Regional-scale controls on dissolved nitrous oxide in the Upper Mississippi River

The U.S. Corn Belt is one of the most intensive agricultural regions of the world and is drained by the Upper Mississippi River (UMR), which forms one of the largest drainage basins in the U.S. While the effects of agricultural nitrate (NO₃^-) on water quality in the UMR have been well documented, its impact on the production of nitrous oxide (N₂O) has not been reported. Using a novel equilibration technique, we present the largest data set of freshwater dissolved N₂O concentrations (0.7 to 6 times saturation) and examine the controls on its variability over a 350 km reach of the UMR. Driven by a supersaturated water column, the UMR was an important atmospheric N₂O source (+68 mg N₂O N m^-2 yr^-1) that varies nonlinearly with the NO₃^- concentration. Our analyses indicated that a projected doubling of the NO₃^- concentration by 2050 would cause dissolved N₂O concentrations and emissions to increase by about 40%.