Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification.

Measurements of the stable isotope ratios of nitrogen (N/N) and oxygen (O/O) in nitrate (NO) enable identification of sources, dispersal, and fate of natural and contaminant NO in aquatic environments. The O/O of NO 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 (NO) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO O/O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO produced from the nitrification of ammonia (NH), we incubated streamwater supplemented with ammonium (NH) and increments of O-enriched water. Resulting NO O/O ratios showed (1) a disproportionate sensitivity to the O/O ratio of water, mediated by isotopic equilibration between water and NO, as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO O/O ratios thus produced fortuitously converge near the O/O ratio of water. More elevated NO O/O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO reduction.