Thermal adaptation mitigates the warming-induced decline of microbial carbon use efficiency in global soils

The response of soil microbial carbon use efficiency (CUE) to global warming is a critical uncertainty in projecting soil carbon feedbacks, with conflicting empirical data obscuring a clear mechanism. We integrated a global meta-analysis with a laboratory warming experiment across a subtropical climate gradient to mechanistically isolate CUE responses. We found that warming consistently suppressed CUE in both field-synthesized and controlled laboratory settings. Structural equation modeling revealed that this CUE decline was driven by reduced carbon substrate accessibility, as warming accelerated the depletion of labile pools and strengthened organo-mineral associations, shifting microbial metabolism toward costly acquisition. Critically, however, this CUE suppression systematically diminished with both increasing experimental duration and warmer baseline climates, providing convergent evidence for a potent thermal adaptation mechanism. Global model projections show warming reduces CUE at +2 °C and +4.8 °C, with declines attenuating over 5–15 years; under +4.8 °C sub/tropical CUE increase over time (up to ~17.3%), while boreal and arctic continue to decline, identifying high-latitude soils as vulnerable carbon loss hotspots. Our findings establish that CUE responses are governed by the dynamic interplay between substrate accessibility and microbial adaptive capacity, offering a mechanistic framework vital for constraining soil carbon-climate feedbacks in Earth system models.