Drought effects on microbial carbon use efficiency regulated by particulate organic carbon and microbial life history strategy in a subtropical forest

Microbial carbon use efficiency (CUE) is a critical determinant of soil carbon storage. However, the long-term effects of drought on microbial CUE, particularly across soil profiles, remain poorly understood and represent a major source of uncertainty in ecosystem modeling. Here, based on a 12-year throughfall reduction experiment in a subtropical forest, we investigated the effects of prolonged drought on microbial CUE in both topsoil (0–15 cm) and subsoil (15–30 cm). Our results showed that drought significantly decreased microbial CUE by 20.14% overall, but exerted divergent effects between the topsoil and subsoil layers. Specifically, drought significantly reduced CUE in the topsoil, whereas it unexpectedly increased CUE in the subsoil. This contrasting pattern reveals a previously underappreciated depth-dependent response of microbial carbon metabolism to long-term drought. This increase in subsoil CUE was attributed to the greater stress tolerance of subsoil microbial communities, as indicated by a significant increase in the bacterial yield-to-acquisition (Y/A) ratio under drought. Mechanistically, drought-induced changes in microbial CUE across soil depths were jointly regulated by shifts in substrate availability, particularly particulate organic carbon (POC), and changes in microbial life-history strategies, reflected in the trade-off between growth yield and resource acquisition. Across both soil depths, the relative abundance of Proteobacteria emerged as the strongest microbial predictor of CUE, while both bacterial and fungal richness were positively correlated with CUE (P < 0.05). These findings highlight the importance of incorporating depth-dependent POC dynamics and microbial Y/A strategy shifts into land surface models to improve predictions of soil carbon climate feedbacks.