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Wetland Soils

Denitrification in Marsh-Pond-Marsh Constructed Wetlands Treating Swine Wastewater at Different Loading Rates

^a USDA-ARS, Coastal Plain Soil, Water, and Plant Research Center, 2611 W. Lucas St., Florence, SC 29501
^b North Carolina A&T State Univ., Greensboro, NC 27411

^* Corresponding author (Hunt{at}Florence.usda.ars.gov)

Denitrification in constructed wetlands can be very important in the treatment of swine lagoon effluent when land application areas are limited. The objectives of this investigation were to determine (i) the denitrification enzyme activity (DEA) in the marsh sediments of marsh-pond-marsh (MPM) constructed wetlands, (ii) changes in DEA with additions of carbon and nitrate, and (iii) the response of DEA to different wastewater N loading rates. Swine wastewater was applied to six MPM wetlands located at North Carolina A&T State University, Greensboro, NC, at rates of 4 to 35 kg N ha^–1 d^–1. Soil samples were obtained from the top 25 mm of the marsh sections on four dates for determination of DEA via the acetylene blockage method (blocked at N₂O). Headspace N₂O was measured via gas chromatography. In the control treatment, they ranged from 0.06 to 1.13 and 0.16 to 0.79 mg N₂O-N kg^–1 soil hr^–1 in the first and second marshes, respectively. In both marshes, the DEA rate was significantly increased with the addition of nitrate but not by glucose, indicating that nitrate was a clear limiting factor for denitrification. The DEA in both the control and the amended treatments increased dramatically with increased wastewater N loading, and the increases were generally more pronounced in the first marsh. The DEA values produced in the absence of acetylene blockage did not increase with wastewater N loading rate. Denitrification enzyme activity levels in the marsh sections of the MPM were generally consistent with a highly denitrifying environment.

Abbreviations: DEA, denitrification enzyme activity