Research advances in mechanisms of climate change impacts on soil organic carbon dynamics

Soil, as the largest terrestrial carbon pool, has garnered significant attention concerning its response to global warming. However, accurately estimating the stocks and dynamics of soil organic carbon (SOC) remains challenging due to the complex and unclear influence mechanisms associated with biogeochemical processes in above- and belowground ecosystems, as well as technical limitations. Therefore, it is imperative to facilitate the integration of models and knowledge and promote dialogue between empiricists and modelers. This review provides a concise SOC turnover framework to understand the impact of climate change on SOC dynamics. It covers various factors such as warming, precipitation changes, elevated carbon dioxide, and nitrogen deposition. The review presents impact mechanisms from the perspective of organismal traits (plants, fauna, and microbes), their interactions, and abiotic regulation. Although valuable insights have been gained regarding SOC inputs, decomposition, and stabilization under climate change, there are still knowledge gaps that need to be addressed. In the future, it is essential to conduct systematic and refined research in this field. This includes standardizing the organismal traits most relevant to SOC, studying the standardization of SOC fractions and their resistance to decomposition, and focusing on the interactions and biochemical pathways of biological communities. Through further investigation of biotic and abiotic interactions, a clearer understanding can be attained regarding the physical protection, chemical stability, and biological driving mechanisms of SOC under climate change. This can be achieved by integrating multidisciplinary knowledge, utilizing novel technologies and methodologies, increasing in-situ experiments, and conducting long-term monitoring across multi-scales. By integrating reliable data and elucidating clear mechanisms, the accuracy of models can be enhanced, providing a scientific foundation for mitigating climate change.


Introduction
The search for literature was conducted on the Web of Science, using the following query: '((TS = (soil organic carbon) OR TS = (SOC)) AND (TS = (climate change) OR TS = (global warming))) AND PY = (1900-2022)' .A total of 14 176 articles were obtained for the period between 1900 and 2022, with the number of articles increasing annually (as illustrated in figures 1 and 2).Published articles from 2013 to 2022 accounted for approximately 76.12% (10 791), indicating that soil C and climate change were prominent research areas.Since 2018, the annual number of publications has surpassed 1000, with approximately 10% of the articles being published within the last three years.Over the past ten years, the top ten categories were environmental sciences, soil science, ecology, geochemistry & geophysics, agronomy, plant sciences, biodiversity conservation, water resources, meteorology & atmospheric science, and forestry.The top ten subject words included soil, carbon, ecosystem, environmental monitoring, nitrogen, China, climate change, soil microbiology, agriculture, and CO 2 .
Although extensive research has been conducted from various perspectives, the response of soil C pools to climate change in the Earth system remains unknown.The capabilities of climate models in predicting future soil C stocks are characterized by significant uncertainties (Tian et al 2015, Farina et al 2021).SOC stocks refer to the quantity of soil C that remains in soils after the C sink-shifting process occurs.The budget of SOC is primarily determined by SOM input (e.g.dead organismal residues, litter material, and root exudates) and degradation (e.g.autotrophic respiration of roots, heterotrophic respiration of soil fauna and microbial communities, and chemical oxidation of C-containing substances).However, current soil biogeochemical models used for projecting climate change do not typically account for SOC's formation and stabilization mechanisms.The assumptions made in these models regarding the potential response mechanism of SOC to climate change often conflict with emerging understandings (Wieder et al 2013, Verheijen et al 2015, Bradford et al 2016).Therefore, it is essential to regularly review and summarize studies conducted globally on different scales and over extended periods to improve soil C-climate feedback models (Huang et al 2021b).
This study aims to provide a comprehensive summary of research advances about the mechanisms of SOC dynamics in response to climate change and to generate new knowledge and future research directions.Given that SOC performance's response varies temporally and spatially, multi-level research has been conducted employing in-situ or laboratory-controlled experiments.The existing literature involves numerous cases of varying ecosystems, altitudes, soil depths, and time scales.We have first synthesized the SOC turnover process under natural conditions and subsequently focused on the impact of climate change as an external stressor from three main perspectives: organisms (plants, fauna, and microbes) and their interactions, as well as the abiotic environment of the soil.The primary climate change processes includewarming, precipitation changes, elevated CO 2 (eCO 2 ), and nitrogen (N) deposition.Having comprehensively summarized the research situation and progress regarding impact mechanisms, we have proposed future research focus and direction.

Essential links and processes of SOC formation and turnover
The Soil C cycle refers to the flow and circulation of C in soil among the biosphere, physical, and chemical spheres, which never stops, even without external stress.Lehmann and Kleber (2015) highlight the importance of C flows over C stocks.Understanding the primary links and processes of SOC turnover can help us gain insight into the impact mechanisms of climate change on SOC dynamics.SOC is highly integrated on spatiotemporal and biotic/abiotic scales (Dignac et al 2017,Wiesmeier et al 2019).Substances related to SOC turnover in terrestrial ecosystems can be divided into biotic factors (plants, fauna, and microbes) and abiotic factors (as shown in figure 3).Additionally, SOC turnover is influenced by interactions with and between organisms and abiotic environments (Jackson et al 2017, Luo et al 2017, Zhang et al 2020).

