Nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent, is produced intensely in oxygen minimum zones (OMZs) predominantly through nitrate reduction \(\left({{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}\right)\) . However, mechanisms and controls of this pathway remain unclear. Here, we investigate the microbial ecology governing this pathway using experiments and an ecosystem model. We experimentally confirm a critical hypothesis: most \({{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}\) denitrifiers do not utilize extracellular nitrite, an intermediate of the pathway. Model results demonstrate that the \({{{{\rm{NO}}}}}_{3}^{-}\to {{{{\rm{N}}}}}_{2}{{{\rm{O}}}}\) pathway is compatible with oxygen, and that its response to oxygen is heterogeneous because it is governed by niche partitioning of distinct microbial types and thus may not follow a smooth curve. Lastly, experiments demonstrate that this pathway is sensitive to the type of organic matter, its electron acceptor, in addition to organic matter availability. These findings advance our mechanistic understanding of the primary N2O production pathway, necessary for predictions of marine N2O emissions.
