Elevated soil organic carbon content limits microbial carbon use efficiency under manure amendment in Mollisols

Soil organic carbon (SOC) sequestration under organic amendments is strongly mediated by microbial processes, especially microbial carbon use efficiency (CUE). However, how amendment effects vary with baseline SOC content and microbial carbon–nitrogen (C:N) stoichiometry remains unclear. In the Mollisol zone, Northeast China, long-term SOC decline persists despite widespread manure and straw inputs, suggesting unresolved constraints in microbially driven carbon stabilization. This study examined how SOC content and microbial stoichiometry regulate carbon sequestration responses to manure and straw across six long-term (>5 years) field experiments. In soils with SOC < 18 g kg −1 , manure significantly increased SOC accumulation (by 9.6–26.7%) compared to that of straw, whereas in soils with SOC > 18 g kg −1 , manure reduced SOC (by 9.8–22.3%) and carbon sequestration relative to the unamended control. Manure increased dissolved organic carbon at all sites and alleviated microbial nitrogen limitation, as indicated by the elevated ratios of C:N-acquiring enzyme activity and reduced abundance of nitrogen acquisition genes. This improved microbial CUE in soils with SOC < 18 g kg −1 (+2.55%). However, in soils with SOC > 18 g kg −1 , characterized by elevated SOC:TN ratios and weaker nitrogen limitation, manure inputs likely increased carbon and nitrogen availability beyond microbial assimilative capacity. This imbalance was consistent with enhanced overflow respiration, as indicated by the increased relative abundance of respiration-related genes and enzymes, resulting in reduced CUE (−2.97%). Thus, SOC-dependent microbial strategies govern the efficiency of organic amendments, emphasizing tailoring residue return practices in enhancing site fertility and microbial nutrient constraints to optimize long-term carbon sequestration.