Initiatives to restore soil fertility and mitigate global warming rely on rebuilding soil organic carbon (SOC). Nitrogen (N) fertilization is crucial for crop yields but affects SOC unpredictably due to varying responses of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) pools to initial SOC levels. To clarify these effects, here, by combining a global meta-analysis with continental-scale field experiments, we determine that an initial SOC threshold of 15 g C kg⁻¹ controls the effect of N fertilization on POC and MAOC. In SOC-poor soils (< 15 g C kg⁻¹), N fertilizer increases plant-derived C inputs and promotes soil aggregation, favouring POC accumulation. Conversely, in SOC-rich soils, N fertilizer stimulates microbial metabolic efficiency, leading to larger necromass production and stabilization by mineral protection, observed as more pronounced MAOC accrual. Our findings reveal how SOC thresholds shape the response of active and stable carbon pools to N fertilization, with consequences for SOC accrual in cropland soils globally.