Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/26/2005
Publication Date: 3/7/2006
Citation: Hunt, P.G., Poach, M.E., Matheny, T.A., Reddy, G.B., Stone, K.C. 2006. Denitrification in marsh-pond-marsh constructed wetlands treating swine wastewater at different loading rates. Soil Science Society of America Journal. 70:487-493. Interpretive Summary: While nitrogen is an essential component of plant live and proper fertilization of agronomic crops, it can be produced in excess of the farm needs by concentrated livestock production. One of the most effective and natural ways of managing excess nitrogen is a microbial process called denitrification. This process converts nitrogen in plant available forms to inert dinitrogen gas. Denitrification is a major process in both natural and constructed wetlands. The research of this investigation assessed the extent and controlling factors for denitrification on constructed wetlands used to treat swine lagoon effluent. Denitrification was found to be occurring at very significant levels, and the rate increased as the amount of nitrogen applied in the swine effluent increased. As generally found in constructed wetland, the limiting factor was sufficient oxygen to create the form of nitrogen necessary to initiate denitrification. These findings indicate that progress can be made by finding better methods of increasing oxygen transfer into the wetlands.
Technical Abstract: Denitrification in constructed wetlands can be very important in the treatment of swine lagoon effluent when land applications areas are limited. The objectives of this investigation were to determine 1) the denitrification enzyme activity (DEA) in the marsh sediments of marsh-pond-marsh (MPM) constructed wetlands, 2) changes in denitrification activity with additions of carbon and nitrate, and 3) 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/d. Each wetland cell was 40m by 11m with two marsh sections [10 by 11m; cattail (Typha latifolia L.) dominated] separated by a pond section (20 by 11m). Soil samples were obtained from the top 25 mm of the marsh sections on four dates for determination of DEA via the acetylene blockage (blocked at N2O) method. Five treatments were used: 1) control, 2) nitrate addition, 3) glucose addition, 4) nitrate and glucose addition, and 5) nitrate and glucose addition without acetylene blockage ' to access background N2O production. Headspace N2O was measured via gas chromatography. DEA levels in the marsh section of the MPM were consistent with a highly denitrifying environment. In the control treatment, they ranged from 0.06 to 1.13 and 0.16 to 0.79 mg N2O-N/kg soil/hr, 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, but 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 indicating that a smaller percentage of incomplete denitrification was occurring at higher loading rates.