Fertilizer‐Induced Nitrogen Accumulation Contributes to One Fifth of Topsoil Nitrogen in Chinese Upland Croplands

Abstract Fertilizer nitrogen (N) can accumulate in the soil‐vadose zone‐groundwater continuum as legacy N, exerting a consistent influence on key biogeochemical processes such as crop N uptake, N leaching, and gaseous N emissions. However, the regional stock of legacy N in soils and its contribution to soil N pools remain unknown due to the lack of effective estimation methods, hindering the understanding of its environmental and agricultural effects. Here, we proposed a method for quantifying legacy N stock at the regional scale by identifying an exponential decay pattern in multi‐year N retention rates and establishing a validated power function fitting between a parameter related to the initial retention rate and the N application rate. We further proposed a novel N isotope model to constrain the contribution of legacy N to soil N pools. Overall, China's upland croplands have accumulated legacy N of 54.7 ± 24.9 Tg N in top 30 cm soils over the past six decades (1961–2020). Croplands in southern China are hotspots for legacy N accumulation, likely due to their rich soil organic carbon. Legacy N may account for 43%–89% of soil organic N sequestration. The N isotope model estimates that legacy N constitutes 21.4% ± 6.9% of total soil N, comparable with the compiled proportion of total N increases due to fertilization (16.0%). These findings suggest that the legacy N from historical fertilizers substantially enhances soil fertility, and may inform biogeochemical parameterization given that the components of soil N pools may differ in stability.