related to changes in the C-degrading enzymes. Surprisingly, the SOC molecular structure remained unchanged in all N enrichment and warmed plots, suggesting that both treatments had affected all forms of SOC, from simple and complex polymeric in a similar way. Our results suggest that long-term warming stimulates soil C loss but no preference in SOC loss with different chemical structure.

Peatlands are an important global carbon (C) reservoir storing at least one-third of global soil organic carbon (SOC), but little is known about the stability of these vast C stocks under climate change. Here, we examine the impact of four years of warming (+0, +2.25, +4.5, +6.75, +9 °C) and two years of elevated atmospheric CO2 concentration (eCO2) on the molecular composition of SOC to infer SOC sources (microbe-, plant- and fire-derived) and stability in a boreal peatland. We show that while warming alone decreased plant- and microbe-derived SOC due to enhanced decomposition, warming combined with eCO2 increased plant-derived SOC compounds. Further, using biopolymers distinct to either leaf/needle (cutin) or root (suberin), we observed increasing root-derived inputs and declining leaf-derived C inputs into SOC under warming and eCO2. Unsurprisingly, SOC derived from historical pyrolysis (pyrogenic C) was unaffected by warming or eCO2. The decline in SOC compounds with warming and gains from new root-derived C under eCO2, suggest that warming and eCO2 may shift peatland C budget towards pools with faster turnover. Together, our results indicate that climate change may increase inputs and enhance decomposition of SOC potentially destabilising C storage in peatlands.

Anoxic wetland soils are the biggest natural source of methane (CH4) globally. Climate-driven changes to soil moisture regimes are expected to alter wetland CH4 production, consumption, and transport in a variety of different ways. At the same time, moisture changes also influence plants and their traits, especially belowground. Wetland plants provide key carbon substrates for methanogenesis, and transport CH4 to the atmosphere but also oxygen into the soils, which can promote CH4 consumption. We tested hypotheses on these complex abiotic and biotic interactions to understand how belowground plant traits influence net CH4 emissions in wetlands. Specifically, we address the following questions 1) which root traits are most important for wetland CH4 processes? 2) how does water table manipulation influence the link between root biomass and CH4? 3) how does soil moisture influence the amount of root-derived carbon emitted as CH4? First, using a literature review, we developed a conceptual framework describing root traits that would be most related to CH4 processes, highlighting trait categories regulating CH4 substrate provision and transport. Second, using a field manipulation in an experimental rice system, we found that root biomass and CH4 emissions are positively linked, but only under low moisture conditions. Lastly, in a lab incubation, we found that the amount of root-derived C-CH4 increases with increased fresh root litter, in water saturated peat soils. Overall, our studies identify root traits and their moisture interactions that should be considered in wetland CH4 measurements and models.

Background: Malignant melanoma is the most common form of oral cancer in canines with a median survival of 3 months for dogs with stage III or IV disease. Currently, there are limited effective systemic treatment options for these patients with advanced disease. We have developed an anchored immunotherapy approach in which canine interleukin-12 is stably linked to aluminum hydroxide (cANK-101). The anchored IL-12 forms a stable, functional depot of IL-12 and is expected to increase therapeutic responses with limited systemic toxicity. We report the preliminary results of an exploratory Phase I study of cANK-101 in dogs with advanced melanoma. Methods: The clinical study was approved by the University of Illinois IACUC, conducted at the College of Veterinary Medicine, and all pet owners provided written informed consent. The primary objective of the trial was to determine the safety and tolerability of cANK-101 in dogs with advanced melanoma. A standard 3+3 dose-escalation design was used with three dose levels (1, 3, and 10 µg/kg) cANK-101 given by intratumoral injection every three weeks for 4 cycles. In the absence of overt clinical progression, dogs were allowed to receive a second course of four cycles. Dogs were monitored for adverse events via VCOG-CTCAE and clinical responses were measured using RECISTv1.1. Serum was collected for pharmacokinetic (PK) and immunogenicity analyses. In addition, serial tumor biopsy and lymph node cytology were performed. Serum cytokines were assayed via ELISA while immunophenotyping of PBMC and lymph node aspirates were assessed by flow cytometry. Tumor-infiltrating lymphocyte (TIL) analysis was performed by immunohistochemistry (IHC) and gene expression profiling (Nanostring). Descriptive statistics were used for analyses. Results: As of November 16, 2022, 8 dogs have been enrolled and all dogs remain on treatment. Thirteen adverse events have been reported in 6 dogs, all being grade 1. However, only two are considered related to the study drug (tumor site inflammation and pain). Treatment was associated with increases in serum IFNγ and IL-10, as well as increases in peripheral CD4+ T and CD21+ T cells. Additional data on PK analyses, anti-drug antibody levels, gene expression, and clinical responses will be presented. Conclusions: Thus far, cANK-101 appears to be safe and tolerable in dogs with advanced melanoma. Data from this trial will help inform human clinical trials and may represent a new therapeutic option for dogs with advanced melanoma and perhaps other solid tumors. Our experience further suggests that companion animal trials could serve as relevant translational models for early immuno-oncology drug development. Citation Format: Matheus Moreno Passos Barbosa, Angel J. Lopez, Rachel Uyehara, Rebecca L. Kamerer, Michael Schmidt, Sailaja Battula, Howard L. Kaufman, Timothy M. Fan. Preliminary results of an exploratory phase I clinical trial of anchored canine interleukin-12 (cANK-101) in dogs with advanced oral malignant melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6347.

