Nitrous Oxide Production and Fluxes from Coastal Sediments under Varying Environmental Conditions

Abstract:

Although coastal zones represent important contributors to the increasing levels of atmospheric nitrous oxide (N₂O), it is still unclear which role benthic processes play and whether marine sediments represent sinks or sources for N₂O, since interactions among closely associated microbial groups lead to a high degree of variability. In addition, coastal areas are extremely dynamic regions, often exposed to increased nutrient loading and conditions of depleted oxygen. We investigated benthic N₂O fluxes and how environmental conditions affect N₂O production in different sediments at 2 different geographical locations (German Wadden Sea, a California coastal lagoon). At each location, a total of 32 sediment cores were taken in areas that differed in sediment type, organic content and pore-water nutrient concentrations, as well as in bioturbation activity. Parallel cores were incubated under in-situ conditions, low oxygen and increased nitrate levels for 10 days. Zones of N₂O production and consumption were identified in intact cores by N₂O microprofiles at the beginning and end of the experiments. In a collaborative effort to determine the dominant sources of N₂O, samples were taken throughout the course of the experiments for the determination of the isotopic composition of N₂O (as well as nitrate, nitrite and ammonium). Our results indicate that both, nitrate addition and low oxygen conditions in the overlying water, caused an increase of subsurface N₂O production in most sediments, with a high variability between different sediment types. N₂O production in the sediments was accompanied by N₂O consumption, reducing the fluxes to the water column. In general, organic rich sediments showed the strongest response to environmental changes with increased production and efflux of N₂O into the overlying water. Bioturbation activity added to the complexity of N₂O dynamics by an increase in nitrification-denitrification processes, as well as enhanced pore-water transport. The results will be used in a metabolic modeling approach that will allow numerical simulation and prediction of sedimentary N₂O dynamics.