Field evidence confirms that stream processing of reactive nitrogen (Nr), primarily ammonium and nitrate, is a potentially important source of the potent greenhouse gas nitrous oxide (N2O). Stream emissions may account for up to 10% of global anthropogenic N2O production and N2O has 310 times more warming potential per unit weight than carbon dioxide. In streams, the production of N2O occurs primarily in streambed sediments, and the mass transport of reactive species, i.e., dissolved oxygen, Nr, and organic carbon, via hyporheic flow strongly influences reaction rates, residence times, and subsequent N2O emissions. Previous research has shown a strong interaction between hydraulics in the hyporheic zone (HZ) and streambed morphology. The project couples a novel Lagrangian modeling approach based on residence time distributions differentiated for channel reach types and local hyporheic components of the stream network with seasonal synoptic sampling in two watersheds with contrasting land use.
Improved understanding of watershed and network scale controls on potent greenhouse gas emissions will be societally-relevant to policy makers addressing elevated Nr concentrations in surface waters. Research results will be applicable to land use management, non-point source pollution, and river restoration projects. The products from this research will provide a model for estimating the fate of Nr at the stream network scale, offer a new understanding of the role of HZ at the stream network scale, and clarify the effect of stream network structure and stream morphology on HZ processes. Results from this research can be extended to study transport of other solutes and pathogens along streams. With the advance in remote sensing and GIS tools, data for this approach will be more readily available and thus applicable to provide predictions when minimal field survey data are available. The broader impacts also involve the outreach to high school students, and the work with the McCall Outdoor Science School in regards to communication of the work in a climate change context.