Integrated Laboratory and Field Studies on Effects of Oil on Key Wetland Soil/Sediment Biogeochemical Processes

On April 20, 2010, the BP Deepwater Horizon (DWH) offshore oil platform experienced an explosion that triggered the largest marine oil spill in US history, resulting in the release of ~795 million liters of southern LA light sweet crude oil into the Gulf of Mexico. Approximately 7.9 million liters of dispersant, Corexit EC9500A, was used during the spill for remediation. We examined the effect of DWH crude oil and Corexit EC9500A, the dispersant used during the spill, on coastal Louisiana marsh soil microbial populations and processes. Surface soil samples were collected from an unimpacted salt marsh site proximal to areas that suffered light to heavy oiling in Barataria Bay, LA. The 1:10 ratio of crude oil:wet soil fully coated the soil surface, mimicking a heavy oiling scenario. Potential denitrification rates at the 1:10 oil:wet soil ratio, for weathered south LA light sweet crude oil, were 46 ± 18.4% of the control immediately after exposure and 62 ± 8.0% of the control following a two week incubation period. Denitrification rates of soil exposed to fresh crude oil were 51 .5 ± 5.3% of the control after immediate exposure and significantly lower at 10.9 ± 1.1 % after a 2 week exposure period. Microbial biomass nitrogen(N) values were below detection for the 1:10, 1:100 and 1:1,000 Corexit:wet soil treatments. Potentially mineralizable N correlated with microbial biomass with decreased activity for 1:10 and 1:100 Corexit:wet soil treatments. Potential denitrification rates after immediate exposure to Corexit:wet soil ratios of 0:10, 1:10, 1:100, 1:1,000, and 1:10,000 were below detection for the 1:10 treatment while the 1:100 was 7.6 ± 2.7% of the control and the 1:1,000 was 33 ± 4.3% of the control. The 1:10,000 treatment was not significantly different from the control. Denitrification rates measured after 2 weeks exposure showed the 1:10 treatment still below detection limit and the 1:100 was 12 ± 2.6% of the control. Results suggest that while both crude oil and Corexit impact denitrification, which is an important microbial process for water quality, Corexit is more detrimental at identical concentrations.