Exogenous organic carbon inputs drive a temporal shift in microbial carbon use efficiency from chemical constraints to microbial adaptation

Soil microbial carbon use efficiency (CUE) governs the partitioning of carbon (C) between anabolic biomass synthesis and catabolic respiration, thereby exerting a central control on soil organic carbon (SOC) sequestration. Because CUE is fundamentally constrained by resource stoichiometry, exogenous inputs of inorganic nitrogen (N) and organic C have the potential to substantially alter soil nutrient availability and composition globally. However, whether and how microbial CUE responds differently to these two input pathways across gradients of input rate and experimental duration remains poorly resolved. Here, we synthesized a global dataset comprising 250 paired observations from field experiments and employed hierarchical meta-analysis and linear mixed-effects models to quantify the independent and interactive effects of nutrient input rate and experimental duration on microbial CUE. Both N addition and exogenous organic C inputs significantly increased microbial CUE, by 8.6% and 29.4%, respectively. However, microbial CUE responses to N addition were largely decoupled from addition rate and duration, whereas exogenous organic C inputs exhibited a distinct biphasic temporal pattern. Specifically, short-term responses (≤ 24 yrs) were primarily regulated by chemical constraints, especially initial soil total N and soil pH changes, whereas long-term responses (> 24 yrs) were predominantly driven by mean annual temperature and microbial adaptation, as indicated by microbial biomass C. Collectively, these findings reveal fundamentally different rate–duration response patterns of microbial CUE under inorganic N enrichment versus exogenous organic C inputs, and highlight the importance of incorporating time-dependent controls when predicting soil C dynamics under sustained organic C inputs.