The Impact of Fe(II) on NO2- Isotopic Composition During Denitrification by Natural Sediment

Abstract:

The role of Fe(II) on Nitrite (NO₂^-) isotopic composition during denitrification was investigated in anaerobic, closed-system batch incubations of tidal marsh sediments. Fe(II) is often found in similar redox conditions and can rapidly reduce NO₂^- to N₂ and/or nitrous oxide (N₂O), a potent greenhouse gas, through a process called chemodenitrification. Microbial communities can also reduce nitrate (NO₃^-) to NO₂^- and eventually to N₂O through the anaerobic process of biological denitrification. This study compared the impact on NO₂^- accumulation when Fe(III)-containing minerals ferrihydrite and goethite were added to natural sediments. The presence of dissolved Fe(II), presumably produced by microbial iron reduction, significantly limited the amount of NO₂^- accumulation, suggesting that Fe(II) may have chemically reduced NO₂^-. Changes in the δ¹⁵N of the intermediate NO₂^- pool in each bottle was measured, but the apparent isotope effects of NO₂^- reduction were indistinguishable among treatments, suggesting that the reaction of Fe(II) and NO₂^- imparts an isotope effect on the NO₂^- pool of a similar magnitude to that of biological NO₂^- reduction. The isotopic composition of N₂O and its ¹⁵N site preference will be measured to determine if chemodenitrification truly occurred, and if so, the relative contributions to N₂O production from biological denitrification and chemodenitrification will be determined.