Oceanic oxygen-minimum-zones (OMZs), despite only occupying roughly 1% of ocean volume, are estimated to account for up to 50% of the oceanic emissions of nitrous oxide (N2O), a powerful greenhouse gas and ozone-depleting agent. While most of the N2O in the ocean is produced as a byproduct during nitrification, N2O also forms as an intermediary during stepwise denitrification (the reduction of NO3  NO2  NO  N2O  N2) as oxygen approaches anoxia in OMZs. Recent metagenomic studies have revealed denitrification to be predominantly a modular process, with most denitrifying microorganisms possessing the genetic capabilities for only partial denitrification pathways or ‘steps’. Thus, intermediates such as N2O may escape further denitrification if ecological, environmental, and/or physical factors decouple their microbial-mediated production and consumption. Here we expand on previous work to construct a trait-based ecosystem model of microbial functional types, which include a suite of single and multi-step denitrifiers. The traits of each functional type are informed by the underlying reduction-oxidation (redox) reactions that supply energy for their respective metabolisms, allowing for OMZ nitrogen transformations to be represented as respiratory fluxes fueling microbial growth. We embed the ecosystem model in an eddy-resolving ocean model of the Eastern Tropical South Pacific OMZ to demonstrate that gradients in chemical redox potential and organic matter availability allows for the coexistence of diverse denitrifying functional types at different points within the water column. Furthermore, their ecological interactions are used to explain observed patterns of N2O in OMZs, such as undersaturated concentrations in the anoxic core and the predominance of N2O production by multi-step NO3-reducing denitrifiers.
 

biogeochemical cycles, nitrous oxide