Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 10, 2004
Publication Date: November 15, 2004
Citation: Hunt, P.G., Matheny, T.A., Stone, K.C. 2004. Denitrification in a coastal plain riparian zone contiguous to a heavily loaded swine wastewater spray field. Journal of Environmental Quality. 33:2367-2374. Interpretive Summary: Riparian zones are recognized worldwide as important landscape features that are able to buffer streams from pollutants, particularly nitrogen. This buffering capacity can be especially important for fields that have received considerable amounts of animal waste application. One of the most important processes in riparian zone is a microbial process called 'denitrification.' It is most active in soils that have high moisture and low oxygen, conditions which are common to riparian zones. The level of this microbial process can be assessed by analyzing for the enzyme activity of these microorganisms. When we did these enzyme analyses in the investigated riparian zone, we found that it had very high levels of the enzyme present. In fact, they were much higher than commonly found in riparian zones next to row crops. The level of the enzyme in a particular location within the riparian buffer was highly related to the nitrogen content of the soil. The levels of enzymes were highest in the surface layer and next to the stream. These high levels of the denitrifying enzyme activity were consistent with the large reduction in nitrogen as groundwater moved from the waste application field to the stream.
Technical Abstract: Riparian zones are recognized worldwide as important landscape features that are able to buffer streams from pollutants, particularly nitrogen. The objectives of this experiment were to 1) assess the denitrification potential within this riparian zone and 2) determine the influence of various physical, chemical, and landscape features on denitrification. This experiment was conducted from 1994 to 1997 in North Carolina on a riparian zone contiguous to a spray field that was heavily loaded with swine lagoon wastewater. The acetylene blockage method was used for denitrification enzyme assays (DEA) on soils collected in cores (25 by 155 mm) on 13 dates from 1) the soil surface, 2) midway between the soil surface and water table, and 3) above the water table. Values of DEA ranged from 3 to 1660 g/kg/hr. DEA values were highest next to the stream and lowest next to the spray field. Throughout the riparian zone, nitrate was generally found to be the limiting factor for denitrification. Values of DEA generally decreased with soil depth; means for the surface, middle, and bottom depths were 147, 83, and 67 g/kg/hr, respectively. These DEA values are much higher than those commonly reported for riparian zones adjoining cropland of the Southeastern USA, but they are lower than those reported for constructed wetland used for treatment of swine wastewater in the region. In a stepwise regression of log DEA, soil total nitrogen was found to be the most highly correlated factor (r2 = 0.64). However, inclusion of water table depth, soil sample depth and distance from the spray field gave improvements in the predictive capability (r2 = 0.86). This riparian zone possessed sufficient soil area with high denitrifying conditions to be a significant factor in the removal of excess nitrogen in the groundwater.