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

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 environmentally relevant conditions. We observe a range of kinetic isotope effects that were 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 thought to reflect complex interactions between Fe(II) and reaction intermediates. Analysis of the 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) likely represents an important abiotic source of N₂O, especially in iron-rich environments that experience dynamic redox variations. This study provides a multi-compound, multi-isotope framework in which to evaluate the occurrence of abiotic NO₂^- reduction and N₂O formation under environmental conditions and should help future studies shed light on relative roles of abiotic and biological N₂O production pathways across many environments.