Disentangling the complexity of nitrous oxide cycling in coastal sediments: Results from a novel multi-isotope approach

Abstract:

Although marine environments contribute approximately 30% of the global atmospheric nitrous oxide (N₂O) flux, coastal systems appear to comprise a disproportionately large majority of the ocean-atmosphere flux. However, there exists a wide range of estimates and future projections of N₂O production and emission are confounded by spatial and temporal variability of biological sources and sinks. As N₂O is produced as an intermediate in both oxidative and reductive microbial processes and can also be consumed as an electron acceptor, a mechanistic understanding of the regulation of these pathways remains poorly understood. To improve our understanding of N₂O dynamics in coastal sediments, we conducted a series of intact flow-through sediment core incubations (Sylt, Germany), while manipulating both the O₂ and NO₃^- concentrations in the overlying water. Steady-state natural abundance isotope fluxes (δ¹⁵N and δ¹⁸O) of nitrate, nitrite, ammonium and nitrous oxide were monitored throughout the experiments. We also measured both the isotopomer composition (site preference (SP) of the ¹⁵N in N₂O) as well as the Δ¹⁷O composition in experiments conducted with the addition of NO₃^- with an elevated Δ¹⁷O composition (19.5‰), which provide complementary information about the processes producing and consuming N₂O.

Results indicate positive N₂O fluxes (to the water column) across all conditions and sediment types. Decreasing dissolved O₂ to 30% saturation resulted in reduced N₂O fluxes (5.9 ± 6.5 μmol m₂ d^-1) compared to controls (17.8 ± 6.5 μmol m^-2 d^-1), while the addition of 100 μM NO₃^- yielded higher N₂O fluxes (49.0 ± 18.5 μmol m^-2 d^-1). In all NO₃^- addition experiments, the Δ¹⁷O signal from the NO₃^- was clearly observed in the N₂O efflux implicating denitrification as a large source of N₂O. However, Δ¹⁷O values were always lower (1.9 to 8.6‰) than the starting NO₃^- indicating an important role for nitrification-based N₂O production and/or O isotope exchange with water in influencing the O isotope composition of N₂O from the sediment-water interface. A steady-state multi-isotope flux model will help to constraining rates and isotope effects of these processes and improve our understanding of the dynamics and pathways of N₂O production and emission under varying scenarios of environmental change.