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

Microbial communities that reduce nitrous oxide (N 2 O) are divided into two clades, nosZI and nosZII. 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 nosZI- and nosZII-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 nosZI marginally dominates more often in rivers. While microbes containing either nosZI and nosZII were crucial in reducing N 2 O emissions, the net contribution of nosZII-containing microbes was more significant. This can be attributed to the nir gene co-occurring more frequently with the nosZI gene than with the nosZII gene. The diversity within each clade also played a role, with nosZII 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.