Nitrogen isotopic fractionations during nitric oxide production in an agricultural soil

[摘要] 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 ( δ 15 N ) of soil-emitted NO and its utility in providing mechanistic information on soil NO dynamics. However, interpretation of soil δ 15 N -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  %   O 2 ), oxic (20  %   O 2 ), and hypoxic (0.5  %   O 2 ) incubations of an agricultural soil were conducted to quantify the net N isotope effects ( 15 η ) for NO production in denitrification, nitrification, and abiotic reactions of nitrite ( NO 2 - ) using a newly developed δ 15 N -NO analysis method. A sodium nitrate ( NO 3 - ) containing mass-independent oxygen-17 excess (quantified by a Δ 17 O notation) and three ammonium ( NH 4 + ) fertilizers spanning a δ 15 N gradient were used in soil incubations to help illuminate the reaction complexity underlying NO yields and δ 15 N dynamics in a heterogeneous soil environment. We found strong evidence for the prominent role of NO 2 - re-oxidation under anoxic conditions in controlling the apparent 15 η for NO production from NO 3 - in denitrification (i.e., 49  ‰ to 60  ‰ ). These results highlight the importance of an under-recognized mechanism for the reversible enzyme NO 2 - oxidoreductase to control the N isotope distribution between the denitrification products. Through a Δ 17 O -based modeling of co-occurring denitrification and NO 2 - re-oxidation, the 15 η for NO 2 - reduction to NO and NO reduction to nitrous oxide ( N 2 O ) 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 NH 4 + oxidation and NO 3 - consumption, with both processes having a significantly higher NO yield under O 2 stress. Under both oxic and hypoxic conditions, NO production from NH 4 + oxidation proceeded with a large 15 η (i.e., 55  ‰ to 84  ‰ ) possibly due to expression of multiple enzyme-level isotopic fractionations during NH 4 + oxidation to NO 2 - that involves NO as either a metabolic byproduct or an obligatory intermediate for NO 2 - production. Adding NO 2 - to sterilized soil triggered substantial NO production, with a relatively small 15 η (19  ‰ ). Applying the estimated 15 η values to a previous δ 15 N measurement of in situ soil NO x emission (NO x = NO + NO 2 ) provided promising evidence for the potential of δ 15 N -NO measurements in revealing NO production pathways. Based on the observational and modeling constraints obtained in this study, we suggest that simultaneous δ 15 N -NO and δ 15 N - N 2 O measurements can lead to unprecedented insights into the sources of and processes controlling NO and N 2 O emissions from agricultural soils.

[学科分类] 大气科学