Effects of nitrogen fertilizer on soil microbial residues and their contribution to soil organic carbon and total nitrogen in a rice-wheat system

Soil microbial residues have an impact on the stability of the soil organic carbon (SOC) pool, since they are crucial and stable sources of SOC. However, it is not clear how soil microbial residues in aggregates affect the SOC and total nitrogen (TN) at different gradients of N fertilization. Therefore, we investigated the effect of N fertilizer on microbial residues and their contribution to SOC and TN in soil aggregates under a rice-wheat (Oryza sativa-Triticum aestivum) system with four rates of N fertilizer. The results indicated that the addition of N fertilizer clearly increased the phospholipid fatty acids of bacteria and enhanced the activities of soil enzymes (β-1,4-glucosidase, β-N-acetyl-glucosaminidase and phenol oxidase) in the macroaggregates. The content of bacterial residual carbon increased noticeably when N fertilizer was added, while the content of fungal residual carbon did not appear to change. The addition of N fertilizer increased the contribution of bacterial residual carbon to SOC and TN. There was no significant difference in the contribution of bacterial residues on SOC and TN at different N fertilizer rates. Nevertheless, their contribution gradually decreased with the decrease in aggregate size. Structural equation modeling showed that the contribution of soil microbial residue to TN was influenced by activities of soil enzymes, suggesting that N is an important factor that affects soil ecology and limits the availability of nitrogenous organic matter in agroecological systems. This finding is crucial to understanding how N fertilizer influences soil C and N cycle in rice-wheat systems.