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Constraining the oxygen isotopic composition of nitrate produced by nitrification

March 14, 2019

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.

Publication Year 2019
Title Constraining the oxygen isotopic composition of nitrate produced by nitrification
DOI 10.1021/acs.est.8b03386
Authors Danielle S. Boshers, Julie Granger, Craig R. Tobias, John K. Böhlke, Richard L. Smith
Publication Type Article
Publication Subtype Journal Article
Series Title Environmental Science & Technology
Index ID 70202638
Record Source USGS Publications Warehouse
USGS Organization WMA - Earth System Processes Division