Predicting nitrous oxide emissions through riverine networks

Nitrous oxide (N₂O) is currently the leading ozone-depleting gas and is also a potent greenhouse gas. Predictions of N₂O emissions from riverine systems are difficult and mostly accomplished via regression equations based on dissolved inorganic nitrogen (DIN) concentrations or fluxes, although recent studies have shown that hydromorphological characteristics can influence N₂O emissions in riverine reaches. Here, we propose a predictive model for N₂O riverine concentrations and emissions at the reach scale. The model is based on Damköhler numbers and captures the primary effects of reach-scale biogeochemical and hydromorphological characteristics in flowing waters. It explains the change in N₂O emissions from small streams to large rivers under varying conditions including biome, land use, climate, and nutrient availability. The model and observed data show that dimensionless N₂O concentrations and emission rates have higher variability and mean values for small streams (reach width <10 m) than for larger streams due to high spatial variability of stream hydraulics and morphology.