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 N₂O-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. by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.