Nitrogen fertilization increases soil organic carbon through distinct pathways in contrasting cropland soils.

Nitrogen (N) fertilization influences soil organic carbon (SOC) formation by regulating plant inputs and microbial activity; however, the relative contributions of plant- versus microbial-derived carbon (C) to SOC accumulation remain unclear, largely due to site-specific variations in soil properties and the complex transformation pathways governing C stabilization. Here, we used amino sugars and lignin phenols as molecular tracers to quantify microbial necromass- and lignin-derived C contributions to SOC under N fertilization across four long-term maize field experiments in Quzhou and Changwu (alkaline, low-fertility soils) and Lishu and Yaan (acidic, high-fertility soils). Although N fertilization increased SOC across all sites, the dominant pathways for C accumulation differed in contrasting soils. In alkaline, low-fertility soils, SOC accumulation was primarily associated with greater lignin-derived C, regulated by soil geochemical properties and aggregate protection rather than increased plant inputs. In acidic, high-fertility soils, microbial necromass contributed more to SOC accumulation and was associated with greater lignin oxidation, reduced oxidase activity, and elevated oxalate-extractable Fe/Al oxides. These divergent mechanisms explain variability in SOC responses to N fertilization and emphasize the need for soil-specific nutrient management strategies to maximize C retention in croplands.