Factors influencing soil carbon stocks in secondary subtropical forests of eastern China

Soil organic carbon (SOC) stocks in forest ecosystems are a critical component of the global carbon cycle. Although multiple drivers affect SOC stocks, few studies have simultaneously tested the relative contributions of forest type, tree species richness and soil properties to SOC stocks in subtropical forests. We addressed this gap by investigating how these factors regulate SOC stocks across 25 sites spanning five forest types (bamboo, coniferous, deciduous broadleaf, evergreen broadleaf, and mixed forests) in subtropical forests. At each site, we measured SOC stocks in the topsoil and subsoil, along with assessments of soil properties, above- and belowground biomass, litter quantity and quality. The results revealed that evergreen broadleaf forests exhibited the highest SOC stocks among all forest types. Tree species richness correlated positively with topsoil carbon stocks, especially in deciduous stands, but showed a negative correlation in evergreen broadleaf forests. Greater belowground biomass also enhanced SOC in the topsoil, whereas high phosphorus in litter and soil could lead to carbon loss. These relationships were stronger in the topsoil. Bayesian structural equation modeling revealed that soil properties and phosphorus content in litter were key mediators of SOC stocks. Forest type exerted a direct positive effect on SOC stocks, while tree species richness promoted SOC stocks indirectly by altering litter quality and soil properties. These findings suggest that the impact of tree species richness on SOC is dependent on forest type and soil conditions. Therefore, forest management strategies aiming to promote SOC through biodiversity should consider forest type comprehensively, especially since phosphorus availability emerges as a key regulatory factor.