Fine-scale Estimates of Net Nitrous Oxide Production Rates and Sediment Fluxes Along Vertical Gradients of Dissolved Oxygen in Muddy Sediments from a Temperate Estuary

Brett Jameson1, Peter Berg2, Damian Grundle3 and Kim Juniper1,4, (1)University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada, (2)University of Virginia, Department of Environmental Sciences, Charlottesville, VA, United States, (3)Bermuda Institute of Ocean Science, St. George's, Bermuda, (4)Ocean Networks Canada, Victoria, BC, Canada
Estuarine sediments are active sites of nitrogen cycling, and can act as a buffer to the eutrophication of coastal environments through the biological reduction of fixed nitrogen to gaseous end products. However, increased rates of nitrogen cycling are also associated with elevated fluxes of the potent greenhouse gas, nitrous oxide (N2O). Sediment cores taken from Cowichan Bay, a temperate estuary on Southern Vancouver Island, were incubated under ambient and elevated nutrient (NH4+ or NO3-) regimes. Porewater N2O and O2 profiles were obtained using microsensors to estimate sediment net N2O production rates and vertical fluxes in relation to O2 penetration depth and consumption rates. Production of N2O under the ambient and elevated NH4+ conditions was below the sensor detection limit (100 nM). Production of N2O following addition of NO3- was highly variable across both short (30-60 min) and long-term (>20 hrs) incubation times. Outward flux of N2O from the sediment interface was observed in long-term incubations under elevated NO3- conditions. These results implicate estuarine sediments as a potential source of N2O to the atmosphere under elevated nutrient scenarios, with denitrification serving as the predominant production process. The short response time of sediment microbes to nutrient additions also suggests that tidal pulses may result in short-term bursts in sedimentary N2O production. Future research will attempt to link N2O production and consumption rates via denitrification with microbial community dynamics, including the diversity and expression of enzyme-encoding functional genes.