Soil microbial necromass regulation of long-term fertilizer N retention influenced by maize stover mulching

Microbial immobilization of nitrogen (N) fertilizer is critical for N retention in soils and is mainly influenced by agricultural management, including crop residue return. However, the underlying relationship between microbially mediated processes and long-term fertilizer N dynamics has not been characterized well in arable soil. Here, by tracing in situ the temporal changes of soil amino sugars initially synthesized from the 15N fertilizer applied in the 1st year, we investigated the dynamic contribution of microbial necromass to long-term fertilizer N retention in soil influenced by maize stove mulching. After the 1st year of fertilizer application, microbial necromass contributed to 36.3-43.9 % of the remaining fertilizer N in the topsoil (0-10 cm), but the remaining fertilizer N in the subsoil (10-20 cm) was predominantly attributed to microbial necromass accumulation, regardless of maize stover mulching. During the following 9 years, the fertilizer-derived microbial necromass N decreased at a slower rate (42.3-52.8 %) than the remaining fertilizer N (73.5-76.7 %) in the topsoil layer, while the former decreased synchronously with the latter in the subsoil layer, indicating that microbial necromass played a critical role in long-term fertilizer N stabilization though it could serve as a transitional N pool for N supply. For the microbial necromass of different origin, the residence time of fertilizer N in fungal necromass (34.7-48.7 y) was longer than that in the bacterial necromass pool (21.4-27.1 y) in the cultivated soil layer (0-20 cm), explicitly indicating that the retention of fungal necromass was inclined to contribute to long-term fertilizer N stabilization while bacterial necromass was associated with fertilizer N turnover. Compared with fertilizer application alone, maize stover mulching enhanced the initial fertilizer N retention by increasing microbial necromass accumulation but had no influence on the subsequent dynamics of both the remaining fertilizer N and fertilizer-derived microbial necromass N, thus improving long-term fertilizer N retention. Specifically, maize stover mulching increased the stability of microbial necromass N by enhancing the retention and prolonging the residence time of fungal necromass over bacterial necromass, thus improving the contribution of microbial necromass to the long-term stabilization of fertilizer N in the cultivated soil.