Different responses of soil bacterial necromass carbon and fungal necromass carbon to nitrogen deposition in meadow steppe

Microbial necromass carbon (MNC) is a key contributor to the accumulation and stabilization of soil organic carbon (SOC). However, the effects of nitrogen (N) deposition on MNC dynamics and SOC sequestration in grasslands remain poorly understood. We conducted a 12-year, multi-level N addition experiment with eight fertilization gradients (0, 15, 30, 50, 100, 150, 200, and 300 kg N ha⁻¹ yr⁻¹) in a Stipa baicalensis steppe to simulate N deposition. Our results revealed that MNC constituted up to 60 % of SOC, significantly enhancing its contribution under N enrichment. We found that fungal necromass carbon (FNC) accounted for 75 % to 82 % of MNC in the S. baicalensis steppe. However, we observed that the concentrations of FNC, the FNC/SOC ratio, and the FNC/ BNC ratio peaked in the N30 treatment and gradually decreased thereafter. The concentration of bacterial necromass carbon (BNC) and the BNC/SOC ratio showed an increasing trend along the N addition gradient, significantly increasing in the N150 ~ N300 treatments. Using a ¹³C pulse-labeling approach, we tracked microbial carbon flow: newly assimilated ¹³C was initially dominated by fast-growing r-strategist bacteria and arbuscular mycorrhizal fungi (AMF), then progressively transferred to slow-growing saprotrophic fungi and actinobacteria. This microbial succession was tightly coupled to N-induced shifts in soil nutrient availability. N addition altered the soil pH, nutrient availability, and mineral content, driving changes in microbial community composition and enzyme activities. These factors were strongly associated with increased accumulation of BNC and its contribution to SOC, whereas FNC exhibited a negligible response. Long-term N addition significantly increased the necromass accumulation coefficient (NAC) and microbial carbon use efficiency (CUE). Regression analyses revealed significant positive correlations between BNC content, the contribution of BNC to SOC, NAC, and CUE while FNC and the contribution of FNC to SOC were not directly affected. The structural equation modeling (SEM) also demonstrated that increased CUE and higher Fe and Al contents directly enhanced BNC accumulation but did not directly affect FNC. Our results showed that N addition predominantly drives necromass carbon dynamics and SOC stabilization through accelerating bacterial turnover and altering BNC stabilization mechanisms. GP bacteria play a central role in these processes. Our findings demonstrate that N deposition preferentially stabilizes BNC through accelerated turnover and mineral association, fundamentally reshaping SOC persistence in grasslands. This study provides new insights into the differential responses of microbial necromass pools to N deposition.