Subsoils contain more than half of soil organic carbon (SOC) and are expected to experience rapid warming in the coming decades. Yet our understanding of the stability of this vast carbon pool under global warming is uncertain. In particular, the fate of complex molecular structures (polymers) remains debated. Here we show that 4.5 years of whole-soil warming (+4 °C) resulted in less polymeric SOC (sum of specific polymers contributing to SOC) in the warmed subsoil (20–90 cm) relative to control, with no detectable change in topsoil. Warming stimulated the subsoil loss of lignin phenols (−17 ± 0%) derived from woody plant biomass, hydrolysable lipids cutin and suberin, derived from leaf and woody plant biomass (−28 ± 3%), and pyrogenic carbon (−37 ± 8%) produced during incomplete combustion. Given that these compounds have been proposed for long-term carbon sequestration, it is notable that they were rapidly lost in warmed soils. We conclude that complex polymeric carbon in subsoil is vulnerable to decomposition and propose that molecular structure alone may not protect compounds from degradation under future warming.

Peatlands are an important carbon (C) reservoir storing one-third of global soil organic carbon (SOC), but little is known about the fate of these C stocks under climate change. Here, we examine the impact of warming and elevated atmospheric CO2 concentration (eCO2) on the molecular composition of SOC to infer SOC sources (microbe-, plant- and fire-derived) and stability in a boreal peatland. We show that while warming alone decreased plant- and microbe-derived SOC due to enhanced decomposition, warming combined with eCO2 increased plant-derived SOC compounds. We further observed increasing root-derived inputs (suberin) and declining leaf/needle-derived inputs (cutin) into SOC under warming and eCO2. The decline in SOC compounds with warming and gains from new root-derived C under eCO2, suggest that warming and eCO2 may shift peatland C budget towards pools with faster turnover. Together, our results indicate that climate change may increase inputs and enhance decomposition of SOC potentially destabilising C storage in peatlands.

Soils store more carbon than other terrestrial ecosystems 1 , 2 . How soil organic carbon (SOC) forms and persists remains uncertain 1 , 3 , which makes it challenging to understand how it will respond to climatic change 3 , 4 . It has been suggested that soil microorganisms play an important role in SOC formation, preservation and loss 5 – 7 . Although microorganisms affect the accumulation and loss of soil organic matter through many pathways 4 , 6 , 8 – 11 , microbial carbon use efficiency (CUE) is an integrative metric that can capture the balance of these processes 12 , 13 . Although CUE has the potential to act as a predictor of variation in SOC storage, the role of CUE in SOC persistence remains unresolved 7 , 14 , 15 . Here we examine the relationship between CUE and the preservation of SOC, and interactions with climate, vegetation and edaphic properties, using a combination of global-scale datasets, a microbial-process explicit model, data assimilation, deep learning and meta-analysis. We find that CUE is at least four times as important as other evaluated factors, such as carbon input, decomposition or vertical transport, in determining SOC storage and its spatial variation across the globe. In addition, CUE shows a positive correlation with SOC content. Our findings point to microbial CUE as a major determinant of global SOC storage. Understanding the microbial processes underlying CUE and their environmental dependence may help the prediction of SOC feedback to a changing climate.