Oxygen minimum zones (OMZs) are major sites of marine fixed nitrogen loss, where denitrification and anammox convert bioavailable nitrogen to N₂. Because these pathways depend on different substrates, intermediates, and microbial interactions, shifts in their balance alter the coupling between nitrogen loss and organic matter remineralization, the recycling of reactive nitrogen, and the response of OMZs to environmental change. Yet the ecological and biogeochemical mechanisms governing this partitioning remain poorly constrained. To address this gap, we use a regional physical–biogeochemical model spanning the eastern tropical North and South Pacific OMZs. Nitrogen cycling in the model emerges from the metabolic activity, competition, and cross-feeding among explicit redox-constrained microbial functional types: aerobic ammonia oxidizers, nitrite oxidizers, and heterotrophs; nitrate-, nitrite-, and nitrous oxide-reducing denitrifiers; anammox bacteria; and dissimilatory nitrate/nitrite reducers to ammonium (DNRA). Building on trait-based theory that links microbial metabolism to redox chemistry and resource competition, we test whether contrasting biogeochemical gradients across the two Pacific OMZs select for distinct microbial communities whose interactions shape the anammox:denitrification ratio of N₂ production. Including DNRA closes the major anaerobic fates of nitrate and nitrite, allowing us to test whether localized nitrogen retention and ammonium recycling can relieve ammonium limitation of anammox and influence its balance with denitrification. This work links microbial community structure to regional patterns of marine nitrogen loss and provides a mechanistic basis for predicting how OMZ nitrogen cycling may respond to changes in productivity, circulation, and deoxygenation.