Climate change impact on organismal traits and their effects on SOC
The stocks and biodegradation resistance of SOC in soils is influenced by both external and internal determinants (Quesada et al 2020).The stability of SOM is principally reliant on climate and land use (Kurganova et al 2019).In particular, this paper explores the impacts of climate change as an external factor, elaborating on its effects on organismal traits, soil environmental conditions, and relationships between living and non-living components, which can ultimately influence SOC stocks.The feedback response of SOC to climate change may be positive or negative (Ofiti et al 2022).Moreover, Rillig et al (2023) report that eight environmental stress factors (including the four climate factors discussed in this paper) surpass the critical threshold (over 75% of maximum observed levels), resulting in a considerable decline in soil biodiversity and functionality across all samples in the global soil biology database, which may subsequently impact SOC content.To better illustrate the SOC dynamics under climate change and the primary framework of this review, please refer to figure 4.

Plants
Plant-derived SOC inputs include above-and belowground components, with vegetation's belowground contribution potentially surpassing above-parts (Basile-Doelsch et al 2020, Xu et al 2021).C inputs to soil from belowground sources include root biomass and rhizodeposition (Hirte et al 2018), which Both warming and insufficient precipitation have been found to reduce the photosynthesis rate, while elevated eCO 2 tends to increase it, impacting biomass.However, Wilschut et al (2022) emphasized that the combined effects of warming and drought Climate change affects root traits, exudate secretion rate, types, and contents, influencing SOC stability.A meta-analysis shows that warming generally increases fine root biomass, productivity, respiration, and N concentration, while decreasing the root C/N ratio and non-structural carbohydrates.However, it has little impact on fine root length, morphology, mortality, lifespan, and turnover rate.The effect of warming on fine root biomass weakens with higher warming intensity (Wang et al Climate and litter quality both drive litter decomposition, with their impact varying over time, climate, and traits (Canessa et al 2020).Guo et al (2021) found a positive correlation between key chemical characteristics of deciduous and fine-rooted leaves, such as C/N ratio, phosphorus (P) content, N/P ratio, lignin, cellulose, non-structural carbohydrates, and tannins.Sugars, amino acids, and starch decompose more easily, while cellulose and hemicellulose decompose moderately, and lignin is the most resistant (Jia 2019).Labile compounds in litter reduce the sensitivity of decomposition to warming and altered precipitation (Suseela et al 2013).Leaching, influenced by precipitation frequency and intensity, affects litter decomposition by dissolving organic compounds, altering litter's physical structure through wetting and infiltration, and changing particle distribution and size through erosion.Higher temperatures generally accelerate litter decomposition, supporting the 'C quality-temperature' hypothesis (Jia 2019).However, limited precipitation can slow down the decomposition process (Yu et al 2019).ECO 2 impacts soil C cycling by increasing photosynthesis, altering C/N ratios, and changing substrate quality (Rai et al 2020).Coexistence of eCO 2 and warming reduces decomposition rates (Amani et al 2019).Optimal N levels are crucial, as both excess and deficiency can affect decomposition (Li et al 2022).Future research should consider the vegetation-litter-soil system comprehensively, employ ecological chemometrics, and systematically study soil biogeochemical cycling mechanisms under multiple climatic factors.

Fauna
Soil fauna play a crucial role in SOM turnover, necessitating their explicit consideration for accurate simulation of SOM dynamics and predicting the nonlinear response to global change (Filser et al 2016).Fauna, through biological activities and physiological processes such as respiration, feeding, digestion, secretion, excretion, excavation, and death, directly or indirectly influence the content and stability of SOC (Nie et al 2019).Climate change has altered the intensity and functional traits of faunal activity.The impact of faunal activity on SOC stability is manifested through changes in respiratory rates, feeding patterns driven by plant growth restrictions and increased food demand, enzyme activity levels, nutrient release and transformation, promotion of excretion function, and the contribution or stability impact of excreta on SOC.Excavation and other behavioral activities consider their intensity, scope, and effects on soil properties.Death may result from the inability of fauna to adapt to extreme climate conditions, changes in food chains, and habitat alterations.Different faunal populations exhibit varying responses to climate change, with biomass and population diversity depending on species traits and adaptability.
Currently, research mainly focuses on limited species, such as large organisms like earthworms and ants, medium-sized organisms like collembola, and small organisms like mites and nematodes.Warming will significantly impact ant communities in tropical forests, favoring species with moderate heat tolerance and invasiveness (Bujan et al 2022) Hohberg et al (2015) investigated the impact of eCO 2 on collembola and nematode community structure, indicating potential benefits for species adapted to high CO 2 environments through altered food supply or reduced competition.Wang et al (2022d) observed increased abundance and species richness of faunal communities in temperate natural secondary forests with low N addition, while high N addition led to a decrease.However, four years of N addition did not alter the faunal community structure in mature subtropical forests (Tian et al 2020).Current research predominantly focuses on the 'phenomenon' of faunal impact on the C cycle, requiring further in-depth mechanistic understanding.Researchers have increasingly focused on understanding SOC turnover from a trait perspective (Wieder et al 2015, Wang et al 2022b).In a study by Ren et al (2022), microbial traits were found to be crucial in explaining PE in forests, with rapid C metabolism identified as a key factor.Zhou et al (2021b) found warming reshapes the hierarchical interactions among SMC, altering the ecosystem functions mediated by microbes through nutritional interactions.Sokol et al (2022a), focusing on microbial traits, identified factors such as microbial CUE, total biomass, extracellular products, MGR, and MTR as influencing SOC.They also summarized indicators related to MBC allocation, biochemical properties, and morphological classification traits based on MAOM formation characteristics.

