Salt marsh denitrification is impacted by oiling intensity six years after the Deepwater Horizon oil spill.

Coastal salt marshes provide the valuable ecosystem service of removing anthropogenic nitrogen (N) via microbially-mediated denitrification. During the 2010 Deepwater Horizon (DWH) spill, oil exposure killed marsh plants in some regions and contributed to rapid compositional shifts in sediment microbial communities, which can impact ecosystem denitrification capacity. Within 3-5 years of the spill, plant biomass and microbial communities in some impacted marshes can recover to a new stable state. The objective of this study was to determine whether marsh recovery 6 years after the DWH oil spill results in subsequent recovery of denitrification capacity. We measured denitrification capacity (isotope pairing technique), microbial 16S rRNA gene composition, and denitrifier abundance (quantitative PCR) at sites subjected to light, moderate, and heavy oiling during the spill that were not targeted by any clean-up efforts. There were no differences in plant belowground biomass, sediment extractable NH4+, inorganic nitrogen flux, 16S rRNA composition, 16S rRNA diversity, or denitrifier functional gene (nirS, norB, and nosZ) abundances associated with oiling status, indicating that certain drivers of ecosystem denitrification capacity have recovered or achieved a new stable state six years after the spill. However, on average, denitrification capacities at the moderately and heavily oiled sites were less than 49% of that of the lightly oiled site (27.7 ±â€¯14.7 and 37.2 ±â€¯24.5 vs 71.8 ±â€¯33.8 µmol N m-2 h-1, respectively). The presence of heavily weathered oiled residue (matched and non-matched for MC252) had no effect on process rates or microbial composition. The loss of function at the moderately and heavily oiled sites compared to the lightly oiled site despite the comparable microbial and environmental factors suggests that oiling intensity plays a role in the long-term recovery of marsh ecosystem services.

Denitrification Capacity of a Natural and a Restored Marsh in the Northern Gulf of Mexico.

Anthropogenic pressures, such as diking, construction of dams, and oil spills negatively impact coastal marshes creating growing pressure to preserve and to restore salt marshes due to their critical role in permanently removing nitrate runoff through denitrification as well as other ecosystem services they provide. This study determined denitrification rates across a typical northern Gulf of Mexico salt marsh landscape that included a natural marsh, a tidal creek, and a 21-year-old restored salt marsh. Denitrification capacity, measured with the isotope pairing technique on a membrane inlet mass spectrometer, was comparable across the sites despite significant differences in above and below ground characteristics. Total extractable ammonium concentrations and sediment carbon content were higher at the natural marsh compared to the restored marsh. Hydrogen sulfide concentrations were highest at the creek compared to the vegetated sites and lowest at the restored marsh. This suggests that marsh restoration projects reestablish nitrogen removal capacity at rates similar to those in natural systems and can help to significantly reduce nitrogen loads to the coastal ocean.

Vegetation Loss Decreases Salt Marsh Denitrification Capacity: Implications for Marsh Erosion.

Salt marshes play a key role in removing excess anthropogenic nitrogen (N) loads to nearshore marine ecosystems through sediment microbial processes such as denitrification. However, in the Gulf of Mexico, the loss of marsh vegetation because of human-driven disturbances such as sea level rise and oil spills can potentially reduce marsh capacity for N removal. To investigate the effect of vegetation loss on ecosystem N removal, we contrasted denitrification capacity in marsh and subtidal sediments impacted by the Deepwater Horizon oil spill using a combination of 29N2 and 30N2 production (isotope pairing), denitrification potential measurements (acetylene block), and quantitative polymerase chain reaction (qPCR) of functional genes in the denitrification pathway. We found that, on average, denitrification capacity was 4 times higher in vegetated sediments because of a combination of enhanced nitrification and higher organic carbon availability. The abundance of nirS-type denitrifers indicated that marsh vegetation regulates the activity, rather than the abundance, of denitrifier communities. We estimated that marsh sediments remove an average of 3.6 t N km-2 y-1 compared to 0.9 t N km-2 y-1 in unvegetated sediments. Overall, our findings indicate that marsh loss results in a substantial loss of N removal capacity in coastal ecosystems.