Presence of vegetation and tidal heights influence rates of denitrification in a natural and restored marsh in the northern Gulf of Mexico

Abstract

Salt marshes act as coastal filters, permanently removing nitrate through denitrification. This study determined denitrification rates in a typical northern Gulf of Mexico (GOM) salt marsh and the factors that influenced these rates. Salt marsh denitrification rates were compared to rates within an adjacent tidal creek to determine the impact of vegetation on denitrification and to rates at a nearby restored salt marsh to evaluate the extent to which restored habitats contribute to N removal 21 years post construction. Rates of denitrification were measured with the isotope pairing technique on a membrane inlet mass spectrometer. The highest overall denitrification rates were measured in the restored marsh followed by the natural marsh site, with the lowest rates at the creek site (p=0.0005). Rates displayed a temporal pattern with peaks in the summer (Restored: 63±11 μmol m-2 hr-1; Marsh: 46±4 μmol m-2 hr-1; Creek: 35±3 μmol m-2 hr-1). The presence of vegetation likely enhanced denitrification by supplying oxygen supply to the rhizosphere, thereby allowing for increased coupled nitrification-denitrification. Porewater hydrogen sulfide concentrations remained low at the Marsh (0-184 μM) and Restored Marsh (0-2 μM) sites, but were greater at the Creek site (50-860 μM). Lower H2S concentrations in vegetated sediments likely resulted from its greater oxidation through root oxygen inputs in vegetated zones, a process, which also promotes coupled denitrification. Overall, the restored marsh efficiently removed N at rates comparable to, or higher than, a natural marsh. However, the height of the restored marsh platform limited its inundation period to half as much as the natural marsh, reducing its effectiveness in removing anthropogenically-derived nitrate from infiltrating waters and mitigating nitrogen loading to offshore waters. (Published By University of Alabama Libraries