Microbes
Climate factors significantly impact SOC through their regulation of microbial physiology (Allison et al 2010, Wang et al 2021a).In response to environmental changes, microbes adapt, become dormant, or die based on their genetic and physiological states.Experiments have studied the impact of climate change on extracellular enzyme activity, as changes in microbial traits often correspond with enzyme changes (Meng et al 2019).Q 10 is a measure of temperature sensitivity, indicating the rate at which respiration increases for every 10 • C rise.High temperatures denature enzymes by reducing substrate and water availability in microbial processes.The large molecular rate theory predicts the optimal temperature for microbial processes by analyzing thermodynamic changes and enzyme catalytic rates (Schipper et al 2014).Chen et al (2018) found heating increases lignin enzyme activity but has no effect on cellulase activity.Short-term warming impacts unstable C and bacteria, while long-term warming affects recalcitrant C and fungi (Wang et al 2022a).According to Fanin et al (2022), improving enzyme representation in soil C models requires long-term studies on hydrolytic and oxidative enzymatic traits and SMC responses.
In recent years, scholars have increasingly recognized the importance of studying microbial growth, metabolism, and death within the context of the history of life (Abs et al 2019, Sokol et al 2022b).The Y-A-S framework, proposed by Malik et al (2018), highlights the importance of SMC strategies, including growth and yield, resource acquisition, and stress tolerance, in regulating microbial functions.Li et al (2021) explored the relationship between Q 10 and microbial ecological strategies, using genetic information to understand the complex link between Q 10 -SOC quality and microbial ecological functions.Microbes adapt to ecological changes by regulating gene transcription, accumulation mutations, or acquiring new genes.He et al (2020) proposed a conceptual framework connecting SMC functional genes, responsiveness to elevated CO 2 , and ecosystem processes.However, further research is needed to fully understand the impact of climate change on SMC structure (resistance, resilience, and functional redundancy) and metabolic activity (Zhou et al 2020, Dove et al 2021, Bhattacharyya et al 2022).

Climate change impacts on inter-organism interactions and their effects on SOC
The relationships between organisms can be competitive, exploitative, neutral, commensal, or mutualistic.Plants interact with soil minerals, aggregates, and microbes (Merino et al 2015, Jackson et al 2017, Feng et al 2022b).The decomposer role of fauna and microbes profoundly impacts the degradation of organic matter from microbial and plant compounds, with potential ramifications for soil health and ecosystem functioning (Angst et al 2021).Soil microbiomes profoundly impact the biogeochemical cycles of macronutrients, micronutrients, and the global food web, significantly influencing plant and faunal health (Cavicchioli et al 2019, Jansson and Hofmockel 2019, Elshafei 2022, Mishra et al 2023).Food webs strongly affect SOM decomposition at shallower depths (Jackson et al 2017).However, the food web is not currently considered in SOC estimation models, and researchers are attempting to acquire more knowledge to integrate it into future models.

