Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

Measurements of the stable isotope ratios of nitrogen (15N/14N) and oxygen (18O/16O) in nitrate (NO3–) enable identification of sources, dispersal, and fate of natural and contaminant NO3– in aquatic environments. The 18O/16O of NO3– produced by nitrification is often assumed to reflect the proportional contribution of oxygen atom sources, water, and molecular oxygen, in a 2:1 ratio. Culture and seawater incubations, however, indicate oxygen isotopic equilibration between nitrite (NO2–) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO3– 18O/16O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO3– produced from the nitrification of ammonia (NH3), we incubated streamwater supplemented with ammonium (NH4+) and increments of 18O-enriched water. Resulting NO3– 18O/16O ratios showed (1) a disproportionate sensitivity to the 18O/16O ratio of water, mediated by isotopic equilibration between water and NO2–, as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO3– 18O/16O ratios thus produced fortuitously converge near the 18O/16O ratio of water. More elevated NO3– 18O/16O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO3– reduction