The effects of straw returning and nitrogen fertilizer application on soil labile organic carbon fractions and carbon pool management index in a rice-wheat rotation system

Straw returning and nitrogen (N) fertilizer application are important agricultural measures to improve soil carbon pool. However, the combined effect of various straw-returning amounts and N-fertilizer application rates on soil carbon pool is still unclear. Therefore, a field experiment with three straw-returning amounts and three N-fertilizer application rates was conducted for 4 years in central China. The results revealed that the addition of N fertilizer clearly enhanced the contents of soil organic carbon (SOC) by 2.2%-2.7%, dissolved organic carbon (DOC) by 20.1%-39.2%, microbial biomass carbon (MBC) by 37.9%-44.2%, and easily oxidizable carbon (EOC) by 18.4%-23.6%. Moreover, it also enhanced the activities of invertin by 33.4%-46.4%, cellulase by 23.5%-32.4%, and polyphenol oxidase by 20.5%-27.1%, as well as the carbon pool management index (CPMI) by 21.3%-27.6%. The CPMI increased gradually as the N-fertilizer application rate increased. However, straw returning did not significantly change the contents of SOC, EOC, and MBC; invertin activity, or CPMI. Based on the structural equation model, it was observed that N fertilizer application enhances MBC and subsequently improves CPMI by increasing EOC. This highlights the vital role of soil microorganisms in the soil carbon cycle and the significance of supplementing N to improve and sustain agricultural soil carbon reservoirs. Additionally, this study indicates that the combined application of N fertilizer and straw returning has a more positive effect on improving the soil carbon pool.