Regulation of reactive nitrogen transport and transformation in contaminated environments

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Agricultural activities have led to a rapid growth in the release of a number of chemically, radiatively, and biologically reactive nitrogen (N) species within terrestrial watersheds. These reactive N compounds play important roles in the deterioration of water quality through leaching losses from soil surfaces of aqueous inorganic N species, including ammonium (NH4+) and nitrate (NO3-), and through changes to the climate system via increased emissions of nitrous oxide (N2O), a powerful and long-lived greenhouse gas and stratospheric ozone depleting compound. Nitrous oxide is a product of the transformation of both NH4+ and NO3- in watersheds, so watershed zones that facilitate improved water quality through losses of these species may show enhanced emissions of this gas. These N transformations and exchanges are closely regulated by redox conditions and temperature when inorganic N is abundant. ***Emissions of N2O in agricultural watersheds arising from leached N are seldom quantified in field studies, which tend to focus upon the point of immediate impact (i.e., soil surfaces). However, a significant proportion of reactive N released to soils through anthropogenic land use activities is leached into linked aquatic systems, where it is subsequently removed or transformed. Nitrogen transformation processes that generate N2O can be differentiated by their unique isotopic fingerprints (i.e., d15N and d18O). As stable isotopes provide the only means by which N2O can be apportioned to sources and/or transformation processes in the field, they are likely to factor in as crucial monitoring tools over the next decade, particularly in light of recent advances in the development of field portable and high-sensitivity N2O isotopic analyzers. ***The objective of the proposed research program is to quantify rates and characterize isotopic patterns of reactive N transformations that lead to N2O emissions from leached N in temperate zone agriculturally dominated watersheds experiencing elevated N loading. This will be accomplished by: selecting field sites that provide model systems for characterizing leached N loading conditions associated with agricultural activities; investigating the transport, transformation, and isotopic signals of aqueous inorganic N compounds that are precursors to N2O at these sites; studying the rates and isotopic signatures associated with N2O released under varying redox states, thermal conditions, and N loading levels at these sites; placing N2O release in the context of overall global warming potentials for the watershed points of study through concurrent analysis of carbon-based greenhouse gases; and identifying the utility of the observed chemical patterns for use as longer-term N2O emission monitoring and mitigation tools.**