Counteracting impacts of increasing atmospheric nitrogen and iron deposition on global marine N2 fixation in the Anthropocene

Marine N₂ fixation delivers the majority of bioavailable nitrogen to the ocean that promotes primary productivity. Despite its importance, the factors that determine the spatial distribution of N₂ fixation as well as their sensitivity to environmental change remain poorly understood. Here we use the coupled ocean-biogeochemical component in the UVic Earth System Climate Model to estimate the impact of increasing atmospheric nitrogen and iron deposition due to anthropogenic sources on spatial distribution and temporal trends of N₂ fixation from preindustrial into the Anthropocene. We simulate sensitivity experiments that isolate the effects from global warming, nitrogen deposition, and iron deposition. Our simulations show that nitrogen deposition primarily reduces nitrogen limitation and thus the niche for N₂ fixation in the northern hemisphere, whereas iron deposition relieves iron limitation enhancing N₂ fixation in the southern hemisphere. The model result suggests that anthropogenically-derived atmospheric iron deposition may already be significantly increasing N₂ fixation the tropical South Atlantic and Pacific Oceans, thereby increasing productivity and contributing to expanding oxygen minimum zones in the model. Our study highlights the important role for atmospheric nitrogen and iron deposition on N₂ fixation and that future studies must better constrain their anthropogenic-derived sources as well as the response of N₂ fixation.