Constraining the role of iron in environmental nitrogen transformations: Dual stable isotope systematics of abiotic NO2− reduction by Fe(II) and its production of N2O

Despite mounting evidence for biogeochemical interactions between iron and nitrogen, our understanding of their environmental importance remains limited. Here we present an investigation of abiotic nitrite (NO₂⁻) reduction by Fe(II) or ‘chemodenitrification’, and its relevance to the production of nitrous oxide (N₂O), specifically focusing on dual (N and O) isotope systematics under a variety of environmental conditions. We observe a range of kinetic isotope effects that are regulated by reaction rates, with faster rates at higher pH (∼8), higher concentrations of Fe(II) and in the presence of mineral surfaces. A clear non-linear relationship between rate constant and kinetic isotope effects of NO₂⁻reduction was evident (with larger isotope effects at slower rates) and is interpreted as reflecting the dynamics of Fe(II)–N reaction intermediates. N and O isotopic composition of product N₂O also suggests a complex network of parallel and/or competing pathways. Our findings suggest that NO₂⁻ reduction by Fe(II) may represent an important abiotic source of environmental N₂O, especially in iron-rich environments experiencing dynamic redox variations. This study provides a multi-compound, multi-isotope framework for evaluating the environmental occurrence of abiotic NO₂⁻ reduction and N₂O formation, helping future studies constrain the relative roles of abiotic and biological N₂O production pathways.