Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification

作者：Danielle S. Boshers, Julie Granger, Craig Tobias, J. K. Böhlke, Richard L. Smith · 发表于：Environmental Science & Technology · 年份：2019 · DOI：10.1021/acs.est.8b03386 · 被引用次数：109 · 研究领域：Groundwater and Isotope Geochemistry、Wastewater Treatment and Nitrogen Removal、Soil and Water Nutrient Dynamics

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.