Straw incorporation and nitrogen fertilization enhance soil carbon sequestration by altering soil aggregate and microbial community composition in saline-alkali soil

Background and aims Straw incorporation and optimized fertilization application are considered as effective strategies for agricultural sustainable development. However, the interaction of straw incorporation and N fertilization on labile carbon fraction, soil aggregate stability and microbial community composition are less known, especially in saline-alkali soil. Methods A 4-year experiment was conducted in the Yellow River Delta with two factors: straw incorporation (0, 4 500 and 9 000 kg ha −1 ; C0, C1 and C2, respectively) and N fertilization (255 and 400 kg N ha −1 ; N1 and N2). Results Soil organic carbon (SOC), microbial biomass carbon (MBC), dissolved organic carbon (DOC) and particulate organic carbon (POC) contents were also increased. Meanwhile, the proportion of macroaggregate was enhanced with straw incorporation through the increase of MBC and POC. Compared with N2 level, DOC:SOC ratio was decreased with straw application under N1 level, reflecting the improvement of SOC adsorption capacity. Additionally, the proportion of straw-C converted to SOC under N1 level was about 21–25%, 15–20% under N2 level, and the highest conversion rate was in N1C1 treatment. We also found DOC and SOC were the most important factors to affect the abundance of bacteria ( Firmicutes ) and fungi ( Ascomycota ) involved in straw-C degrading, which were lower under N1 level. Conclusions It is feasible to achieve higher carbon content while increasing soil aggregate stability by optimizing straw incorporation and N fertilizer in saline-alkali soil. These findings provide important insights for improving saline-alkali soil.