Objectives Enhancing straw decomposition rate in soil facilitates the promotion and application of straw return technology. We investigated changes in microbial resource limitations and microbial carbon use efficiency (CUE) under combined application of decomposition agents and nitrogen fertilizer, and elucidated the mechanism of straw incorporation with decomposition agents on soil microbial metabolism. This study provides a theoretical basis for the rational use of decomposition agents in farmland straw management.

Methods An indoor incubation experiment was conducted with four factors: straw (S), decomposition agent (D), nitrogen fertilizer (N), and incorporation method. Treatments included: CK, control (no additions); S, straw incorporation (chopped and mixed); N, nitrogen fertilizer; S+N, straw+nitrogen fertilizer; S+D, straw+decomposition agent; S+N+D, straw+nitrogen fertilizer+decomposition agent; SⅡ+N+D, straw in nylon bag+nitrogen fertilizer+decomposition agent. Soil samples were destructively collected on days 30, 60, and 90 of the 90-day incubation period. Straw fiber structure was observed via electron microscopy, and soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and enzyme activities were analyzed. Stoichiometric ratios of enzyme activities were calculated.

Results By days 60 and 90, the S+N+D treatment exhibited significantly more pore structures on straw surfaces compared to CK and S treatments. Microbial biomass and extracellular enzyme activities in all treatments increased initially and decreased over time. At peak activity (incubation day 60), S+N+D significantly increased MBC, MBN, and MBP by 154.0%, 55.0%, and 38.4%, respectively, relative to S+D. The SⅡ+N+D treatment showed significantly lower microbial biomass and enzyme activities than S+N+D. Stoichiometric analysis revealed carbon (C) and phosphorus (P) limitations across all treatments. The lowest microbial C limitation occurred in S+D (vector length 1.35±0.01), while the lowest P limitation was in S+N+D (vector angle 49.08°±1.42°). Except for day 30, S+N+D exhibited higher microbial CUE, which was significantly and negatively correlated with microbial resource limitations.

Conclusions Combined application of decomposition agents and nitrogen fertilizer effectively increases microbial biomass and enzyme activities, alleviates C and P limitations imposed by straw, accelerates straw decomposition, and enhances CUE. Therefore, we recommend integrating decomposition agents and nitrogen fertilizer with straw return practices in agricultural fields.