Nitrogen Fertilizers Prime Bacterial Necromass Formation and Carbon Sequestration in Soil

Carbon (C) input is essential for the formation, turnover, and sequestration of soil organic carbon (SOC) in terrestrial ecosystems. Nitrogen (N) fertilization controls microbial turnover, thereby altering the rate of C conversion from crop residues to SOC. However, it is not yet clear how C and N inputs affect the contributions of plant- and microbially-derived C to SOC sequestration. To elucidate this question, a long-term (18 years) field experiment was conducted to examine the response of SOC sequestration to straw return and N fertilization assessed by plant and microbial biomarkers, living microbial biomass, and enzyme activities. Fungal necromass C (39%) contributed most to SOC sequestration, compared to bacterial necromass (5.2%) and lignin phenols (2.1%). Higher straw return increased SOC, lignin phenols, and fungal necromass, as well as the proportion of vanillyl and syringyl in SOC, but decreased the proportion of cinnamyl phenols. Thus, higher straw return did not alter the contribution of plant- and microbially-derived C to SOC. N fertilization increased SOC, lignin phenols, and microbial necromass, as well as the contribution of bacterial necromass to SOC. N fertilization also accelerated the conversion of crop residues to microbial necromass. Fungi, actinomycetes, and Gram-negative bacteria were key microorganisms in this process because their biomass correlated closely with C-acquiring enzyme activities and necromass content. The slope between bacterial necromass and living biomass (0.06-0.13) was much lower than that of fungi (1.9). Bacterial necromass was closely correlated with the activities of N-acquiring enzymes, indicating intense recycling of bacterial necromass and nutrients. In conclusion, N fertilization and higher straw return accelerated crop residue utilization, microbial turnover, and necromass formation, thereby increasing SOC sequestration, especially from fungal necromass.