Oceanic nitrogen cycling and N₂O flux perturbations in the Anthropocene

There is currently no consensus on how humans are affecting the marine nitrogen (N) cycle, which limits marine biological production and CO 2 uptake. Anthropogenic changes in ocean warming, deoxygenation, and atmospheric N deposition can all individually affect the marine N cycle and the oceanic production of the greenhouse gas nitrous oxide (N 2 O). However, the combined effect of these perturbations on marine N cycling, ocean productivity, and marine N 2 O production is poorly understood. Here we use an Earth system model of intermediate complexity to investigate the combined effects of estimated 21st century CO 2 atmospheric forcing and atmospheric N deposition. Our simulations suggest that anthropogenic perturbations cause only a small imbalance to the N cycle relative to preindustrial conditions (∼+5 Tg N y −1 in 2100). More N loss from water column denitrification in expanded oxygen minimum zones (OMZs) is counteracted by less benthic denitrification, due to the stratification‐induced reduction in organic matter export. The larger atmospheric N load is offset by reduced N inputs by marine N 2 fixation. Our model predicts a decline in oceanic N 2 O emissions by 2100. This is induced by the decrease in organic matter export and associated N 2 O production and by the anthropogenically driven changes in ocean circulation and atmospheric N 2 O concentrations. After comprehensively accounting for a series of complex physical‐biogeochemical interactions, this study suggests that N flux imbalances are limited by biogeochemical feedbacks that help stabilize the marine N inventory against anthropogenic changes. These findings support the hypothesis that strong negative feedbacks regulate the marine N inventory on centennial time scales.