Nitrogen Fertilizer Builds Up Soil Organic Carbon Under Straw Return Via Microbial Necromass Formation

Carbon (C) and nitrogen (N) input strongly influence the formation, turnover, and sequestration of soil organic carbon (SOC) in agricultural ecosystems. However, it is not yet clear how N inputs regulate the contributions of plant- and microbially-derived C to SOC sequestration under straw return. To fill this knowledge gap, a long-term (18 years) field experiment was conducted to elucidate the response of SOC sequestration to straw return and N fertilization assessed by plant and microbial biomarkers, and enzyme activities. Straw returns increased SOC content and the proportion of total lignin (mainly vanillyl and syringyl) phenols in SOC. In contrast, straw return decreased the contribution of cinnamyl phenols to SOC. In addition, straw return increased phospholipid fatty acids (PLFA) content and enzyme activities, which indicated the acceleration of straw decomposition. Therefore, this contradiction means that lignin alone is not sufficient to assess the contribution of plant-derived C to SOC under straw return. N fertilization increased the contents of SOC, PLFA and microbial necromass, as well as activity of C and N-acquiring enzymes. N fertilization decreased the maximal plant residue contribution to SOC, but raised the portion of microbial necromass (especially bacterial necromass) C. Accordingly, N fertilization accelerated the microbial utilization of straw and consequently microbial necromass formation. In terms of PLFA composition and necromass content, Ascomycota and Basidiomycota, Actinobacteria, and Gram-negative bacteria were the key groups forming microbial necromass and consequently SOC. N fertilization boosted the involvement of bacterial necromass in nutrient cycling, which in turn stimulated plant and microbial growth. Although the bacterial necromass contribution to SOC was increased under N fertilization, the relative importance of fungal necromass C in SOC sequestration was highest compared to bacterial necromass and lignin phenols. Concluding, N fertilization and straw return accelerated straw utilization, microbial turnover and necromass formation, thereby increasing SOC sequestration.