Divergent changes in river nitrogen export to coastal waters worldwide in the Anthropocene

River transport of reactive nitrogen from land to sea is an important component of the nitrogen cycle, significantly influencing freshwater and coastal water quality and their ecosystem health. Human activities have markedly accelerated the Earth’s nitrogen cycle since pre-industrial times. Despite estimates of river total nitrogen export, the speciation of river nitrogen export to global coastal waters and its spatiotemporal changes remain poorly understood. Assessing the long-term changes in the river export of different nitrogen forms to coastal systems worldwide in response to their different trajectories of human perturbations is crucial for developing effective mitigation strategies for nitrogen pollution and improving water quality. In this study, we quantify the river export of different nitrogen forms to global coastal waters from 1900 till 2010 using the spatially explicit, mechanistic, coupled hydrology and biogeochemistry model IMAGE-DGNM. This model keeps track of nutrient supply from the land, perturbations of river network, and hydroclimate change, and describes the dynamic biogeochemical nitrogen transformations and transport along the terrestrial-freshwater continuum. Results show that although the river export of all major nitrogen forms increased during 1900-2010 at the global scale, some regions have shown stable or decreasing trends in recent decades. Moreover, the composition of different forms in river nitrogen export differs across different regions. Not only the fluxes but also the fractions of different forms changed differently across systems during 1900-2010, which emphasizes the importance of taking into account varied human impacts, climates and hydrological conditions to address the complexity of mitigating local nitrogen pollution.