N2O Production and Removal in Streambed Sediments: Understanding Physicochemical Influences on N2O Fluxes From Agricultural Streams on the Eastern Shore of Virginia

N2O production and removal within sediments of gaining, low-relief coastal streams proximal to agricultural fields was examined in the context of how changes in temperature, NO3- concentration, and pore water velocity can affect the concentration of N2O in the groundwater and efflux of N2O from the sediment. Sediment cores extracted from Cobb Mill Creek (CMC), a 2nd order stream, on the Eastern Shore of Virginia were used as vertical-flow columns and operated under conditions that varied, in turn, each of the aforementioned parameters systematically, resulting in 36 scenarios. Pore water samples were extracted after equilibration in each scenario from ports in the columns and were analyzed for major anions and N2O. Nitrate concentration was the strongest control on N2O efflux followed by temperature, where increasing NO3- concentration and temperature each resulted in an increase of N2O efflux and N2O yield. As NO3- concentration increased from 3.5 to 18 mg N L-1, mean N2O fluxes increased from 91 to 284 µg N m-2 h-1 and N2O yield increased from 0.15% to 1.75%, respectively. Pore water velocity had minimal effect on N2O efflux due to a net balance of production and removal along the flow path and advection rates. Within the columns distinct areas of N2O production followed by areas of removal were observed. These zones were positioned deeper in the column at higher temperature and at slower pore water velocities. The overall mean N2O flux for all 36 environmental scenarios was 156 µg N m-2 h-1. In addition to the column study, N2O efflux and production at depth was studied in situ at CMC for each season of 2013. Denitrification was found to occur prior to the shallow biologically active zone in the sediments causing a buildup of N2O at 70 cm depth. A seasonal lag in groundwater temperature resulted in warm groundwater temperatures in the winter which coincided with increased denitrification and mean N2O fluxes of 568 µg N m-2 h-1. All other mean seasonal fluxes were between 4 and 8 µg N m-2 h-1. On average, concentrations of N2O were less at 5 cm depth than at 70 cm, indicating N2O removal along a vertical flow path. Despite overall removal of N2O, singular locations of concentrated N2O production at the sediment surface were found to contribute 37 to 97% of the N2O efflux to the surface water. Overall, projected increases in groundwater NO3- in agricultural areas suggest that there could be significant impacts to enhancing N2O emissions from biologically active streambed sediments, especially when coupled with projected temperature increases.