Impacts of Long-Term Oiling from the 2010 Deep Water Horizon Oil Spill on Salt Marsh Soil Denitrification.

In coastal Louisiana (LA), demands for ecosystem services are increasing while human activities continue to deteriorate coastal systems.  On April 20, 2010, the largest offshore oil spill in United States history occurred in the Gulf of Mexico, known as the Deep Water Horizon (DWH) oil spill.  Approximately 795 million L of crude oil were released, consequently oiling 1,773 km of Gulf Coast shoreline.  Four years later, oil from the spill was found buried in the soil and seeping at the salt marsh surface in Bay Jimmy, LA.  Previous studies found that immediately following oil exposure, wetland soils have suppressed microbial activity.  This study seeks to understand effects of the long-term presence of oil on soil microbes and associated impacts to wetland soil denitrification.  Bulk soil and intact soil cores were collected four years after the DWH spill from a heavily impacted salt marsh and a proximal site deemed unoiled in Barataria Bay, LA.  Oil present in the soil subsurface increased dry weight bulk density, and decreased moisture content.  Potential denitrification (acetylene block) in the top 10 cm of soil was 38% lower for oiled samples versus unoiled controls.  Areal nitrate reduction rates were significantly lower in oiled samples in an intact core flux experiment under environmentally relevant nitrate conditions (2mg/L NO₃-N), P-value <0.1.  The presence of oil lowered potential denitrification rates in extruded sections of soil from the intact core study at 0-5 cm and 5-10 cm depths.  These results demonstrate that after four years, buried oil is still toxic to denitrifying bacteria and may have significantly negative impacts on the ecosystem service of water quality improvement.  Future studies should investigate impacts of oil being rebroadcasted onto marshes as land erodes in the study area.