Understanding the Fate of Submerged Coastal Wetland Soils.

Coastal wetlands have been shown to respond to rapid rates of relative sea level rise with submergence, which occurs when the wetland platform erodes or is unable to accrete vertically to keep pace with sea level rise. As coastal wetlands submerge, previously sequestered soil organic matter is exposed to oxygen-rich seawater, which has the ability to increase soil mineralization rates and potentially release large stores of carbon (C), nitrogen (N), and phosphorus (P) into coastal ecosystems. Barataria Basin, LA, where high rates of both relative sea level rise and submergence have been observed, was used as a proxy to determine the potential release of CO₂, CH₄, dissolved organic C, and potentially mineralizable N and P. Brackish marsh soil cores were obtained in triplicate from three sites within Barataria Basin to a depth of 1 m and sectioned into 11 depth intervals. Soil were incubated in the laboratory under anaerobic (mimicking an intact marsh) and aerobic (mimicking exposure to oxygen-rich seawater following submergence) conditions for 14 days. In addition C production measurements, samples were also analyzed for enzyme activity and extractable nutrients before and after the 14 day incubation period. Preliminary results show that aerobic conditions contribute to increased CO₂ production, particularly in deep soils (60+ cm), as well as increased concentrations of extractable nitrate and soluble reactive phosphorus throughout the soil profile. The activity of several soil enzymes also responded to changes in soil oxygen availability. These findings suggest buried soil organic matter is be more labile than previously thought and may be rapidly mineralized following marsh collapse.