Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

Abstract

Measurements of the stable isotope ratios of nitrogen ( 15 N/ 14 N) and oxygen ( 18 O/ 16 O) in nitrate (NO 3- ) enable identification of sources, dispersal, and fate of natural and contaminant NO 3- in aquatic environments. The 18 O/ 16 O of NO 3- 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 2- ) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO 3-18 O/ 16 O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO 3- produced from the nitrification of ammonia (NH 3 ), we incubated streamwater supplemented with ammonium (NH 4+ ) and increments of 18 O-enriched water. Resulting NO 3-18 O/ 16 O ratios showed (1) a disproportionate sensitivity to the 18 O/ 16 O ratio of water, mediated by isotopic equilibration between water and NO 2- , as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO 3-18 O/ 16 O ratios thus produced fortuitously converge near the 18 O/ 16 O ratio of water. More elevated NO 3-18 O/ 16 O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO 3- reduction.