Pathways of soil organic carbon accrual affected by manure combined with different nitrogen application rates: Highlighting microbial life history strategies and biomarker accumulation

• Bacterial necromass formation declined when N addition exceeded 120 kg N ha −1 . • Mineral nitrogen above 32.5 mg kg −1 reduced bacterial necromass by lowering biomass. • Enhanced resource acquisition strategists correlate with lower residue stability in manure. • Growth yield strategists linked to higher residue accrual with N and manure. • MNC buildup capacity was driven by the microbial metabolic efficiency. Microbial life history strategies, which describe how microbes allocate their limited resources among growth, nutrient acquisition, and stress tolerance, regulate plant- and microbial-derived carbon turnover and determine soil organic carbon (SOC) accrual. However, the effects of manure amendment with nitrogen (N) fertilizer application on microbial life history strategies and SOC formation are unclear. This study was conducted in the low-fertility dryland agricultural soils of the Loess Plateau in Northwest China, a region characterized by low precipitation, limited nitrogen availability, and high sensitivity to organic matter inputs. This study analysed a 20-year field experiment with seven treatments. The treatments were categorized into two groups: (1) a manure-amended group (no fertilization + two manure-only application rates) and (2) a manure plus N-amended group (two manure application rates + two N addition levels). Manure application increased the C/N imbalance, suppressed the expression of recalcitrant-to-labile carbon-degrading genes (R/L ratio), and reduced lignin oxidase activity. Specifically, manure application increased the contribution of total lignin to the SOC by 3.7–7.3 %. Concurrently, N-hydrolase activity increased, and microbial carbon use efficiency (CUE) decreased, but microbial resource acquisition increased with increasing manure application rates. Furthermore, manure amendment reduced the contribution of microbial necromass C (MNC) by 20.7–29.4 %. Manure application enhanced the selective retention of lignin phenols and increased microbial resource demands, which, along with reduced microbial CUE, limited microbial necromass accumulation and altered the relative contributions of plant- and microbe-derived carbon to SOC. Fertilization decreased bacterial and fungal K/r ratios and the ECM/saprotroph ratios. Manure application alone decreased the abundance of both K- and r-strategist fungi. Conversely, combined manure and N fertilization stimulated the growth of r-strategist bacteria and fungi. Moderate N fertilization (120 kg N ha −1 ), combined with manure, alleviated microbial nitrogen limitation, increased microbial biomass, and elevated CUE. This synergy stimulated lignin decomposition and promoted microbial necromass stabilization within the SOC pool. Excessive N input (240 kg N ha −1 ), however, induced ammonium toxicity, suppressing bacterial biomass synthesis and MNC accumulation. These findings indicate that plant- and microbial-derived carbon fluxes can be balanced by optimizing the coapplication of manure and N, effectively increasing SOC accrual and stability in dryland agricultural systems.