Climate and forest type dominate altitudinal variations in soil organic carbon in subtropical montane forests

Despite the recognized complexity of biotic-abiotic interactions in regulating soil organic carbon (SOC) dynamics, their relative contributions in subtropical montane forests remain poorly quantified. This study examined SOC and its drivers across an altitudinal gradient in four forest types—evergreen-deciduous broad-leaved forest (EDF), deciduous broad-leaved forest (DF), coniferous-broadleaved mixed forest (MF), and dark coniferous forest (DC) within the Shennongjia Forestry District. Using a stratified random sampling design, we established 160 plots across elevation zones to capture forest-type and elevational variation. Here, we hypothesized that SOC increases with elevation, with higher content in subalpine forests, abiotic factors exert stronger direct effects on SOC along elevation gradients than biotic factors and biotic factors dominate SOC dynamics in mixed and coniferous forests. We integrated biological (litter quality, stand attributes, tree diversity) and abiotic (climate, soil properties) factors, applying piecewise structural equation modeling (pSEM) to evaluate fixed effects and forest-type random effects on SOC. Linear SEM was further used to examine pathways across elevation and forest types. Results indicated that SOC increased significantly with elevation and showed substantial variation among forest types. Climate, in particular mean annual temperature (MAT), served as the primary driver of SOC variation along elevational gradients, followed by litter carbon-to-nitrogen ratio and soil pH. Forest types explained more SOC variability than environmental factors and played a significant mediating role in biogeochemical relationships. Biological factors, for example litter quality, species richness, and tree diameter at breast height (DBH), were the dominant influences across all forest types except deciduous broadleaved forests. Species richness indirectly enhanced SOC by reducing litter C/N, although this effect varied by forest type. These findings support a hierarchical framework wherein climate and forest type jointly regulate SOC, underscoring the importance of incorporating biotic-abiotic interactions to accurately predict carbon cycling under global change.