Volume 35, Issue 2 e8979
RESEARCH ARTICLE

Influence of δ18O of water on measurements of δ18O of nitrite and nitrate

Kanae Kobayashi

Kanae Kobayashi

Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628 Japan

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Keitaro Fukushima

Keitaro Fukushima

Center for Ecological Research, Kyoto University, Otsu, Shiga, 520-2113 Japan

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Yuji Onishi

Yuji Onishi

Center for Ecological Research, Kyoto University, Otsu, Shiga, 520-2113 Japan

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Kazuya Nishina

Kazuya Nishina

Center for Regional Environmental Research, National Institute for Environmental Studies, Onogawa, Tsukuba, 305-8506 Japan

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Akiko Makabe

Akiko Makabe

Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, 237-0061 Japan

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Midori Yano

Midori Yano

Center for Ecological Research, Kyoto University, Otsu, Shiga, 520-2113 Japan

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Scott D. Wankel

Scott D. Wankel

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543-1050 USA

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Keisuke Koba

Keisuke Koba

Center for Ecological Research, Kyoto University, Otsu, Shiga, 520-2113 Japan

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Satoshi Okabe

Corresponding Author

Satoshi Okabe

Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628 Japan

Correspondence

S. Okabe, Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan.

Email: [email protected]

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First published: 14 October 2020
Citations: 5

Abstract

Rationale

Oxygen isotope ratio measurements of NO2 and NO3 by the azide method and denitrifier method are sensitive to the δ18O value of the sample water. However, the influence of δ18OH2O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ18OH2O of a sample on the δ18O analysis of NO2 and NO3.

Methods

We prepared NO2 and NO3 standards (with known δ18ONO2- and δ18ONO3- values) dissolved in waters having different δ18OH2O values (δ18OH2O = −12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted into N2O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N2O were measured with a Sercon purge-and-trap gas chromatography/isotope ratio mass spectrometry (PT-GC/IRMS) system. The measured δ18O values of the produced N2O were plotted against known δ18ONO2- and δ18ONO3- values to evaluate the influence of exchange of an oxygen atom with H2O during the conversion of NO2 into N2O and NO3 into N2O, respectively.

Results

The degree of oxygen isotope exchange was 10.8 ± 0.3% in the azide method and 5.5 ± 1.0% in the denitrifier method, indicating that the azide method is more susceptible to artifacts arising from differences in the δ18OH2O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ18OH2O. Abiotic NO2–H2O equilibrium isotope effect experiments yielded a rate constant of (1.13 ± 007) × 10−2 (h−1) and an equilibrium isotope effect of 11.9 ± 0.1‰ under the condition of pH = 7.5, 30°C, and 2.5% salinity.

Conclusions

Oxygen isotope ratio measurements of NO2 by the azide method are highly sensitive to δ18OH2O as a result of significant oxygen isotope exchange between NO2 and H2O. Therefore, to obtain the most accurate measurements water with the same δ18OH2O value as that of the sample must be used to make the NO2 and NO3 standards.

PEER REVIEW

The peer review history for this article is available at https://publons.com/publon/10.1002/rcm.8979.