Global Oceanic Nitrogen Deposition under Future Emission Pathways and Responses to Nitrogen Emission Reductions

Oceanic nitrogen deposition influences marine ecosystem eutrophication and the global carbon cycle. Its future global spatiotemporal features still remain unclear driven by changing anthropogenic emissions. Furthermore, existing studies reported air quality and climate benefits of ambitious nitrogen emission reductions, while consequent impacts for global marine ecosystems through atmospheric nitrogen deposition are unexplored. Here we utilize the global atmospheric chemistry transport model GEOS-Chem to evaluate changes in global oceanic nitrogen deposition between 2015 and 2050 under three CMIP6 SSP-RCP emission scenarios and its responses to multiple levels of NH₃ and NO_x emission reductions. We find that global oceanic nitrogen deposition is projected to change by −24%-+6% between 2015-2050, with a substantially increasing share contributed by NH_x-N across all scenarios. Coastal regions respond much more drastically to nitrogen emission reductions than open ocean areas. Ocean carbon sink related to nitrogen-contributed marine primary productivity is projected to decrease from 290 Tg C in 2015 to 222 Tg C (-23%) in SSP1-RCP2.6 scenario in 2050, posing challenges to climate mitigation and affecting global carbon budget. Our findings highlight nitrogen management and the overlooked climate mitigation impacts on marine ecosystems through atmospheric nitrogen deposition and call for increasing attention for holistic assessments of nitrogen management impacts on air, terrestrial and ocean systems.