Date of Completion
Julie Granger, Craig Tobias, David Lund
Field of Study
Master of Science
Identifying sources of nitrate (NO3-) in the environment is important to elucidate causes of water quality impairment and eutrophication. Measurements of naturally occurring stable isotope ratios of nitrogen (15N/14N) and oxygen (18O/16O) in NO3-, can be used to determine the sources, dispersal, and fate of natural and contaminant NO3- in aquatic environments. To this end, it is necessary to know how NO3- isotopologues are modified by biological reactions, as heavy and light isotopes have different reaction rates. One important microbial reaction that influences isotope ratios of NO3- in the environment is nitrifcation, the biological oxidation of ammonium (NH4+) to nitrite (NO2-) then NO3-, the influence of which is not well understood in freshwater systems. The purpose of this study was to determine the influence of the d18O of ambient water on the isotopic composition of NO3- produced by freshwater nitrification. Water was collected from two streams in New England during the fall and spring, which were amended with NH4+ and with increments of 18O-enriched water, and then monitored the isotopic composition of NO2- and NO3- produced by natural consortia of nitrifiers. Although oxidation rates differed between the two stream waters, the final d18O of NO3- produced in both experiments revealed a sensitivity to the d18O of water mediated by (a) isotopic equilibration between water and NO2- and (b) kinetic isotope fractionation during O-atom incorporation from molecular oxygen and water into NO2- and NO3-. Our results concur with seawater incubations and nitrifying culture experiments that have demonstrated analogous sensitivity of the d18O of nitrified NO3- to equilibrium and kinetic O isotope effects (Buchwald et al. 2012). These findings have important implications for interpretations of O isotopes in NO3- source apportionation studies.
Boshers, Danielle, "Oxygen Isotopic Composition of Nitrate Produced by Freshwater Nitrification" (2017). Master's Theses. 1162.