Fertilizer-N recovery links to SOC stabilization and fractionation in straw-mulched soil: Insights from 15N-tracing and 13C natural abundance

• 1% macroaggregate rise boosts SOC stock by 0.17 Mg ha −1 and N recovery by 0.41% • Fertilizer-N retention in aggregates is associated with increased SOC stabilization • Stable SOC exhibited strongest link to fertilizer-N distribution in soil profile • Straw mulching plus reduced N maintained yield and increased fertilizer-N recovery • Straw amendment enhanced SOC accumulation and microbial transformation While straw mulching has the potential to reduce fertilizer-nitrogen (N) losses in intensively managed cropland, how soil organic carbon (SOC) regulates this fate of fertilizer-N at soil aggregate or profile scales remains unresolved. Here, micro-plots were nested within a four-year field experiment to assess fertilizer-N fates and their linkages with SOC fractions and stabilization processes via 15 N-tracing and 13 C natural abundance analyses. Three treatments were included: (i) conventional N application (FN), (ii) reduced N application (RN), and (iii) reduced N with straw mulching (RS). While RN reduced crop yields compared to FN, RS achieved comparable yields and 7.71% higher N recovery efficiency ( P <0.05). The δ 13 C fractionation between aggregates and bulk soil was significantly positively correlated with the fertilizer-N content in the >2 mm and <0.053 mm fractions, indicating that N retention was coupled with SOC stabilization processes. Compared with RN, RS resulted in a 2−3.4 times greater SOC conversion probability into the <0.053 mm fraction and a 1.4 times higher aggregate-associated fertilizer-N content. SOC fractions differentially regulated the profile distribution of fertilizer-N, with nonlabile organic carbon (C) correlated positively, while dissolved organic C correlated negatively but increased plant N recovery. Compared with RN, RS increased the SOC stock by 24%, reduced NO 3 − -N accumulation by 37%, and immobilized 36% more N into the microbial biomass ( P <0.05). Our findings demonstrate that straw mulching increases N recovery by mediating SOC fractionation, stabilization, and microbial N immobilization. These results provide new insights into SOC‒N interactions that could aid in the development of optimal soil C and N management strategies.