Soil organic carbon thresholds control fertilizer effects on carbon accrual in croplands worldwide

Abstract 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.…