Stable Isotopes and Iron Oxide Mineral Products as Markers of Chemodenitrification.

When oxygen is limiting in soils and sediments, microorganisms utilize nitrate (NO₃^–) in respirationthrough the process of denitrificationleading to the production of dinitrogen (N₂) gas and trace amounts of nitrous (N₂O) and nitric (NO) oxides. A chemical pathway involving reaction of ferrous iron (Fe²⁺) with nitrite (NO₂^–), an intermediate in the denitrification pathway, can also result in production of N₂O. We examine the chemical reduction of NO₂^– by Fe­(II)chemodenitrificationin anoxic batch incubations at neutral pH. Aqueous Fe²⁺ and NO₂^– reacted rapidly, producing N₂O and generating Fe­(III) (hydr)­oxide mineral products. Lepidocrotite and goethite, identified by synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy, were produced from initially aqueous reactants, with two-line ferrihydrite increasing in abundance later in the reaction sequence. Based on the similarity of apparent rate constants with different mineral catalysts, we propose that the chemodenitrification rate is insensitive to the type of Fe­(III) (hydr)­oxide. With stable isotope measurements, we reveal a narrow range of isotopic fractionation during NO₂^– reduction to N₂O. The location of N isotopes in the linear N₂O molecule, known as site preference, was also constrained to a signature range. The coexistence of Fe­(III) (hydr)­oxide, characteristic ¹⁵N and ¹⁸O fractionation, and N₂O site preference may be used in combination to qualitatively distinguish between abiotic and biogenically emitted N₂Oa finding important for determining N₂O sources in natural systems