Nonlinear Response of Riverine N₂O Fluxes to Oxygen and Temperature

One-quarter of anthropogenically produced nitrous oxide (N₂O) comes from rivers and estuaries. Countries reporting N₂O fluxes from aquatic surfaces under the United Nations Framework Convention on Climate Change typically estimate anthropogenic inorganic nitrogen loading and assume a fraction becomes N₂O. However, several studies have not confirmed a linear relationship between dissolved nitrate (NO₃^–) and river N₂O fluxes. We apply recursive partitioning analysis to examine the relationships between N₂O flux and NO₃^–, dissolved oxygen (DO), temperature, land use and surficial geology in the Grand River, Canada, a seventh-order river in an agricultural catchment with substantial urban population. Results suggest that N₂O flux is high when hypoxia exists. Temperature, not NO₃^–, was the primary correlate of N₂O flux when hypoxia does not occur suggesting NO₃^– is not limiting N₂O production and further increases in NO₃^– may not lead to comparable increases in N₂O flux. This work indicates that a linear relationship between NO₃^– and N₂O is unlikely to exist in most agricultural and urban impacted river systems. Most N₂O is produced during hypoxia so quantifying the extent of hypoxia is a necessary first step to quantifying N₂O fluxes in lotic systems. Predicted increases in riverine hypoxia via eutrophication and increased temperature due to climate change may drive nonlinear increases in N₂O production