Forests store substantial amounts of soil organic carbon (SOC), but SOC stocks differ strongly between forest ecosystems dominated by arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi. In temperate forests, nearly all tree species associate with either AM or EcM fungi, but it is unclear if variation in SOC stocks is linked to the dominance of AM versus EcM trees at local scales. However, SOC stocks are also influenced by many other factors, including plant diversity, plant traits, soil properties and microbial community composition, that can vary substantially at small spatial scales. Thus, elucidating how tree mycorrhizal associations interact with other drivers of SOC formation at local scales could improve our understanding of forest SOC storage. Using multivariate data from a 25‐ha temperate forest plot, we hypothesised that SOC stocks would be greater in AM‐dominated than in EcM‐dominated subplots, due to the rapid decomposition of labile litter inputs from AM trees. We expected that variation in SOC stocks would be explained by interactions between AM associations, plant diversity or traits, and soil microbial communities. We also accounted for differences in soil abiotic conditions and used piecewise structural equation models to identify potential causal pathways. Forest stand attributes played a primary role in predicting forest SOC stocks (45% relative contribution), compared to microbial community composition (26%) and abiotic factors (20%). Forest SOC stocks increased with plant and litterfall diversity and root diameter but declined with soil clay‐silt content. Importantly, forest SOC stocks were lower in subplots with a high ratio of EcM to saprotrophic fungi and subplots with a high predominance of bacterial functions related to carbon and nitrogen cycling. Our models suggest that smaller SOC stocks in stands with high EcM dominance are indirectly linked to lower tree species diversity, greater litter carbon input, and distinct soil microbial community composition. Our findings highlight that tree mycorrhizal type also influences SOC storage at local scales, but stand‐level relationships between SOC stocks and mycorrhizal dominance differ from regional and global patterns. We show that mycorrhizal associations regulate complex relationships between producers, decomposers, and soil properties at small spatial scales. Considering these linkages could therefore improve estimates and management of temperate forest SOC stocks. Read the free Plain Language Summary for this article on the Journal blog.

Soil respiration in forests contributes to significant carbon dioxide emissions from terrestrial ecosystems but it varies both spatially and seasonally. Both abiotic and biotic factors influence soil respiration but their relative contribution to spatial and seasonal variability remains poorly understood, which leads to uncertainty in models of global C cycling and predictions of future climate change. Here, we hypothesize that tree diversity, soil diversity, and soil properties contribute to local-scale variability of soil respiration but their relative importance changes in different seasons. To test our hypothesis, we conducted seasonal soil respiration measurements along a local-scale environmental gradient in a temperate forest in Northeast China, analyzed spatial variability of soil respiration and tested the relationships between soil respiration and a variety of abiotic and biotic factors including topography, soil chemical properties, and plant and soil diversity. We found that soil respiration varied substantially across the study site, with spatial coefficients of variation (CV) of 29.1%, 27.3% and 30.8% in spring, summer, and autumn, respectively. Soil respiration was consistently lower at high soil water content, but the influence of other factors was seasonal. In spring, soil respiration increased with tree diversity and biomass but decreased with soil fungal diversity. In summer, soil respiration increased with soil temperature, whereas in autumn, soil respiration increased with tree diversity but decreased with increasing soil nutrient content. However, soil nutrient content indirectly enhanced soil respiration via its effect on tree diversity across seasons, and forest stand structure indirectly enhanced soil respiration via tree diversity in spring. Our results highlight that substantial differences in soil respiration at local scales was jointly explained by soil properties (soil water content and soil nutrients), tree diversity, and soil fungal diversity but the relative importance of these drivers varied seasonally in our temperate forest.

Plant diversity is often expected to enhance soil organic carbon (SOC) storage through greater and more heterogeneous plant inputs, but its relationships with functionally distinct SOC fractions in natural forests remain uncertain. This study compiled published SOC fraction data from 341 surface soil observations in natural forests across China and spatially matched these records with gridded plant alpha diversity, forest age, climate, topographic and soil properties datasets to evaluate biotic and abiotic associations with SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and MAOC/SOC. Linear regression, multiple regression, piecewise structural equation modelling and stratified analyses were used to evaluate whether plant diversity was associated with the absolute accumulation and relative stabilization of SOC fractions. Plant alpha diversity was negatively associated with ln[SOC], ln[POC] and ln[MAOC] at the national scale, whereas its bivariate relationship with MAOC/SOC was weak. After accounting for forest age and environmental covariates, plant alpha diversity remained negatively related to the absolute contents of SOC fractions while showing a positive association with MAOC/SOC. Forest age was positively associated with ln[SOC], ln[POC] and ln[MAOC], and POC was more strongly related to plant diversity and forest age than MAOC. In contrast, MAOC and MAOC/SOC were more strongly associated with mineral protection potential, soil pH and precipitation background. Structural equation models indicated that mineral protection potential and mean annual precipitation were associated with greater MAOC accumulation and SOC allocation to the mineral-associated fraction, whereas temperature and topography were linked to MAOC partly through indirect associations with soil physicochemical conditions. Stratified analyses showed that plant diversity associations varied among forest types and climatic backgrounds. Additional interaction models showed that mineral protection potential significantly moderated the associations between plant alpha diversity and ln[SOC], ln[POC] and ln[MAOC], with negative diversity associations weakening under higher mineral protection potential. These findings indicate that plant diversity associations with SOC fractions in natural forests cannot be interpreted as universally positive input relationships. Instead, their direction and strength depend on hydroclimatic context and soil mineral protection, especially for the absolute accumulation of SOC fractions.