Effects of nitrogen management and straw return on soil organic carbon sequestration and aggregate‐associated carbon

SUMMARY: Soil organic carbon (SOC) sequestration and aggregate‐associated carbon (C) can be strongly affected by nitrogen (N) management and straw return. However, an in‐depth understanding of the effect of N and straw application on SOC and aggregate‐associated C is lacking. After 6 years of five inorganic and organic N amounts under straw removal and straw return, we investigated SOC sequestration at a depth of 0–60 cm and soil aggregate‐associated C in the topsoil (0–20 cm). The results were as follows: (i) the 0–60‐cm depth SOC stocks for all treatments except N₀ increased by 2.5–25.5% after 6 years, (ii) sequestered SOC was linearly correlated (P < 0.01) with estimates of C input and the rate of C input required to maintain the SOC stock level was 3.68 Mg ha⁻¹ year⁻¹, (iii) application of chemical N increased SOC mainly through an increase in coarse particulate organic matter (POM) and silt‐ and clay‐bound C in macroaggregates, whereas with the organic N‐incorporated treatments SOC increased mainly through an increase in intra‐microaggregate POM within macroaggregates, (iv) the main mechanism for the formation of macroaggregates was POM bound to primary particles (sand, silt and clay) that formed microaggregates and eventually macroaggregates and (v) intra‐microaggregate POM within macroaggregates (imMPOM) is a good indicator of the efficacy of N fertilization and straw management in stabilizing soil aggregates. In conclusion, chemical N incorporated with straw and organic fertilizer can enhance SOC stocks, aggregate stability and aggregate‐associated C, which might promote long‐term C sequestration. HIGHLIGHTS: Effects of N and straw management on SOC sequestration and aggregate‐associated C were investigated. Consolidated SOC fractionation was used to evaluate the effects of N and straw on aggregate‐associated C. Intra‐microaggregate POM within macroaggregates is a good indicator of management on soil aggregate stability. Chemical N mixed with organic N and straw increased the amount and quality of SOC.