Abstract
Subsoils below 30 cm store more than half of global soil carbon. Microbial carbon use efficiency (CUE) serves as a key indicator of microbial control over soil carbon turnover, but the general patterns and drivers of microbial CUE across soil depths remain poorly understood. Here, we report a decreasing trend in microbial CUE with increasing soil depths through large-scale soil sampling across 60 sites spanning tropical to boreal forests. Using multiple analytical and statistical approaches complemented by experiments, we further identify depth-dependent drivers of microbial CUE. In the topsoil (0-10 cm), microbial CUE is primarily regulated by microbial diversity, whereas in deep subsoil (70-100 cm), it is predominantly driven by soil physicochemical protections. Our findings underscore the need to incorporate depth-specific microbial CUE drivers into carbon cycle models for more accurate predictions of whole-soil carbon storage and its feedback to climate change.