Long-term effect of crude oil and dispersant on denitrification and organic matter mineralization in a salt marsh sediment.

The 2010 BP oil spill has an unprecedented impact on coastal wetland ecosystem along the northern Gulf of Mexico. A two-dimensional analysis (dispersant concentration and duration of exposure) was conducted by pre-incubation of a salt marsh sediment under an open or closed condition. Denitrification activity was characterized by NO production using an acetylene (CH) blockage technique, and organic matter (OM) mineralization by CO production. The results show that even trace amount of the dispersant could significantly inhibit the denitrification activity by 20% (p < 0.05). However, the sediment was resilient to the oil/dispersant contamination, likely due to shift of its microbial communities, by recovering the denitrification activity within 46 days in the open incubation. Inhibitory effect of the oil/dispersant on denitrification persisted beyond 46 days in the closed incubation, and the recovery could take up to 137 days depending on the dispersant concentration. The dispersant continuously stimulated OM mineralization that lowered the sediment redox status. Mobilization of N in the sediment from the OM mineralization forms a positive feedback loop, leading to deterioration of the coastal ecosystem. The study concludes that minimum dispersant should be applied for oil spill remediation, and oil cleanup operations should avoid moving the oil/dispersant from surface into deeper layers of the sediment. Synergistic interactions between the crude oil and dispersant and their biodegradation products deserves future examinations.