Ecological Differentiation Among Nitrous Oxide Reducers Enhances Temperature Effects on Riverine N2O Emissions

ABSTRACT Nitrous oxide (N 2 O) reductase, the sole natural microbial sink for N 2 O, exists in two microbial clades: nosZ I and nosZ II. Although previous studies have explored inter‐clade ecological differentiation, the intra‐clade variations and their implications for N 2 O dynamics remain understudied. This study investigated both inter‐ and intra‐clade ecological differentiation among N 2 O reducers, the drivers influencing these patterns, and their effects on N 2 O emissions across continental‐scale river systems. The results showed that both nosZ I and nosZ II community turnovers were associated with similar key environmental factors, particularly total phosphorus (TP), but these variables explained a larger proportion of variation in the nosZ I community. The influence of mean annual temperature (MAT) on community composition increased for more widespread N 2 O‐reducing taxa. We identified distinct ecological clusters within each clade of N 2 O reducers and observed identical ecological clustering patterns across both clades. These clusters were primarily characterized by distinct MAT regimes, coarse sediment texture as well as low TP levels, and high abundance of N 2 O producers, with MAT‐related clusters constituting predominant proportions. Intra‐clade ecological differentiation was a crucial predictor of N 2 O flux and reduction efficiency. Although different ecological clusters showed varying or even contrasting associations with N 2 O dynamics, the shared ecological clusters across clades exhibited similar trends. Low‐MAT clusters in both the nosZ I and nosZ II communities were negatively correlated with denitrification‐normalized N 2 O flux and the N 2 O:(N 2 O + N 2 ) ratio, whereas high‐MAT clusters showed positive correlations. This contrasting pattern likely stems from low‐MAT clusters being better adapted to eutrophic conditions and their more frequent co‐occurrence with N 2 O‐producing genes. These findings advance our understanding of the distribution and ecological functions of N 2 O reducers in natural ecosystems, suggesting that warming rivers may have decreased N 2 O reduction efficiency and thereby amplify temperature‐driven emissions.