Anthropogenic Perturbations of Nitrogen Cycling and Budgets Across the Land–Inland Water Continuum: Insights from ORCHIDEE3_Nlat

Quantifying lateral nitrogen (N) transfers and riverine N₂O emissions is essential for closing the global N budget. We present ORCHIDEE3-N_lat, which couples lateral N routing with ORCHIDEE3 to simulate NH₄⁺, NO₃^-, and DON fluxes through the land ocean aquatic continuum, from canopy to ocean. Aquatic transformations, including nitrification, denitrification and DON decomposition, occur within the routing framework, and riverine N₂O emission is estimated to using an emission-factor scheme tied to nitrification and denitrification rates. Evaluation against global observation-based datasets shows that the model reproduces the global magnitudes and broad spatial patterns of DIN and DON concentrations and fluxes, as well as N₂O emission rates across major river systems spread across the world. The model was then applied to reconstruct the historical evolution (1901–2020) of global N lateral transfers from land to rivers, and ultimately to the ocean, together with associated N₂O emissions. Globally, DIN inflow to rivers and export to oceans increased by ~245% and ~151% from 1901–1920 to 2001–2020, whereas DON increased more modestly (~38% and ~32%), implying a century-scale shift towards inorganic N cycling. Riverine N₂O emissions increased substantially, with a strong acceleration after the mid-1960s, with contemporary hotspots in intensively managed subtropical regions. Attribution analysis indicates that DIN trends were dominated by atmospheric deposition and sewage injection before the 1960s, while fertilizer inputs dominated the increase after the 1960s. The analysis also revealed that due to the fertilization effect on vegetation, increasing atmospheric CO₂ decreased the DIN exports to the global river network. In contrast, DON variability and trends were governed primarily by manure application and hydroclimate, showing weaker sensitivity to anthropogenic N inputs than for DIN. Together, these results provide a comprehensive picture of how human activities have reshaped riverine N composition, downstream N export, and the spatial distribution of N₂O emissions over the twentieth century, offering a robust baseline for global N-cycle assessments and mitigation planning.