Plant communities strongly influence soil microbial communities and, in turn, soil carbon (C) cycling. Microbial carbon use efficiency (CUE) is an important parameter for predicting soil C accumulation, yet how plant and soil microbial community traits influence microbial CUE remains poorly understood. Here, we determined how soil microbial CUE is influenced by plant and soil microbial community traits, by studying a natural gradient of plant species diversity in a subtropical forest. Our results showed that microbial CUE increased with increasing tree species diversity, suggesting a correlation between plant community traits and soil C storage. The specific soil properties that explained the greatest variation in microbial CUE were associated with microbial communities (biomass, enzyme activities and the ratio of oligotrophic to copiotrophic taxa); there were weaker correlations with plant‐input properties, soil chemistry and soil organic C quality and its mineral protection. Overall, high microbial CUE was associated with soil properties correlated with increased tree species diversity: higher substrate availability (simple SOM chemical structures and weak mineral organic associations) and high microbial growth rates despite increased community dominance by oligotrophic strategists. Our results point to a mechanism by which increased tree species diversity may increase the forest C sink by affecting carbon use with the soil microbial community.

Predicting soil carbon dynamics under warming is constrained by limited understanding of microbial thermal adaptation, particularly whether microbial carbon use efficiency (CUE) can adapt to warming and how plant diversity modulates this response. Using soils from a natural tree species diversity gradient in a subtropical forest, we combined a 365‐day laboratory incubation with regular substrate amendment and ¹⁸ O‐H ₂ O labeling to quantify thermal responses of microbial respiration, growth, and CUE. High tree species diversity was associated with a strengthened compensatory thermal adaptation of microbial respiration and growth, effectively dampening their response to warming. Simultaneously, diversity promoted an enhanced thermal response of CUE, increasing microbial carbon retention capacity under warming. This dual regulation was mechanistically linked to a cascade of processes: higher tree species diversity was associated with lower soil organic matter stability (i.e., higher lability), minimizing bioenergetic costs of enzyme synthesis, facilitating a community‐wide shift toward r‐selected bacteria, and intensifying microbial competition as evidenced by network topology. Our findings reveal the potential of biodiversity to buffer soil carbon losses: conserving and restoring plant diversity can enhance soil capacity to mitigate climate change, both by reducing respiratory carbon losses and by increasing the potential for microbial carbon sequestration under warming.