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https://doi.org/10.5194/bg-2020-344
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-344
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Oct 2020

20 Oct 2020

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This preprint is currently under review for the journal BG.

Nitrogen isotopic fractionations during nitric oxide production in an agricultural soil

Zhongjie Yu1,2 and Emily M. Elliott1 Zhongjie Yu and Emily M. Elliott
  • 1Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
  • 2Department of Natural Resources and Environmental Sciences, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA

Abstract. Nitric oxide (NO) emissions from agricultural soils play a critical role in atmospheric chemistry and represent an important pathway for loss of reactive nitrogen (N) to the environment. With recent methodological advances, there is growing interest in the natural abundance N isotopic composition (δ15N) of soil-emitted NO and its utility in providing mechanistic information on soil NO dynamics. However, interpretation of soil δ15N-NO measurements has been impeded by the lack of constraints on the isotopic fractionations associated with NO production and consumption in relevant microbial and chemical reactions. In this study, anoxic (0 % O2), oxic (20 % O2), and hypoxic (0.5 % O2) incubations of an agricultural soil were conducted to quantify the net N isotope effects (15η) for NO production in denitrification, nitrification, and abiotic reactions of nitrite (NO2) using a newly developed δ15N-NO analysis method. A sodium nitrate (NO3) containing mass-independent oxygen-17 excess (quantified by a Δ17O notation) and three ammonium (NH4+) fertilizers spanning a δ15N gradient were used in soil incubations to help illuminate the reaction complexity underlying NO yields and δ15N dynamics in a heterogeneous soil environment. We found strong evidence for the prominent role of NO2 oxidation under anoxic conditions in controlling the apparent 15η for NO production from NO3 in denitrification (i.e., 49 to 60 ‰). These results highlight the importance of an under-recognized mechanism for the reversible enzyme NO2 oxidoreductase to control the N isotope distribution between the denitrification products. Through a Δ17O-based modeling of co-occurring denitrification and NO2 re-oxidation, the 15η for NO2 reduction to NO and NO reduction to nitrous oxide (N2O) were constrained to be 15 to 22 ‰ and −8 to 2 ‰, respectively. Production of NO in the oxic and hypoxic incubations was contributed by both NH4+ oxidation and NO3 consumption, with both processes having a significantly higher NO yield under O2 stress. Under both oxic and hypoxic conditions, NO production from NH4+ oxidation proceeded with a large 15η (i.e., 55 to 84 ‰) possibly due to expression of multiple enzyme-level isotopic fractionations during NH4+ oxidation to NO2 that involves NO as either a metabolic byproduct or an obligatory intermediate for NO2 production. Adding NO2 to sterilized soil triggered substantial NO production, with a relatively small 15η (19 ‰). Applying the estimated 15η values to a previous δ15N measurement of in situ soil NOx emission (NOx = NO + NO2) provided promising evidence for the potential of δ15N-NO measurements in revealing NO production pathways. Based on the observational and modeling constraints obtained in this study, we suggest that simultaneous δ15N-NO and δ15N-N2O measurements can lead to unprecedented insights into the sources of and processes controlling NO and N2O emissions from agricultural soils.

Zhongjie Yu and Emily M. Elliott

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Zhongjie Yu and Emily M. Elliott

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Short summary
In this study, we demonstrated distinct nitrogen isotope effects for nitric oxide (NO) production from major microbial and chemical NO sources in an agricultural soil. These results highlight characteristic bond forming and breaking mechanisms associated with microbial and chemical NO production and implicate that simultaneous isotopic analyses of NO and nitrous oxide (N2O) can lead to unprecedented insights into the sources and processes controlling NO and N2O emissions from agricultural soils.
In this study, we demonstrated distinct nitrogen isotope effects for nitric oxide (NO)...
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