Long-term straw-fertilizer interactions drive depth-dependent microbial necromass carbon and soil organic carbon sequestration in a cropland soil

Straw mulch and slow-release urea (SRU) increases topsoil soil organic carbon (SOC) but reduces subsoil SOC stability. Fungal necromass carbon (FNC) dominates topsoil SOC accrual while bacterial necromass carbon (BNC) mediates subsoil SOC loss. SRU reshapes microbial diversity/assembly via depth-dependent resource partitioning. Stratified carbon management resolves agricultural C sequestration trade-offs. Soil carbon sequestration plays a dual role in mitigating climate change and enhancing ecological resilience. Microbial necromass carbon (MNC) constitutes a critical component of soil organic carbon (SOC), yet its vertical distribution under straw mulching with fertilization remain poorly characterized. We conducted a 3 year field experiment on China's Loess Plateau, integrating straw (mulching (S) vs . removal (S 0 )) with nitrogen fertilizer (no fertilizer (W), conventional urea (U), slow-release urea (RU)). The results indicate that SRU treatment drove surface enrichment of labile carbon fractions, triggering microbial diversity and microbial community assembly processes. Additionally, compared to S 0 W, the SRU treatment significantly increased topsoil bacterial necromass carbon (BNC) by 22.2%, fungal necromass carbon (FNC) by 33.4%, and MNC by 28.9%. Conversely, the SRU treatment significantly decreased subsoil FNC content by 9.2%. In the 0-10 cm layer, compared to S 0 W, both straw mulching combined with chemical fertilizer treatments (SU and SRU) significantly reduced the ratios of BNC/SOC, FNC/SOC, and MNC/SOC. Regarding carbon-degrading enzyme activities, at the 0-10 cm depth, the sole fertilization treatments (S 0 U, S 0 RU) significantly increased peroxidase (POD) activity, whereas straw mulching combined with fertilization (SU, SRU) had no significant effect in this layer. Meanwhile, compared to S 0 W, both SU and SRU treatments significantly enhanced β-glucosidase (BG) activity, with increases of 200.5% and 122.2%, respectively. Additionally, relative to S 0 W, the SRU treatment also significantly increased BG activity in the 10-20 cm and 20-30 cm layers, by 106.4% and 110.5%, respectively. Using partial least squares path modeling and optimal multivariate regression model, this study revealed that FNC exhibited a significantly positive correlation with SOC accumulation in the topsoil. In the subsoil layer, the 9.2% decrease in FNC accumulation may be associated with reduced nitrogen availability and the consequent decline in fungal activity. These findings suggest that straw-fertilizer drives vertical gradient interactions between edaphic biotic and abiotic factors, thereby regulating the spatial heterogeneity of carbon sequestration.