Ammonium-derived nitrous oxide is a global source in streams.

Global riverine nitrous oxide (N₂O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N₂O emissions. However, the mechanisms and pathways controlling hyporheic N₂O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N₂O sources (69.6 ± 2.1%) in agricultural streams around the world. The N₂O fluxes are mainly in positive correlation with ammonia. The potential N₂O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N₂O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N₂O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N₂O emissions. NH₄⁺-derived pathways, rather than NO₃^--derived pathway, are the dominant hyporheic N₂O sources in lower-order streams. These findings provide insights into better estimation of N₂O emissions in global models of riverine ecosystems and emphasize the importance of managing ammonium. [ABSTRACT FROM AUTHOR]