Differential Response of Microbial Necromass Carbon in Rhizospheric and Non-Rhizospheric Soil to Mulching Measures and How It Drives C Sequestration Mechanism on Loess Plateau

Microbial necromass carbon (MNC) is recognized as an important and relatively stable component of soil organic carbon (SOC); however, it is often overlooked and poorly understood in soil management practices, particularly in the context of Loess Plateau farmlands. Here, a 13-year field experiment was carried out to examine the differences in MNC distribution, the role of MNC in SOC storage, and the impact of environmental factors under long-term mulching practices. The experiment used four treatments: (1) no mulching (NT), (2) straw mulching (NSM), (3) plastic mulching (NPM), and (4) ridge mulching (NRM). Compared to NT, all mulching methods increased SOC levels, phospholipid fatty acids (PLFAs), and amino sugar (AS) content. Straw mulching enhanced microbial biomass carbon (MBC), reduced the gap in AS content between rhizosphere and non-rhizosphere soils, and significantly increased MNC. Conversely, NPM and NRM primarily increased MBC and MNC within the rhizosphere soil. Generally, the rhizosphere soil had higher AS content than non-rhizosphere soil. However, regarding the proportion of MNC contributing to SOC, non-rhizosphere soil showed a significantly greater contribution than rhizosphere soil (p < 0.05). The contribution of MNC to SOC ranged from 10.70% to 26.38% under different treatments. Fungal-derived MNC generally contributed more to SOC (7.96–19.73%) than bacterial-derived MNC (2.62–6.65%). Soil temperature, the C/N ratio, pH, and total phosphorus influence microbial community structure and MBC, which in turn affect MNC and regulate SOC. These results enhance our understanding of how agricultural management practices on the Loess Plateau affect carbon sequestration.