Abstract: The decomposition of deadwood is a crucial process for the accumulation and sequestration of soil organic carbon (SOC) in forest ecosystems. However, the response of SOC to different decay classes of deadwood and the underlying mechanisms remain poorly understood. Here, we investigated the dynamics of SOC, soil properties, extracellular enzyme activities, and phospholipid fatty acid biomarkers across five decay classes (ranging from 1 to 5) of Masson pine (Pinus massoniana Lamb.) downed deadwood in a subtropical–temperate ecotone forest in Central China. Our results revealed a nonlinear response pattern of SOC along the deadwood decomposition gradient, with the maximum value at the decay class 4. Soil available nitrogen content, bacterial biomass, fungal biomass, the ratio of fungal-to-bacterial biomass, cellulase, activity and ligninase activity all increased with the intensification of deadwood decay, while soil pH decreased. The increase in SOC content was associated with a direct positive effect of bacteria and both direct and indirect positive effects of fungi by cellulose activity, but ligninase activity showed no significant relationship with SOC content. These findings suggest that cellulose and microbial biomass are key determinants of soil C formation and sequestration during deadwood decomposition. This study highlights the importance of the nonlinear response of SOC to deadwood decay, providing valuable insights for predicting future carbon-climate feedbacks.

Key words: C-degrading enzyme activity, decomposition stage, microbial biomass, microbial composition, soil carbon pool