Salt Marsh Soil Denitrification Rates Stimulated Following Oiling in a Large-scale Marsh Mesocosm Experiment

Alizia Barnes1, Brian J Roberts2, Charles A Schutte2,3 and Ryann Rossi2, (1)United States, (2)Louisiana Universities Marine Consortium, Chauvin, LA, United States, (3)Rowan University, Environmental Science, Glassboro, NJ, United States
Coastal wetlands straddle the land-ocean interface providing a myriad of ecosystem services,
including removal of nitrogen through the process of denitrification, yet are also especially
vulnerable to disturbances across a range of temporal and spatial scales. Here we report results
from a large-scale experiment using a salt marsh mesocosm facility to test the impact of oil
exposure on soil denitrification potential rates under controlled and uniform environmental
conditions. The facility consists of 12, hydrologically independent Spartina alterniflora marsh
mesocosms (3m diameter) each with its own paired tidal surge tank capable of generating tidal
cycles with ranges up to 50cm via a water control system of blowers and airlifts. Three marsh
mesocosms were assigned to each of four treatments: control plus light, moderate, and heavily
oiling levels scaled to the SCAT categories observed following the Deepwater Horizon spill.
Denitrification potentials were assessed on homogenized surface soil (0-5cm) collected from 5
random quadrats within each marsh using the denitrification enzyme activity (DEA) assay on
three dates: three weeks prior to oiling and one and two weeks post-oiling. Denitrification
decreased in all tanks the week after oiling as a result of higher salinities experienced during the
passage of Hurricane Barry. Therefore, the responses to oil exposure were assessed by
comparing treatment:control ratios over time. Denitrification rates were higher in all oil
treatment marshes than control marshes in the week immediately following oil exposure, but no
significant differences were detected in week 2. Denitrification rates will continue to be
monitored over time to assess potential longer impacts on salt marsh soil nitrogen cycling.