Plant-microbe interactions
Microbes play a crucial role in the decomposition of plant litter.Soil microbes act as both local-and broad-scale controls on litter decomposition rates, with changing climate variables exerting a decisive influence on these processes (Bradford et al 2017).Drought and plant litter biochemistry can significantly affect microbial gene expression and metabolite production, indirectly impacting litter decomposition rates (Guenet et al 2018, Malik et al 2020).A recent study proposes that promoting plant adaptation to climate change necessitates a systems-based approach (Barnes and Tringe 2022).Determining the precise contribution of different organisms to SR remains challenging, given that roots, symbionts, and heterotrophs often form a functional continuum rather than distinct groups (Högberg and Read 2006).Unraveling the respective roles of the host, its microbes, and the environment in plant adaptation to climate change will be an intricate task.
Recent years have witnessed substantial progress in comprehending the composition and dynamics of rhizospheric microbiomes.Jacoby et al (2017) presented an overview of how plants and rootassociated bacteria interact to enhance plant mineral nutrition.The most common plant symbionts are mycorrhizal fungi and N-fixing bacteria (de Deyn et al 2008).Mycorrhiza-mediated competition between plants and decomposers is a pivotal factor that drives SOC stocks (Averill et al 2014).Given the close relationship between plants and their rhizospheric microbiomes, the latter is deemed to be a plant's second genome (Madawala 2019).Shi et al (2019) stressed that neglecting plant-microbe symbioses could lead to underestimating modeled climate impacts.The growing body of research on hostmicrobe interactions highlights the critical importance of 'Microbiome Conservation' (Banerjee et al 2019).
The production of microbial residue is intimately tied to the input of plant material in SOC accumulation.Plants with higher biomass or litter that can induce more humification-enhancing microbes are referred to as natural high-C plants (Yan et al 2018).Plant species richness accelerated MGR, MTR, and increased SMB and necromass, thus driving SOC accumulation via its positive influence on MBC (Prommer et al 2020).Highquality plant litter can boost MGR, CUE, and MTR, leading to enhanced mineral stability of necrotic components (Craig et al 2022).García-Palacios and Chen ( 2022) have emphasized the interplay among microbes, C dynamics, and climate change, underscoring plant inputs, microbial necromass, and mineral interactions.
Studies have shown that the prolonged warming of temperate forest soil has impacted fine root dynamics, morphology, and ectomycorrhizal fungal community (Kwatcho Kengdo et al 2022).Furthermore, under eCO 2 levels, Arbuscular mycorrhizal (AM) fungi have additive effects on residue decomposition and soil aggregation and exhibit PEs (Wei et al 2019).In a study by Dong et al (2018), different patterns were observed with respect to the symbiotic associations of AM and ectomycorrhizal fungi under eCO 2 .AM fungi play a significant role in alleviating drought stress through root trait modification and increasing the effectiveness of P acquisition (Chandrasekaran 2022).Plants compete directly with free-living decomposers for N through fungal symbionts.Moreover, the mycelial pathway contributes twice as much as the root pathway to SOC accumulation under N deposition.N addition enhances the fungal metabolic activity and increases the soil mineral-binding capacity of fungal residue C in the mycelial path, resulting in the more efficient functioning of the MCP (Zhu et al 2022, Feng et al 2022b).
The rhizosphere, the region surrounding the root surface in contact with the soil, plays a crucial role in plant-microbe interactions.Root exudates are vital links between plants, soil, and microbes, affecting seed germination, plant growth, nutrient activation, soil structure formation, and microbial processes.Ccontaining compounds in the rhizosphere promote nutrient and energy flow for rhizosphere microbes, altering their quantity, composition, and activity and stimulating microbial respiration, significantly affecting SOM decomposition, known as the RPE.The Rhizo-Engine framework has identified three rootdriven mechanisms for SOC stabilization: biochemical stabilization, mineral adsorption fixation, and physical protection mechanisms (Dijkstra et al 2020).
Increased labile C flow in the soil-microbe-plantatmosphere continuum in rice under eCO 2 and temperature may trigger the priming of SOC stocks (Padhy et al 2020).However, whether RPE promotes C release or fixation is still under investigation (Wen et al 2022).

Fauna-microbe interactions
Soil fauna regulates soil microbial activity, functional composition, and their physical-chemical interactions with SOM.Species-specific differences in predator-prey interactions can influence SMC composition and functioning, affecting the impact of protists on litter breakdown.Therefore, it is crucial to understand the role of trophic interactions within the microbiome and their role in controlling decomposition processes and C cycling (Geisen et al 2020).The composition of the nematode community, mainly where bacterivores are more abundant, is associated with SOC stabilization in mid-sized aggregates (Martin and Sprunger 2021).
Climate conditions and seasonal variations can significantly influence faunal structure, activity, and interactions with microbes (Zahorec et al 2021).Increased soil moisture can alter the food web and ecosystem function by changing the proportional abundance of mite and nematode trophic groups (Sylvain et al 2014).The predation pressure of fauna on microbes can affect microbial processes by influencing SMC and SR (Nie et al 2019).Increased temperature can influence bacterial-and fungal-mediated decomposition rates, but the role of trophic interactions in affecting decomposition remains unknown (Geisen et al 2020).A symbiotic relationship exists between fauna and microbes, with earthworm diversity positively correlated with soil and gut bacterial diversity across sites (Gong et al 2023).Recently, a study has focused on the effects of temperature fluctuations on insect-microbe holobiont traits (behavior, physiology, and performance) and evolutionary ecology to predict the fate of mutualistic interactions under climate change (Iltis et al 2021).

