Anthropogenic acceleration of the leaky nitrogen cycle across the global land-river continuum

Lateral nitrogen transfer (LNT) along the land-river continuum plays a critical role in regulating global nitrogen (N) cycling and its feedbacks to climate, yet it remains poorly represented in land surface models. Here, we incorporate LNT into a global land surface model within an Earth System Model framework, enabling a process-based quantification of N transport, transformation, and associated riverine N2O emissions at the global scale. The upgraded model, ORCHIDEE3-Nlat, captures the magnitude and broad spatial patterns of riverine N fluxes and N2O emissions across major global river basins. Simulations for 1901–2020 indicate a strong anthropogenic intensification of LNT, with global dissolved inorganic nitrogen (DIN) delivery to rivers and export to the ocean increasing by ~245% and ~151%, respectively, while dissolved organic nitrogen (DON) shows more moderate increases (~32–38%), indicating a shift toward inorganic N dominance in aquatic systems. Riverine N2O emissions increased substantially, accelerating after the 1960s and forming hotspots in intensively managed subtropical regions. Attribution analyses reveal different controls on N forms: global DIN increases were initially driven by sewage and atmospheric deposition, with fertilizer inputs becoming dominant after the 1960s, whereas DON dynamics were primarily governed by manure inputs and hydroclimatic variability. Attribution patterns vary markedly among continents, with Europe being the only region showing declines in lateral N fluxes and riverine N2O emissions after the 1980s. Overall, anthropogenic perturbations more than doubled terrestrial N inputs, boosting riverine export and tripling riverine N2O emissions—far outpacing changes over land, where terrestrial N2O emissions increased by only ~25%. Despite a dramatic increase in global N export, the fraction of terrestrial N inputs reaching the ocean has decreased from 27% to 18% over the historical period, indicating reduced land-ocean connectivity and an intensified filtering role of the global river network.