Nitrous oxide production in the Grand River, Ontario, Canada: New insights from stable isotope analysis of dissolved nitrous oxide

Nitrous oxide (N2O) is a powerful greenhouse gas, and its atmospheric concentration is increasing dramatically. N2O is produced through the microbially-mediated processes of nitrification and denitrification. Since these processes have difference substrates and isotopic enrichment factors, stable isotope analysis (delta15N and delta 18O) of N2O can be used to study the production of this important greenhouse gas.;To meet the study objectives, an offline "purge and trap" method was developed to collect and purify dissolved N2O for stable isotope analysis. Using this method, delta15N and delta 18O analysis of dissolved N2O is possible for samples with concentrations as low as 6 nmol N2O/L.;Due to the isotopic effects of gas exchange and the back flux of tropospheric N2O, there is a complex relationship between the delta 15N and the delta18O of source, dissolved, and emitted N2O in aquatic environments. A simple box model (SIDNO -- Stable Isotopes of Dissolved Nitrous Oxide) was developed to properly interpret isotopic data for dissolved N2O. Using this model, it was determined that the isotopic composition of emitted N2O is much more representative of N2O production in aquatic environments than the isotopic composition of dissolved N2O. If the concentration, delta15N and delta18O of dissolved N2O are measured, the magnitude and isotopic composition of the N2O flux can be calculated.;Sampling downstream of the major wastewater treatment plants (WWTPs) on the Grand River indicates that nitrification and denitrification in the river are strongly tied to diel changes in dissolved oxygen (DO) concentration. During the day, when DO concentrations are high, nitrification or nitrifier-denitrification is the dominant N2O production pathway, with sediment denitrification also contributing to N2O production. At night, when DO concentrations are low, denitrification in the sediments and at the sediment / water interface is the dominant production pathway. Using the SIDNO model, N2O produced during the day was found to have a delta15N of -22‰ and a delta18O of 43‰. N2O produced at night had a delta15N of -30‰ and a delta 18O of 30‰. The isotopic composition of N2O emitted from the Grand River is dominated by night-time production downstream of the Waterloo and Kitchener WWTPs during the summer. The flux and time weighted annual average isotopic composition of N2O emitted from the Grand River is -18.5‰ and 32.7‰ for delta15N and delta 18O respectively. These values are significantly more depleted than the only other published data for riverine N2O production. If the Grand River is representative of global riverine N2O production, these results will have significant implications for the global isotopic budget for atmospheric N2O.;Although production in rivers accounts for a significant portion of the global N2O budget, the isotopic composition of N2O from this source is poorly characterized. Most of the previous work using stable isotopes of N2O has been conducted in terrestrial or oceanic environments, and only one published study has measured delta15N and delta 18O of N2O produced in a riverine environment. The purpose of this research project was to use stable isotope analysis to characterize the processes responsible for N2O production in the Grand River, Ontario, Canada, and to determine the spatial and temporal variability of the isotopic composition of the N2O flux.