Plants-fauna interactions
Plants provide food sources for fauna, such as roots, exudates, litter, and residues.They also impact faunal communities Temperature and humidity regulate the contribution of fauna to litter decomposition.Higher temperature reduces faunal contribution by increasing litter C/N ratio and density, while increased humidity enhances faunal diversity.Under low precipitation, fauna have a stronger synergistic effect on litter decomposition (Njoroge et al 2022).Ott et al (2012) found that eCO 2 and reduced litter quality, along with the interaction between metabolic theory and ecological chemometrics, constrain decomposition rates in macro-fauna.Faunal response to N addition varies based on type, quantity, and duration.Generally, organic and ecological N benefits fauna, whereas combined fertilizers support higher faunal abundance.N addition simplifies arthropod networks by strengthening specific nutrient paths and weakening others (Zhang et al 2023).Further research is needed to understand the direction, intensity, and mechanisms of faunal effects under climate change and their interactions with plants.2022), using a meta-analysis, discovered that the variability of the PE was more remarkable for the fast SOC pool than for the slow SOC pool.Factors such as soil total N content, substrate C/N ratio, soil moisture, soil clay content, soil pH, substrate addition rate, and SOC content were identified as influencing the PE for the fast SOC pool.Conversely, soil pH and total N content explained the variation in the PE for the slow SOC pool.

Climate change impacts on abiotic factors and their effects on SOC
POM and MAOM differ fundamentally in their formation, persistence, and functioning (Lavallee et al 2020).As MAOM has a long lifespan and accounts for a high proportion of Earth's mineral soils (∼65%), most research on SOC accumulation and stocks focuses on MAOM (Sokol et al 2022a).The persistence of POM is mainly influenced by microbial and enzyme inhibition, whereas the persistence of MAOM is influenced by mineral binding.In the short term, POC is highly responsive and a better diagnostic indicator of soil C changes than bulk SOC and MAOC (Rocci et al 2021).Warming effects on SOM components depend on the nature and complexity of the study system due to combinations of direct and indirect effects of warming on the plant-soil-microbial system (Field et al 2007).To more accurately estimate how different biomes' soil C storage will respond to future climate impacts, Sokol et al (2022a) propose a trait-based framework.Precipitation events can significantly change soil structure and microbial activity.Alternating wet and dry conditions can cause soil aggregates to collapse, exposing protected SOC in aggregates to the air.In turn, it can increase the intensity of SR upon rewetting, promoting SOC mineralization and decomposition.In clay-textured soil with low air permeability, soil microbial activity can be reduced, inducing slower SOM decomposition.The effects of precipitation on the SR rate in terrestrial ecosystems are complex and are typically affected by temperature configurations simultaneously.To minimize uncertainties when predicting the impact of climate change on the C cycle, it is crucial to investigate the interaction between temperature and water restriction (Lyu et al 2021).

Discussion and prospect
Despite extensive research, it is a daunting task to fully understand the impact of climate change on SOC dynamics due to the complexity of above-and belowground ecological processes, simultaneous operation of multiple complex mechanisms such as physical, chemical, and biological processes, as well as limitations in research technologies and experimental methods (Han et al 2016).

Collaborate across disciplines for systematic and refined research
Based on the literature review on organisms, interorganism interactions, and abiotic regulations, we can make a brief summary as follows: (1) climate change influences plant growth and ecological traits, which in turn affect the composition and stability of SOC through changes in biomass and chemical composition.However, the complexity of above-and belowground plant connectivity, spatial heterogeneity of species, and temporal scale characteristics pose challenges in understanding the impact of factors such as soil moisture, soil texture, and plant-microbe interactions.(2) While the role of fauna in SOC dynamics is gaining attention, research has been limited to a few common species.Different faunal species exert varying effects on SOC due to their ecological traits and functional differences.(3) Research on microbial-mediated SOC dynamics is more extensive, but our understanding of the mechanisms underlying climate change effects on SMC composition, functionality, and activity change is limited by species diversity, the complexity of biogeochemical processes, and interactions with other biotic and abiotic factors.(4) Consideration of symbiotic, predatory, and competitive relationships between organisms is increasing.However, due to the exceptional complexity of the interaction network and limitations in experiment and observation, our knowledge of the mechanisms and long-term effects of climate change on biotic interactions remains limited.(5) Extensive research has been conducted on the protective mechanisms of minerals, soil texture, pore structure, soil pH, oxygen, and water conditions in soil physicochemical properties.However, the intricate interactions among these factors make it challenging to unravel how they collectively influence SOC stability.
In the future, it is necessary to strengthen interdisciplinary collaboration in fields such as Earth science, biology, ecology, agriculture, chemistry, and environmental science.From a systematic and refined perspective, research can be conducted in the following directions: ( (3) High-throughput sequencing technology: this technology reveals microbial diversity, community structure, and functioning, as well as interactions and symbiotic relationships between organisms, deepening the understanding of SOC transformation and cycling processes.( 4) Remote sensing technology: by obtaining comprehensive observational data on a spatio-temporal scale, remote sensing technology provides extensive land information and soil attributes, contributing to the study of SOC. ( 5) Data mining and machine learning algorithms: processing large-scale monitoring data extracts hidden patterns and trends in the belowground ecosystem, deepening the understanding of SOC transformation and cycling processes (Chen et al 2022).Significant progress can be made in the coming years by utilizing these integrated approaches.

