Predicting nitrous oxide emissions through riverine networks.

Nitrous oxide (NO) is currently the leading ozone-depleting gas and is also a potent greenhouse gas. Predictions of NO 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 NO emissions in riverine reaches. Here, we propose a predictive model for NO 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 NO 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 NO 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.