Contrasting accumulation of necromass carbon in fertilized soils across China: Plant-derived dominates in the North, microbial-derived prevails in the South

Fertilization alters plant- (PNC) and microbial-derived necromass carbon (MNC) dynamics, affecting soil organic carbon (SOC) accumulation and stabilization, yet the spatial variability and drivers of these effects in fertilized agroecosystems remain unclear. Based on four long-term (> 34 years) field trials across China, we investigated PNC and MNC quantity, composition, and key drivers under conventional fertilization strategies: unfertilized control (CK), chemical nitrogen (N), balanced chemical nitrogen, phosphorus, and potassium (NPK), and NPK with manure (NPKM). Results revealed that NPK and NPKM increased both MNC (38%−146% in Northeast/North China; 36%−134% in Southeast/South China) and PNC (88%−331% and 37%−163%, respectively) compared to CK, whereas N showed decreased or neutral impacts, particularly in North and South China. Compared to N, NPK and NPKM reduced the MNC/PNC ratio in Northeast (from 1.6 to 1.4 and 1.3) and North China (2.3 to 1.6 and 1.3) but increased it in Southeast China (1.8 to 2.1 and 2.0), with no significant change in South China. Random forest analysis identified phosphorus (P) availability as the dominant factor of necromass C dynamics, with available P (AP) mainly controlling MNC and total P (TP) influencing PNC. Regional drivers varied: soil pH and Chloroflexi were pivotal in South China, while clay content and Actinobacteria dominated elsewhere. Structural equation modeling indicated that fertilization (P and manure amendments) primarily regulated necromass C via chemical properties (AP, TP) in Northeast and North China, but via microbial traits (Chloroflexi, P-acquiring enzymes) in Southeast and South China. In summary, balanced chemical or organic fertilization enhanced PNC accumulation over MNC in northern regions, potentially destabilizing SOC, whereas Southeast China favored MNC accumulation, and South China maintained a balanced production. These findings contribute to the accurate prediction of terrestrial C dynamics and the optimization of region-specific agricultural practices for C sequestration and persistence.