Impacts of the long-term presence of buried crude oil on salt marsh soil denitrification in Barataria Bay, Louisiana

Denitrification is typically the dominant process of nitrate removal in coastal wetlands, converting bioavailable N in surface waters into gases released to the atmosphere through microbial respiration. This critical function and valuable ecosystem service performed by soil microbes in coastal wetlands improves water quality and benefits the surrounding ecosystem. Five years after the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico, a layer of crude oil was found ∼5 cm beneath the wetland soil surface in Bay Jimmy, LA. While previous research found potential denitrification in coastal wetland soils was significantly reduced immediately following oil exposure, this study sought to investigate the effects of the long-term presence of spilled crude oil on nitrate removal. Intact soil cores were collected from a salt marsh heavily oiled by the DWH spill, and a nearby, non-oiled site in Barataria Bay, LA. The presence of a layer of buried oil increased nitrate removal from the overlying water column, a proxy measure of denitrification, by 31.8% in intact cores compared to non-oiled cores. However, when the oil was thoroughly mixed through the surface soil intervals, there was an 18-53% decrease in the rate of denitrification. This seemingly contradictory result demonstrates that oil buried 5 cm below the surface, and not in contact with the newly accreted soil, did not negatively affect areal denitrification rates. However, when oil was in direct contact and well mixed in the soil, denitrifiers in that part of the soil profile are still negatively affected.