Nitrous Oxide Dynamics in a Braided River System, New Zealand

Recently the Intergovernmental Panel on Climate Change (IPCC) emission factor EF5‐r was revised downward to a value of 0.0025 kg N2O–N per kg NO3−–N leached. It was not reduced further due to the continued uncertainty surrounding the dynamics of N2O in river systems. There have been few studies where river system N2O yields and fluxes have been measured. In this study, we examined the relationship between NO3−–N and N2O–N fluxes at 10 sites along a braided river system (84 km) over a 397‐d period. Isotopic analysis of NO3−–N river water samples and the potential agricultural nitrogen (N) sources demonstrated that the NO3−–N came from agricultural or sewage sources. Percent saturation of N2O varied with site and date (average, 114%) and correlated with river NO3−–N concentrations. Modeled N2O fluxes (16–30 μg m−2 h−1) from five sites were strongly related to river NO3−–N concentrations (r2 = 0.86). The modeled N2O–N fluxes ranged from 39 to 81% of the IPCC‐derived emissions based on the NO3−–N load in the river over 397 d and do not support further lowering of the EF5‐r. Further in situ river studies are required to verify the N2O–N fluxes and the calculated gas transfer velocity values for these braided river systems.