Stimulatory effects of nutrient addition on microbial necromass C formation depend on soil stoichiometry

Nutrient availability strongly influences soil organic carbon (SOC) accumulation through microbial necromass C (MNC) formation. However, the effects of nutrient addition on regulating MNC formation in soils with distinct stoichiometric ratios are not well understood. Soil samples were collected from a long-term (>40 years) wheat-maize rotation field trial, without and with nutrient management, which exhibited high and low C:nutrient (N and P) ratios, respectively. These soils were incubated with C-13-labeled glucose (labile C) and nutrients (N and/or P) for 63 days. A greater proportion of labile C was converted into MNC, especially bacterial necromass C, in the low C:nutrient soil (averaging 32 %) than in the high C:nutrient soil (averaging 25 %). In the high C:nutrient soil, N and/or P addition significantly increased newly formed MNC (C-13-MNC) by 8-33 %, whereas in the low C:nutrient soil, N and P addition had a marginal and negative effect on C-13-MNC, respectively. The C-13-MNC content was negatively correlated with resource stoichiometric ratios (dissolved organic C (DOC):mineral N, microbial C:N imbalance, and DOC:available P), and enzyme activities (beta-1,4-glucosidase, leucine amino peptidase, N-acetyl-glucosaminidase, and alkaline phosphatase). These results suggest that the increase in MNC is primarily due to the alleviation of N and P limitations. Furthermore, the C-13-MNC content was positively correlated with the C-13-phospholipid fatty acids content, indicating that enhanced microbial anabolism, especially bacterial anabolism, promoted MNC formation. Collectively, our findings emphasize the critical role of soil stoichiometry in regulating microbial metabolism and MNC formation in response to nutrient addition. These insights have significant implications for optimizing nutrient management to enhance SOC sequestration in arable soil.