Explore interactive research involving multi-climate variables
Current studies often focus on the effects of single or dual factors.However, relying solely on understanding individual driving factors is insufficient for accurate prediction.Multiple global change drivers may interact and generate synergistic effects, which often exceed the expected impacts of individual driving factors.Failure to consider interactions among multiple factors would limit our comprehensive understanding of SOC responses, increase prediction errors, and hinder the assessment of risks and vulnerabilities, thus constraining the development of adaptive and management strategies.Therefore, a more comprehensive investigation of multi-factor manipulation experiments in different locations is needed to better understand the impact mechanisms of climate change on SOC (Reich et al 2020, Simon et al 2020, Wu et al 2020).
Areas for improvement could include: (1) climate change modeling: considering multiple climate patterns, such as the frequency of precipitation and dry-wet alternation and focusing on climate-induced increases in wildfires or extreme weather events.
(2) Long-term, systematic, and reliable data collection: this data collection is necessary to quantify the independent and interactive impacts of multiple climate factors on SOC dynamics at a system level.
(3) Broadening the scope of research: current studies on the interaction of multiple climate variables are mainly concentrated in specific research sites, and it is necessary to expand the focus scope.(4) Integrated assessment approaches: integrating field observations Y Guo et al and model simulations can provide more comprehensive results by considering impacts at different scales and time frames.

Conduct long-term observations at multiple spatial scales
Terrestrial ecosystems exhibit significant spatial heterogeneity.O'Rourke et al (2015) highlighted the incomplete understanding of SOC processes at the biosphere to biotic community scale, calling for an integrated framework spanning multi-scales to optimize SOC management.However, the integration of macro-scale environmental/ecological processes with micro-scale chemical processes remains challenging.Determinants of SOC dynamics exhibit significant variations at different spatial scales, with complex finer-scale variations exceeding larger-scale changes.Scaling poses a challenge, as driving factors may not achieve the required resolution.The lack of accurate data inhibits model utilization and improvement, particularly on large spatial scales and vertical profiles.Hence, combining large-scale, longterm, and cross-site in situ experiments and monitoring methods is necessary to unravel the interactions of these mechanisms across multiple spatiotemporal scales (Nottingham et al 2022).
The following suggestions can be implemented: (1) enhance representative sampling by considering the diversity of ecosystem types, geographical locations, elevations, and soil depths at various hierarchical levels.(2) Conduct multi-scale studies encompassing regional, continental, and global perspectives.These cross-scale studies can provide a comprehensive comprehension of SOC responses at different scales.(3) Short-term observations may not capture the dynamic processes of SOC at different time scales.Long-term monitoring, including seasonal and annual changes, is necessary.(4) Encourage data sharing and comparisons by collecting and integrating data from globally representative ecosystems and similar regions to increase sample sizes.This facilitates consensus building, rule extraction, knowledge synthesis, and allows for a deeper understanding of mechanisms and improved applicability.

Conclusion
In summary, fully understanding the mechanisms through which global climate change affects the dynamics of SOC is challenging.This article provides an overview of research progress made in the past decade from organisms, biotic interactions, and abiotic regulations.Future research should focus on the systematic and detailed exploration of mechanisms.Plants exhibit above-belowground connections, spatial heterogeneity, and temporal scale characteristics.In the future, it is necessary to identify and standardize plant traits related to SOC, while enhancing research on root exudate composition and regulation.
More studies are needed to understand the effects of faunal types, density, and behavior patterns on SOC cycling in various habitats, as well as the impact of climate change on the structure, function, and ecological niches of faunal communities.Microbes are involved in nearly all C cycling processes, and further research is required to explore the responses of SMC structure, function, and interactions to climate change.This includes investigating the effects of climate change on microbial metabolic activity, enzymatic activity, and SOC decomposition capacity, as well as studying the combination of microbial ecology and biogeochemistry, microbial adaptation strategies, and tolerance mechanisms.
The exploration of the mechanisms through biotic and abiotic interactions has just begun.Biotic interactions are characterized by complex relationship networks, and future research should focus on multi-community interactions and biochemical pathways, as well as aspects such as species diversity, functional groups, nutrient cycling and energy flow, and ecological niches and competition.Additionally, the refinement, standardization, sources, and resistance of SOC components should not be overlooked.The mechanisms by which the physicochemical properties of climate change-affected soils regulate SOC adsorption, protection, and decomposition, which are currently limited to some common physicochemical properties, should be explored in relation to other properties and their interactions with biota.
To address the current knowledge gaps, interdisciplinary theories and methods can be combined, and the latest technologies and experimental approaches can be employed to establish a multi-scale, crosssite monitoring network for long-term studies.This should take into account the interaction of multiple variables of climatic factors and aim to obtain more refined knowledge and reliable observational data.Through dialogue between empiricists and modelers, SOC prediction models can be improved and uncertainties reduced.This will enable more accurate predictions of future SOC stocks, allowing for the development of effective strategies to mitigate climate change, protect biodiversity, and achieve sustainable development goals.

