| Summary |
The capacity of forested riparian buffer strips to assimilate nutrient pollution in naturally drained coastal plain agricultural watersheds is well documented. However, the potential of vegetated riparian zones to reduce NO3-N concentrations in groundwater flow leaving agricultural fields artificially drained by stream channelization has not been extensively studied. Channelization is thought to reduce the amount of subsurface NO3-N removal by increasing discharge rates, decreasing natural periods of flooding, increasing dissolved oxygen levels, lowering the water table, and reducing contact with bottomland forest. The objectives of this study were: (1) to evaluate the capacity of vegetated riparian buffer strips adjacent to channelized streams to decrease NO3-N concentrations in shallow groundwater leaving sandy/loamy agricultural soils typical of the coastal plain and (2) to identify variables that correlate significantly with changes in NO3-N concentrations. At three sites featuring sandy/loamy soils in Pitt County, NC, identical transects of shallow and deep wells were placed perpendicular to 1 st order channelized streams, in vegetated riparian zones measuring 12 - 13 m in width, and in contiguous zones cultivated to the stream edge. In addition to measuring NO3-N concentrations, dissolved organic carbon, pH, redox potential, temperature, electrical conductivity, water table depth and gradient, and direction of flow were monitored monthly from September 1997 through August 1999. NH4-N was monitored on a bimonthly schedule. The electrical conductivity of groundwater yielded findings that indicated separate horizontal groundwater flow paths for the shallow and deep wells at all three sites. Within the riparian zones the groundwater appeared to be moving parallel to the surface of the ground toward, the stream. Statistically, at two out of three sites, groundwater NO3-N concentrations among shallow wells were significantly lower in the vegetated riparian zone compared to cultivated riparian zone. The differences seen in this study were similar to results seen in prior studies on naturally drained agricultural areas characterized by low hydraulic gradients and velocities. The third site, characterized by markedly higher soil hydraulic conductivity and coinciding high rates of groundwater velocity, failed to show any evidence of NO3-N removal in the shallow wells. The ability of vegetated riparian buffers to remove NO3-N in shallow groundwater moving from agricultural fields to channelized streams appeared to be strongly associated with saturation of the upper, more organically rich soil layers, where the environment and carbon supply are conducive to microbial denitrification, and where NO3-N is available for plant uptake. In turn, the degree and duration of saturation was also significantly dependent on the hydrologic characteristics of the soil at a given site. Findings were suggestive that plant uptake and microbial denitrification were highly variable in their importance from site to site and between seasons within sites. |
