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

Microbial communities that reduce nitrous oxide (N₂O) are divided into two clades, nosZI and nosZII. These clades significantly differ in their ecological niches and their implications for N₂O emissions in terrestrial environments. However, our understanding of N₂O reducers in aquatic systems is currently limited. This study investigated the relative abundance and diversity of nosZI- and nosZII-type N₂O reducers in rivers and their impact on N₂O emissions. Our findings revealed that stream sediments possess a high capacity for N₂O reduction, surpassing N₂O production under high N₂O/NO₃- 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 N₂O 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 N₂O sinks in streams with higher N₂O concentrations. Overall, our findings provide a foundation for a better understanding of the biogeography of stream N₂O reducers and their effects on N₂O emissions.