Constraining the oxygen isotopic composition of nitrate produced by nitrification

Environmental Science & Technology
By: , and 

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Abstract

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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Constraining the oxygen isotopic composition of nitrate produced by nitrification
Series title Environmental Science & Technology
DOI 10.1021/acs.est.8b03386
Volume 53
Issue 3
Year Published 2019
Language English
Publisher ACS
Contributing office(s) WMA - Earth System Processes Division
Description 11 p.
First page 1206
Last page 1216