References
Soil is the largest terrestrial carbon (C) pool (Scharlemann et al 2014), holding C stocks over three times larger than vegetation (Davidson et al 2000, Schmidt et al 2011).The majority of soil C is present in the form of SOC (Conant et al 2011, Scharlemann et al 2014).Given soil's vast C storage capability, even slight changes in SOC in response to climate change may cause significant fluctuations in carbon dioxide (CO 2 ) concentrations, thus intensifying or mitigating global climate change trends (Liang and Zhu 2021).The 4 per 1000 initiative aims to increase SOC stocks to at least 4 per thousand annually to help reduce greenhouse gas emissions (Minasny et al 2017, Rumpel et al 2020).While warming has been found to lead to C loss from soil C pools based on compiled data (Hicks Pries et al 2017, Nottingham et al 2020, Soong et al 2021), the net global balance between C fluxes to and from the soil remains unclear (Crowther et al 2016).SOC pools are made up of complex mixtures of C-containing compounds with varying properties and kinetic capabilities (Angst et al 2021), making it challenging to demonstrate the feedback effects of SOC on climate change (Davidson and Janssens 2006, Terrer et al 2021).Nevertheless, the topic has piqued research interest among scientists worldwide, boosting interdisciplinary collaboration in this field.

YFigure 1 .
Figure 1.Interrelated countries, keywords, and journals in Bibliometrix research.(A) Collaboration network and world map.(B) Thematic evolution over the past three decades.(C) The top 10 countries, keywords, and articles and their connections.

Figure 2 .
Figure 2. Highly cited articles and subject words over time using CiteSpace clustering method.(A) The vertical axis denotes the publication year, while the horizontal axis represents the number of related articles.(B) Authors and years of articles with the greatest impact factor.(C) Significant subject words in current literature.

Figure 3 .
Figure 3. Essential links and processes of SOC formation and turnover.(Note: black arrows signify SOC sequestration, red arrows indicate CO2 emission and blue arrows represent interactions.Numbers on the arrows correspond to: x photosynthesis, y water transport, z nutrient transport, { secretion, | humification, } decomposition, ~death, soil respiration (SR), and reshaping.The filled patterns in the rectangular box represent organismal traits).
2021b).Precipitation can have positive or negative associations with fine root growth (Wang et al 2019, Malhotra et al 2020).Adequate soil moisture promotes fine root growth under higher precipitation levels, but extreme humidity or hypoxic conditions hinder it.Herbaceous and woody plant roots exhibit variations in depth, response mechanisms, and strategies in response to precipitation fluctuations (Wang et al 2019).High temperatures generally reduce the physiological metabolic rate and inhibit root exudate secretion.While the impact of eCO 2 on roots is smaller compared to aboveground vegetation, it increases the production of root exudates, which improves plant tolerance to abiotic stresses (Dong et al 2021).

YFigure 4 .
Figure 4. Schematic diagram of SOC dynamics under climate change.Note: (1) the yellow arrow indicates the impact of climate change.The blue arrow represents interactions between organisms and non-organisms, the red arrow indicates positive feedback of CO2, and the black arrow signifies SOC sequestration.(2) Aboveground section: the chart displays four climatic factors (warming, precipitation, CO2 concentration, and N deposition), six ecosystems (forests, grasslands, wetlands, deserts, tundra, and farmlands), and plants (vegetation, fallen wood, and leaf litter).(3) Belowground section: the left dotted box depicts the abiotic soil environment, while the right represents the soil depth.The middle dotted box depicts SOC turnover between biomass.Adapted from Cavicchioli et al (2019).CC BY 4.0.
' structure and activity by affecting habitat conditions (Zahorec et al 2021, Dutta and Phani 2023).Fauna has preferences for food resources, which amplify the effects of litter traits on decomposition (Fujii et al 2018).They also affect litter decomposition and nutrient supply by fragmenting plant residues (Yin et al 2019, Tan et al 2021) and influence soil structure and plant growth by creating burrows, altering air and water movement pathways (Opute and Maboeta 2022).Fauna can disperse plant seeds and impact plant species abundance.Additionally, they can provide resistance against pathogenic microbes or indirectly affect plant performance by regulating microbes (Zahorec et al 2021, Dutta and Phani 2023).Research typically focuses on resource competition, food chains or food webs, and ecological energy flow to describe the community structure and function of plants and fauna (Eisenhauer et al 2013, Parker et al 2022).Climate change indirectly alters litter decomposition by changing tree community composition.Thakur et al (2017) discovered that warming conditions increased nematode species diversity in complex plant communities but decreased it in single communities.This indicates that diverse plant communities can mitigate warming effects.Caddy-Retalic et al (2018) found inconsistent climate change susceptibility between ant and plant communities.Ant communities exhibited higher susceptibility, potentially decoupling from plant communities and affecting their respective ecosystems.Climate change significantly impacts plant parasitic nematodes, altering their range, distribution, abundance, adaptation, reproduction, and parasitism potential.It may also affect plant biomass and the C cycle (Dutta and Phani 2023).

