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.