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^{- 18}O/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^{- 18}O/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^{- 18}O/¹⁶O ratios thus produced fortuitously converge near the ¹⁸O/¹⁶O ratio of water. More elevated NO₃^{- 18}O/¹⁶O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO₃^- reduction.