Dynamics of microbial necromass in response to reduced fertilizer application mediated by crop residue return

Understanding the response of microbial-derived nitrogen (N) in the soil to N availability is essential for optimizing N fertilizer management. In this study, based on amino sugar biomarker assays in a conservation tillage agroecosystem, we evaluated the effects of 3-year fertilizer N reduction (reduced from 240 to 190, 135, 0 kg N ha−1) and crop residue return on microbial necromass N dynamics. The stock of microbial necromass N declined with the decrease of N input, and the decline of bacterial necromass was greater than that of fungal necromass. However, the decrease of 7.3% in microbial necromass N following the cessation of fertilization indicated a dominant role of microbial necromass in soil N retention despite its compensation ability for N demand. Maize residue return alleviated N deficiency in the soil-crop system and favored the maintenance of the soil organic N pool by preferentially improving the net accumulation of fungal necromass.