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) regulates the partitioning of carbon (C) between anabolic biomass synthesis and catabolic respiration, acting as a key determinant of soil organic carbon (SOC) sequestration. Because CUE is fundamentally constrained by resource stoichiometry, exogenous inputs of inorganic nitrogen (N) and exogenous organic C can substantially alter soil nutrient availability and composition globally. Yet whether and how microbial CUE responds differently to these two input pathways across gradients of input rate and experimental duration remains unclear. Here, we synthesized a global dataset of 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. We found that N addition and exogenous organic C inputs significantly increased microbial CUE by 8.6% and 29.4%, respectively. Notably, the response to N addition was decoupled from addition rate and duration, whereas exogenous organic C inputs exhibited a distinct biphasic temporal pattern. Further analysis revealed that for exogenous organic C inputs, short-term (≤ 24 years) variations were primarily regulated by chemical constraints, specifically initial soil total nitrogen and soil pH fluctuations. In contrast, long-term (> 24 years) responses were dominantly driven by mean annual temperature and microbial adaptation, as indexed by microbial biomass carbon. Collectively, these results indicate fundamentally different rate–duration response patterns between inorganic N enrichment and exogenous organic C inputs, and highlight the need to incorporate time-dependent controls when predicting soil C dynamics under sustained organic C inputs.