High Tree Species Diversity Promotes Thermal Enhancement Response of Microbial Carbon Use Efficiency

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 ¹⁸O-H₂O 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.