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 (NO 2 -) reduction by Fe(II) or 'chemodenitrification,' and its relevance to the production of nitrous oxide (N 2 O), 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 NO 2 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 N 2 O also suggests a complex network of parallel and/or competing pathways. Our findings suggest that NO 2 reduction by Fe(II) may represent an important abiotic source of environmental N 2 O, 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 NO 2 reduction and N 2 O formation, helping future studies constrain the relative roles of abiotic and biological N 2 O production pathways.

17 Under anaerobic conditions, the environmental reduction of nitrate (NO 3 -) and 18 nitrite (NO 2 -2) may therefore suggest the dominance of a single formation mechanism. This pathway 595 may proceed with the formation of a dinitrosyl intermediate (Fe-(O-N=N-O)), followed 596 by preferential breaking of the 14 N-O bond and a resulting 15 N enrichment of the central 597 alpha atom in the product N 2 O (e.g., high SP).598Clearly nitrite reduction by Fe(II) can be catalyzed under environmentally 599 relevant conditions, given sufficient levels and redox cycling of nitrogen and mineral-600 associated Fe. In heterogeneous environments hosting dynamic redox oscillations, a 601 number of interrelated physical, chemical, and biological processes lead to a complex 602 network of feedbacks that directly or indirectly influence reaction favorability. Since 603 green rust often forms under fluctuating redox conditions (and the co-occurrence of Fe(II) 604and Fe(III)) such as those often observed in soils and estuarine, wetland and even 605 hydrothermal vent sediments (Bourrie et al., 1999;Trolard et al., 1997; Wankel et al., 606