Background and aimsMicrobial biomass carbon (MBC) has a significant contribution to soil carbon (C) pool. It has been suggested that plant input, soil C and nutrient condition, and microbial characteristic play crucial roles in MBC accumulation. However, the primary driver of MBC accumulation remains uncertain.MethodsTo fill this knowledge gap, we conducted a greenhouse 13CO2 labelling experiment by planting three pioneer species (Pinus tabuliformis, Betula platyphylla, Populus purdomii) in soils collected from three different depths, i.e., top-soil (0-10 cm), mid-soil (10-30 cm), and deep-soil (30-100 cm).ResultsWe found that MB13C positively correlated with plant aboveground and belowground 13C. The effects of plant 13C on MBC accumulation were mainly mediated by fungal diversity and composition. Specifically, mycorrhizal fungi (e.g., Peziza) and toxigenic genera (e.g., Fusarium and Penicillium) were identified as crucial fungal taxa. Notably, plant 13C allocation (aboveground 13C, belowground 13C, proportion of aboveground 13C, and proportion of belowground 13C) explained a larger proportion (17.50%) of MB13C variation among treatments than did soil available phosphorus contents and microbial community structure (fungal diversity and composition) (1.98%).ConclusionOur study suggests that plant inputs are major determinants of soil C storage, as plant C allocation was identified as the primary driver of MBC accumulation. Given the different roles of fungi and bacteria, separating fungal and bacterial biomass C can refine our understanding of MBC accumulation.