Denitrification in a coastal plain riparian zone contiguous to a heavily loaded swine wastewater spray field

Riparian zones are recognized as landscape features that buffer streams from pollutants, particularly nitrogen. The objectives of this experiment were to (i) assess denitrification activity within a riparian zone and (ii) determine the influence of 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. Denitrification enzyme activity (DEA) was measured on soils collected from (i) the soil surface, (ii) midway between the soil surface and water table, and (iii) above the water table. The DEA ranged from 3 to 1660 microg N(2)O-N kg(-1) soil h(-1). The DEA was highest next to the stream and lowest next to the spray field. Nitrate was found to be the limiting factor for denitrification. The DEA generally decreased with soil depth; means for the surface, middle, and bottom depths were 147, 83, and 67 microg N(2)O-N kg(-1) soil h(-1), respectively. These DEA values are higher than those reported for riparian zones adjoining cropland of the southeastern United States, but are lower than those reported for a constructed wetland used for treatment of swine wastewater. Regression analysis indicated that soil total nitrogen was the highest single factor correlated to DEA (r(2) = 0.65). The inclusion of water table depth, soil depth, and distance from the spray field improved the R(2) to 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 ground water.