Crude Oil Effects on Redox Status of Salt Marsh Soil in Louisiana

Abstract

Core Ideas Crude oil did not lower redox status of wetland soil. Crude oil delayed return to aerobic threshold after draining. Oil in marsh soil can depress redox during low water events. In 2010, the Deepwater Horizon (DWH) oil spill triggered extensive research on crude oil impacts on flora and fauna of the Gulf of Mexico. Little research has investigated impact of spilled oil on redox condition of wetland soil. Redox condition is an excellent proxy for oxygen levels, which control biogeochemical functions linked to valuable ecosystem services. The goal of this study was to quantify effects of crude oil on wetland soil redox conditions in cores collected from a salt marsh in Barataria Bay, LA. Treatments tested were nonoiled control, weathered crude oil at the soil surface, fresh crude oil at the soil surface, and a layer of buried weathered crude oil 5 cm beneath the soil surface to mimic the presence of buried oil found in marshes. No significant differences in redox potentials were detected among treatments at any depth at the end of the 35‐d flooded, anaerobic study. After drainage of the cores began, redox was measured to the time to reach the aerobic threshold (+300 mV) within the rhizosphere. Mean redox values for control cores reached +300 mV after 193 h. Both fresh and weathered crude oil surface treatments reached +300 mV after 316 h while the buried crude oil treatment reached the aerobic threshold after 370 h. These results suggest the presence and location of crude oil in the soil profile can impact soil redox conditions which could alter biogeochemical processes over the long term and induce oxygen stress on wetland vegetation and organisms in the soil.