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

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.