Phosphorus Fertilization Reduces Soil Microbial Necromass Carbon Content in Tillage Layer of Dry Farmland on Loess Plateau

This study examines how nitrogen and phosphorus fertilization influence soil microbial necromass carbon (MNC) content of farmland on the Loess Plateau, central Gansu. Based on an extensive (6 years) experiment, a control (CK, no fertilization) and three treatment groups employing different fertilization methods, namely, nitrogen fertilization (N, 115 kg·ha −1 ), phosphorus fertilization (P, 115 kg·ha −1 ), and combined fertilization of nitrogen and phosphorus (NP, 115 kg·ha −1 each), were set up in this research. The results show that, in the tillage soil layer (within a depth range of 0–20 cm), the application of nitrogen and/or phosphorous fertilizers can significantly reduce the ratio between glucosamine and muramic acid (GluN/MurA) ( p < 0.05), with a reduction range of 12.70–35.29%. Phosphorus fertilization can also reduce the content of fungal necromass carbon (FNC) and MNC and their contributions to SOC ( p < 0.05). In addition, phosphorus fertilization and combined fertilization of nitrogen and phosphorus can both increase the content of bacterial necromass carbon (BNC) and contribute to the content of SOC ( p < 0.05). Primarily because of the reduced accumulation efficiency of FNC, the combined fertilization of nitrogen and phosphorus can significantly decrease the accumulation efficiency of MNC. In the non-tillage soil layer (within depth range of 20–40 cm), both nitrogen fertilization and the combined fertilization of nitrogen and phosphorus can increase the content of FNC and MNC in soils and their impacts on SOC ( p < 0.05). The addition of nitrogen and/or phosphorus fertilizers does not alter the accumulation efficiency of soil MNC. Total phosphorus (TP), total nitrogen (TN), soil pH, nitrogen-to-carbon ratio of microbial biomass (MBN/MBC), leucine aminopeptidase (LAP), and β-glucosidase activities (BG) are the primary factors that affect changes in FNC, BNC, and MNC. In summary, phosphorus fertilization alone decreases soil MNC contribution to SOC and reduces carbon pool stability in the tillage layer. On the contrary, both nitrogen fertilization and the combined fertilization of nitrogen and phosphorus can increase the content of soil MNC in the non-tillage layer and its impact on SOC, thus improving the stability of SOC.