Regional-scale Controls on Dissolved Nitrous Oxide in the Upper Mississippi River

The Upper Mississippi River (UMR) drains a large portion of the U.S. Corn Belt - one of the most intensive agricultural regions in the world. 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), an important greenhouse gas and the primary ozone depleting substance, are yet to be reported. Using a novel equilibration technique, we present the largest dataset 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 non-linearly 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%. However, because N2O concentrations were influenced by variables other than NO3-, this forecast could be diminished. These patterns provide our first insights into UMR basin-wide N2O controls, which lay the foundation for targeted mitigation strategies aimed to reduce downstream N2O emissions.