Ocean Biogeochemical Feedbacks Limit the Impact of Atmospheric Nitrogen Deposition on Marine Productivity

Nitrogen is an essential element for life that limits marine productivity throughout much of the surface ocean. While anthropogenic nitrogen emissions and subsequent atmospheric deposition into the ocean continue to increase at unprecedented rates, impacts on marine ecosystems, productivity and biogeochemistry remain uncertain. Here we performed idealized atmospheric nitrogen deposition simulations to quantify these impacts in a global 3D ocean-biogeochemistry model that includes the predominant source and sink terms of the nitrogen budget, nitrogen fixation and denitrification in the water column and sediments. The model reveals strong stabilizing feedbacks on the marine nitrogen inventory in response to atmospheric deposition. Nitrogen fixation provides a temporally efficient stabilizing feedback by decreasing immediately, whereas water column denitrification increases more gradually in the slowly expanding oxygen minimum zones caused by enhanced productivity from nearby deposition. Counterintuitively, nitrogen deposition causes a net loss of oceanic fixed nitrogen near the oxygen minimum zones in the eastern tropical Pacific. This occurs due to the stoichiometry of water column denitrification, which consumes ~7 moles inorganic nitrogen for each mole of remineralizing organic nitrogen produced by deposition and makes denitrification a globally important nitrogen limiting feedback. The combined feedbacks from N₂ fixation and denitrification largely compensate the nitrogen input via atmospheric deposition and suppress the increase in global marine productivity below 2%, in contrast to a simulation that neglects these nitrogen cycle feedbacks that predicts an increase by over 15%. Our study shows that an adequate representation of the patterns and rates of N₂ fixation and denitrification is required to predict the impact of atmospheric nitrogen deposition on marine productivity and the future evolution of the global marine nitrogen cycle.