Nitrogen Isotopic Fractionations during Nitric Oxide Production in an Agricultural Soil

Abstract:

Nitric oxide (NO) emission from agricultural soils plays a critical role in atmospheric chemistry and represents an important pathway for loss of reactive nitrogen (N) to the environment. There is a 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 at plot to regional scales. However, application and interpretation of soil δ¹⁵N-NO measurements have 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) laboratory incubations of an agricultural soil were conducted to quantify the net N isotope effects (¹⁵η) for NO production using a newly developed NO collection and δ¹⁵N analysis method. Under anoxic conditions, we found strong evidence for the prominent role of nitrite (NO₂^-) oxidation in controlling the measured ¹⁵η for NO production from nitrate (NO₃^-) in denitrification, underscoring a novel mechanism for the reversible enzyme NO₂^- oxidoreductase to shape the N isotope distribution among the denitrification products. Through an isotopic 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 under oxic and hypoxic conditions was driven by ammonium (NH₄⁺) oxidation and NO₃^- consumption, with both processes having a significantly higher NO yield under O₂ stress. Under both conditions, NO production via NH₄⁺ oxidation proceeded with a large ¹⁵η (i.e., 55 to 84‰) possibly due to expression of multiple enzyme-level isotopic fractionations during oxidation of ammonia (NH₃) to NO₂^- that involves NO as either a metabolic byproduct or an obligatory intermediate for NO₂^- production. Adding NO₂^- to sterilized soil triggered pulsive NO production with a relatively small ¹⁵η (19‰). The measured NO production rates and net isotope effects provide insights into how δ¹⁵N-NO measurements can be effectively used to understand reaction mechanisms underlying NO turnover in agricultural soils and to trace the transport and fate of soil-emitted NO in the environment.