The distribution and isotopomeric characterization of nitrous oxide in the Eastern Gotland Basin (central Baltic Sea)

Nitrous oxide (N₂O) is a greenhouse gas with a global warming potential ~300 times that of carbon dioxide. Coastal areas are important sources of N₂O to the atmosphere but the biogeochemical pathways of N₂O production and consumption are not well understood. We measured the concentrations and nitrogen (N) and oxygen (O) stable isotopes (δ¹⁵N and δ¹⁸O) of N₂O in the Baltic Sea to constrain the sources and sinks of N₂O in this system. Further, we used the intramolecular ¹⁵N variation or the site preference (SP) as additional tracer. Samples were taken at 7 stations during a cruise with R/V Elisabeth Mann Borgese to the Eastern Gotland Basin (central Baltic Sea) in May/June 2019. The isotope signatures of N₂O accumulation in the oxycline reflected production predominantly via ammonia oxidation. In the waters where hydrogen sulfide (H₂S) was detected, active N₂O consumption was implied by pronounced decrease in N₂O levels in tandem with enrichments in δ¹⁵Nbulk, δ¹⁸O and SP. High site preference values of N₂O observed in suboxic waters of the stations where H₂S buildup was minimal to non-detectable point to the possibility of non-canonical denitrification pathways mediated by fungi or abiotically. A sedimentary source of N₂O was observed in those stations, which resulted in a decoupling of the δ¹⁵Nbulk and δ¹⁸O of N₂O. Our results reveal that transient oxygenation events have the potential to modify the N cycling within the oxic-anoxic interface even if for shorter time scales.