Increasing ECM tree dominance enhances soil lignin phenols but suppresses microbial necromass contribution to soil organic carbon in a subtropical mountainous forest

• ECM% did not influence SOC content, but significantly altered SOC composition. • Plant-derived C increased with higher ECM% due to reduced litter quality and greater litter and fine root inputs. • Microbial-derived C decreased with higher ECM% due to reduced microbial metabolic efficiency and accelerated degradation. Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) plants exhibit distinct litter quality characteristics and nutrient uptake strategies that critically influence soil carbon and nitrogen cycling. However, the effect of mycorrhizal type on soil organic carbon (SOC) accumulation remains unclear, particularly in tropical and subtropical forests in China. To address this knowledge gap, this study quantified the SOC content and its components across a gradient of ECM tree dominance (ECM%, defined as the proportion of ECM tree basal area relative to the all trees) in a subtropical mountainous forest. Lignin phenols and amino sugars were used as indicators of plant- and microbial-derived C in the soil, respectively. Plant, soil, and microbial properties were analyzed to explore the underlying mechanisms influencing SOC components across different mycorrhizal types. Our results indicated that ECM% did not affect SOC content but significantly altered SOC composition. Specifically, a higher ECM% promoted plant-derived C accumulation and reduced microbial-derived C. The increase in plant-derived C accumulation with ECM% was primarily owing to lower litter quality and higher C inputs from litter and fine roots. Conversely, microbial-derived C accumulation decreased with ECM%, primarily because of the lower metabolic efficiency and higher degradation rate of microbial necromass, as indicated by metabolic quotient and extracellular enzyme activities, respectively. Overall, these findings improve existing understanding of the mechanisms underlying SOC formation and decomposition along the gradient of ECM tree dominance, providing valuable insights into how shifts in tree mycorrhizal dominance under global change may alter the pathways and composition of SOC accumulation in forest soils.