Effects of dispersant used for oil spill remediation on nitrogen cycling in Louisiana coastal salt marsh soil

• We investigated effects of dispersant used during the BP Gulf oil spill.
• Surfactant decreased denitrification rates by >80% over the control.
• Surfactant decreased mineralization rates by over 50% of the control.
• 2 week incubation led to lower denitrification rates suggesting longer term impacts.

On April 20, 2010, the BP Deepwater Horizon (DWH) offshore oil platform experienced an explosion which triggered the largest marine oil spill in US history. Approximately 7.9 million liters of dispersant, Corexit EC9500A, was used during the spill between May 15th and July 12th. Marsh soil samples were collected from an unimpacted marsh site proximal to coastal areas that suffered light to heavy oiling for a laboratory evaluation to determine the effect of Corexit on the wetland soil microbial biomass as well as N-mineralization and denitrification rates. Microbial biomass nitrogen (N) values were below detection for the 1:10, 1:100 and 1:1000 Corexit:wet soil treatments. The potentially mineralizable N (PMN) rate correlated with microbial biomass with significantly lower rates for the 1:10 and 1:100 Corexit:wet soil additions. Potential denitrification rates for Corexit:wet soil ratios after immediate dispersant exposure were below detection for the 1:10 treatment, while the 1:100 was 7.6 ± 2.7% of the control and the 1:1000 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 to the surfactant found the 1:10 treatment still below detection limit and the 1:100 ratio was 12 ± 2.6% of the control. Results from this lab study suggest that chemical dispersants have the potential to negatively affect the wetland soil microbial biomass and resultant microbial activity. Consequences of exposure led to reductions in several important microbial-regulated ecosystem services including water quality improvement (denitrification) and ecosystem primary productivity (N-mineralization). Future studies should investigate the longer-term impacts of dispersant exposure on the microbial consortia to determine if microbial activity recovers over time.