Biogeography and impact of nitrous oxide reducers in rivers across a broad environmental gradient on emission rates

Abstract Microbial communities that reduce nitrous oxide (N 2 O) are divided into two clades, nosZ I and nosZ II. These clades significantly differ in their ecological niches and their implications for N 2 O emissions in terrestrial environments. However, our understanding of N 2 O reducers in aquatic systems is currently limited. This study investigated the relative abundance and diversity of nosZ I‐ and nosZ II‐type N 2 O reducers in rivers and their impact on N 2 O emissions. Our findings revealed that stream sediments possess a high capacity for N 2 O reduction, surpassing N 2 O production under high N 2 O/NO 3 ‐ ratio conditions. This study, along with others in freshwater systems, demonstrated that nosZ I marginally dominates more often in rivers. While microbes containing either nosZ I and nosZ II were crucial in reducing N 2 O emissions, the net contribution of nosZ II‐containing microbes was more significant. This can be attributed to the nir gene co‐occurring more frequently with the nosZ I gene than with the nosZ II gene. The diversity within each clade also played a role, with nosZ II species being more likely to function as N 2 O sinks in streams with higher N 2 O concentrations. Overall, our findings provide a foundation for a better understanding of the biogeography of stream N 2 O reducers and their effects on N 2 O emissions.