5. 1 .
SOC content and fractionsWarming effects are contingent on the size of the initial soil C stock(Crowther et al 2016).Soil with a higher C content can absorb C compounds significantly into mineral soils and has a lower unit soil C respiration rate.Therefore, high-C soil is more susceptible to climate warming (Doetterl et al 2015).C-rich SOC releases more CO 2 into the atmosphere after intense droughts, involving high substrate availability and a shift in the SMC (Canarini et al 2017).Wang et al (2022c) also found that warming resulted Y Guo et al in more proportional SOC losses in the topsoil than in the subsoil, particularly in high-latitudinal, C-rich systems.Although initial SOC is considered one of the main driving factors for SOC changes in European farmland, long-term monitoring results require more reliable data for further research (Gubler et al 2019).Slessarev et al (2023) also pointed out that we need new statistical methods and cross-site research to determine whether C-poor soils tend to gain or retain C more readily than C-rich soils.C fractions are more concerning than C content, although the grouping method for SOC has not been standardized.Typically, SOC is grouped according to chemical, physical, biological, and physicochemical combinations.For many years, research has focused on dividing C pools into active, slow, and passive ones based on the difficulty of SOC decomposition and turnover time (Li et al 2013).However, the recent focus has shifted towards POM and MAOM based on particle size, as they are easy to conceptualize and understand, relatively quick and inexpensive to separate, and already incorporated into newer models (Lavallee et al 2020).Furthermore, SOC components can be classified from other perspectives, including chemical solvents, density, aggregates, and biological and physicochemical collaboration groups.The current research focuses on the differences in protection mechanisms, renewal rates, C source composition, C/N ratios, and their response mechanisms to climate change(Witzgall et al 2021, Sokol et al 2022a).The temperature sensitivity of SOM decomposition increases with the complexity of substrate molecules(Davidson and Janssens 2006).Passive C pools, which have a more intricate chemical structure, exhibit a higher response sensitivity to warming than active C pools (Xu et al 2014, Lin et al 2015).The old pool has a higher SOM decomposition rate k than the new pool (van Groenigen et al 2015), but Tang et al (2017) do not support the view that passive C decomposition is more sensitive to temperature change than active SOC.van Groenigen et al (2016) have demonstrated a faster turnover of new SOC inputs under eCO 2 .Huo et al ( Z, Guo D, Xu X, Lu M, Bardgett R D, Eissenstat D M, McCormack M L and Hedin L O 2018 Evolutionary history resolves global organization of root functional traits Nature 555 94-97 Madawala H 2019 Soil-plant microbiome: a promising frontier for research Ceylon J. 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Microbes are key in soil C and N cycle research, and the influence of climate change relies extensively on responses of microbes (Melillo et al 2017, Cavicchioli et al 2019, Tiedje et al 2022). They play a dual role as decomposers in mineralizing SOC into CO 2 and in
Resistance to SOM decomposition can be achieved through chemical conformation during depolymerization, adsorption/desorption, and physical-chemical protection during aggregate turnover (Conant et al 2011).However, it remains unclear how many soil factors are necessary to effectively capture soil dynamics across scales (Jackson et al 2017).Current modeling approaches consider temperature, soil water content, pH, particle size, and C and N interactions, which regulate SOC turnover time (Basile-Doelsch et al 2020).Combining mineral particles with various extents in soils controls theC pool decomposition rate (Witzgall et al 2021).Roots can stabilize SOC by forming soil aggregates, whereas mineral associations drive stabilization at depths greater than ∼30 cm (Jackson et al 2017).Soil aggregation is critical in SOC accrual and stabilization (Basile-Doelsch et al 2020).The particlesize grouping approach can divide SOC into macroaggregates, microaggregates, and silt/clay-sized particles.Soil clay content and C concentrations are the most significant factors mediating soil C response to residue retention (Wan et al 2018, Quesada et al 2020).Fine mineral fraction is identified as the primary determinant of SOC stabilization in most soils(Wiesmeier et al 2019).Luo et al (2016)found that with increased time for adding fresh soil C, higher clay and moisture content resulted in a faster decline of the PE.Warming induces faster SOC turnover in large aggregates and lower SOC decomposition rates in soils with high binding clay content, Li H et al 2021 Temperature sensitivity of SOM decomposition is linked with a K-selected microbial community Glob.Change Biol.27 2763-79 Li Z, Peng Q, Dong Y and Guo Y 2022 The influence of increased precipitation and nitrogen deposition on the litter decomposition and soil microbial community structure in a semiarid grassland Sci.Total Environ.844 157115 Liang C, Schimel J P and Jastrow J D 2017 The importance of anabolism in microbial control over soil carbon storage Nat.Catena 208 105714 Luo Z, Feng W, Luo Y, Baldock J A and Wang E 2017 Soil organic carbon dynamics jointly controlled by climate, carbon inputs, soil properties and soil carbon fractions Glob.Change Biol.23 4430-9 Luo Z, Wang E and Sun O J 2016 A meta-analysis of the temporal dynamics of priming soil carbon decomposition by fresh carbon inputs across ecosystems Soil Biol.Biochem.101 96-103 Lyu M, Giardina C P and Litton C M 2021 Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest Glob.Change Biol.27 3824-36 Ma