Tree species diversity has been found to promote soil organic carbon (SOC) in forests, but its effects on SOC stability have been poorly studied. Using a six-year-old forest biodiversity experiment with monocultures and mixtures of two, four, and eight tree species, we specifically evaluated how functional diversity (FDis) and community-weighted mean (CWM) of leaf nutrients influence the formation of mineral-associated organic carbon (MAOC) via altering the soil microbial community. We found that FDis of leaf nitrogen (LN_mass) and phosphorus (LP_mass) contents, as well as CWM of LP_mass were negatively associated with MAOC, patterns that were mediated by microbial biomass. In addition, CWM of LN_mass was negatively associated with the MAOC:SOC ratio, a relationship mediated by a decrease in the ratio of fungal to bacterial biomass (F:B ratio), while CWM of LP_mass exhibited a direct positive effect on the MAOC:SOC ratio. We also found that soil nutrient availability mediated the relationship between the diversity of leaf nutrients on the soil microbial community. Our results suggested that the diversity of leaf nutrient contents may shape SOC stabilization through moderating microbial biomass and F:B ratio, offering insights into the ecological importance of plant chemical traits in driving SOC stabilization in forest ecosystems.