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 propo...

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Bibliographic Details
Published in:Environmental science & technology 2019-02, Vol.53 (3), p.1206-1216
Main Authors: Boshers, Danielle S, Granger, Julie, Tobias, Craig R, Böhlke, John K, Smith, Richard L
Format: Article
Language:English
Subjects:
Ammonia
Ammonium
Aquatic environment
Balancing
Composition
Contaminants
Dispersal
Groundwater
Isotope ratios
Isotopes
Nitrates
Nitrification
Nitrogen dioxide
Nitrogen isotopes
Oxygen
Ratios
Seawater
Soil contamination
Stable isotopes
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Summary: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.
ISSN:0013-936X
1520-5851
DOI:10.1021/acs.est.8b03386