Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

Measurements of the stable isotope ratios of nitrogen (N-15/N-14) and oxygen (O-18/O-16) in nitrate (NO3-) enable identification of sources, dispersal, and fate of natural and contaminant NO3- in aquatic environments. The O-18/O-16 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- O-18/O-16 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 O-18-enriched water. Resulting NO3- O-18/O-16 ratios showed (1) a disproportionate sensitivity to the O-18/O-16 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- O-18/O-16 ratios thus produced fortuitously converge near the O-18/O-16 ratio of water. More elevated NO3- O-18/O-16 values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO3- reduction.