Microbial niche partitioning explains N2O distributions and cycling in marine oxygen minimum zones.

Oxygen minimum zones (OMZs) are marine hotspots for cycling and emission of the powerful greenhouse gas nitrous oxide (N2O). Steep redox gradients host diverse microbial communities that produce and consume N2O, generating a characteristic vertical distribution, with low concentrations in anoxic cores and supersaturated accumulation in surrounding oxyclines. Although the accumulation is commonly attributed to stronger O2 inhibition of N2O consumption than production, this differential inhibition is not consistently observed, suggesting that interactions among nitrogen-cycling metabolisms may provide additional mechanisms. To evaluate the ecological controls on the N2O profile, we embed a trait-based ecosystem model into an eddy-resolving simulation of the Eastern Tropical South Pacific OMZ. In this framework, redox energetics, metabolite exchange, competition, and grazing structure microbial growth, setting emergent niches of microbial functional types, particularly aerobic ammonia oxidizers and diverse anaerobic denitrifiers that mediate N2O cycling. In the anoxic core, cross-feeding tightly couples denitrifiers that produce and consume N2O, maintaining low concentrations. As O2 increases in the oxycline, competition with aerobic metabolisms and intensified grazing gradually exclude denitrifiers. However, because they draw on more abundant substrates, N2O-producing denitrifiers persist to higher O2 than N2O consumers, reflecting the vulnerability of populations dependent on intermediate metabolites. This ecological decoupling drives N2O accumulation and captures the characteristic double-peaked vertical structure without imposing physiological O2 intolerance thresholds. More broadly, by revealing how niche partitioning and producer-consumer coupling shape other features of OMZ biogeochemistry, these results provide a basis for projecting future shifts in marine nitrogen loss and N2O emissions.