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N2O production and removal within sediments ofgaining, low-relief coastal streams proximal to agricultural fieldswas examined in the context of how changes in temperature, NO3-concentration, and pore water velocity can affect the concentrationof N2O in the groundwater and efflux of N2O from the sediment.Sediment cores extracted from Cobb Mill Creek (CMC), a 2nd orderstream, on the Eastern Shore of Virginia were used as vertical-flowcolumns and operated under conditions that varied, in turn, each ofthe aforementioned parameters systematically, resulting in 36scenarios. Pore water samples were extracted after equilibration ineach scenario from ports in the columns and were analyzed for majoranions and N2O. Nitrate concentration was the strongest control onN2O efflux followed by temperature, where increasing NO3-concentration and temperature each resulted in an increase of N2Oefflux and N2O yield. As NO3- concentration increased from 3.5 to18 mg N L-1, mean N2O fluxes increased from 91 to 284 µg N m-2 h-1and N2O yield increased from 0.15% to 1.75%, respectively. Porewater velocity had minimal effect on N2O efflux due to a netbalance of production and removal along the flow path and advectionrates. Within the columns distinct areas of N2O production followedby areas of removal were observed. These zones were positioneddeeper in the column at higher temperature and at slower pore watervelocities. The overall mean N2O flux for all 36 environmentalscenarios 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 foreach season of 2013. Denitrification was found to occur prior tothe shallow biologically active zone in the sediments causing abuildup of N2O at 70 cm depth. A seasonal lag in groundwatertemperature resulted in warm groundwater temperatures in the winterwhich coincided with increased denitrification and mean N2O fluxesof 568 µg N m-2 h-1. All other mean seasonal fluxes were between 4and 8 µg N m-2 h-1. On average, concentrations of N2O were less at5 cm depth than at 70 cm, indicating N2O removal along a verticalflow path. Despite overall removal of N2O, singular locations ofconcentrated N2O production at the sediment surface were found tocontribute 37 to 97% of the N2O efflux to the surface water.Overall, projected increases in groundwater NO3- in agriculturalareas suggest that there could be significant impacts to enhancingN2O emissions from biologically active streambed sediments,especially when coupled with projected temperatureincreases.

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