Nitrous Oxide Emissions from Riverine Networks.

Nitrous oxide (N₂O) is a potent greenhouse gas with a warming capacity 300 times higher than carbon dioxide that contributes to stratospheric ozone destruction. Anthropogenic inorganic nitrogen loading to river networks is a potentially important source of this gas to the atmosphere via microbially-mediated denitrification of reactive nitrate (NO₃^-) to N₂O and dinitrogen (N₂) gas. Although riverine environments, such as streams and rivers have been observed as sources of this gas to the atmosphere, we have limited understanding on which processes and which parts of watercourses dominate N₂O emissions. We analyze N₂O emissions from 12 river networks across the world including headwater streams, medium and large rivers and a tidal system with different morphology, land cover, biomes and climatic conditions. We identify and define scaling laws based on Damköhler numbers that allows to upscale processes occurring within a single reach to best quantify emissions at the riverine network scale. Finally, we demonstrate that the primary source of N₂O emissions varies with stream and river size and shifts from the hyporheic-benthic zone in headwater streams to the benthic-water column zone in rivers. By understanding the scaling nature of N₂O production along riverine networks, our framework facilitates predictions of riverine N₂O emissions globally using widely accessible chemical and hydromorphological datasets and allow to quantify the effect of human activity and natural processes on N₂O production.