Abstract: Microbial carbon use efficiency (CUE) is a critical factor affecting soil organic carbon (SOC) dynamics. However, the relative importance of biotic and abiotic drivers of CUE variations remains debated, resulting in large uncertainties in SOC predictions. In this study, we combined field soil sampling with isotope labeling experiments to identify the driving factors of changes in microbial CUE along an elevation gradient (1,200–2, 400 m) in the Qinling Mountains. Our findings demonstrated that higher elevations (2,000–2,400 m) resulted in greater microbial CUE, which partially aligned with the enhancement of SOC sequestration and the total SOC content. Notably, microbial CUE was more closely linked to microbial community composition rather than abiotic factors alone. The fungal community at higher elevation, shaped by soil substrate availability, promoted higher nutrient acquisition function and stronger synergistic interactions of microbes within the microbial network, leading to higher CUE. These results highlight the importance of integrating the taxonomic and functional traits of microbial communities to predict community-level CUE. Our findings provide a potential mechanism underlying community- level metabolic efficiency, which has important implications for understanding the microbial-mediated carbon dynamics under global climate changes.

Key words: Soil organic carbon, microbial carbon use efficiency, nutrient acquisition, microbial community, biotic and abiotic drivers