An alternative pathway for marine nitrous oxide production at oxic-anoxic interfaces from coupled biotic-abiotic reactions

Marine emissions of nitrous oxide (N₂O), a potent greenhouse gas, comprise approximately a third of global sources. Recent evidence suggests that the dominant source of N₂O in seawater is the activity of ammonia-oxidizing Thaumarchaeota that lack characterized N₂O-generating enzymes. Nitrous oxide may arise from a novel enzyme and/or abiotic reactions between nitrification intermediates, hydroxylamine (NH₂OH) and nitric oxide (NO), and redox-active metals in seawater. Isotopic site preference, or difference in δ¹⁵N between the two nitrogen atoms in N₂O, has been used as tracer for microbial N₂O production pathways (-10 to 0‰ for nitrifier-denitrification and denitrification vs. 30-37‰ for nitrification via NH₂OH oxidation). Seawater N₂O site preference falls in between these two characterized end members, suggesting simultaneous production via a combination of both microbial pathways or via a novel mechanism with intermediate site preference. Here we show significant N₂O production in abiotic experiments after addition of iron to seawater containing NH₂OH and NO. The N₂O produced from chemical reduction of NO by Fe(II) had a site preference of 16‰ whereas N₂O produced from abiotic NH₂OH oxidation had a site preference of 31‰. We propose that coupled biotic-abiotic N₂O production pathways could contribute significant sources of N₂O at marine oxic-anoxic interfaces.