Nitrogen addition, not warming, enhances soil microbial necromass carbon accumulation in a desert steppe

Soil microbial necromass carbon (MNC), a critical component of soil organic carbon (SOC), plays a vital role in the formation and stabilization of SOC. Climate warming and increased atmospheric nitrogen (N) deposition are key factors influencing carbon sequestration in grassland ecosystems. However, the impacts of warming and N deposition, as well as their interactions, on soil MNC in arid grasslands remain poorly understood. In this study, we investigated soil MNC, including fungal necromass carbon (FNC) and bacterial necromass carbon (BNC), following a continuous 16-year manipulation of warming and N addition in a desert steppe in Inner Mongolia, China. We also analyzed these parameters in conjunction with soil microbial diversity, plant coverage and soil properties. The results showed that N addition significantly increased soil MNC and FNC, as well as fungal diversity while not affecting BNC or bacterial diversity. Warming did not significantly affect MNC (including both FNC and BNC) or soil microbial diversity and there was no interactive effect between warming and N addition. Soil MNC, FNC, and BNC were found to be higher in the topsoil layer than in the subsoil layer, and FNC contributed more to SOC than BNC. Notably, the increase of soil MNC and FNC under N addition was mainly mediated by increased soil N content. This study clarifies the critical role of MNC in soil carbon storage and offers an empirically grounded basis for forecasting grassland carbon changes under future global change scenarios.