Nitrogen isotopic fractionations during nitric oxide production in an agricultural soil

Zhongjie Yu, Emily M. Elliott
2021 Biogeosciences  
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
more » ... ints 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- re-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.
doi:10.5194/bg-18-805-2021 fatcat:mipqrti72fbarb3iv7tycqsn3e