Nitrogen Cycling in Piedmont Vegetated Filter Zones: II. Subsurface Nitrate Removal

Abstract

AbstractSubsurface flow often constitutes the major pathway for movement of dissolved nutrients such as NO3‐N from agricultural fields. The objectives of this study were (i) to determine the changes in shallow groundwater chemistry along a piezometric gradient from agricultural fields, across grass‐vegetated field edges and through adjacent forest on two Piedmont watersheds and (ii) determine the relative importance of dilution, denitrification, and plant uptake in subsurface NO3 attenuation. We monitored changes in groundwater chemistry at three depths along a piezometric gradient from an agricultural field through a grass field edge and through a forested filter zone (FFZ). We measured marked decrease in nitrate concentrations from 8 to 10 mg L−1 at the field edge to almost 0 at the forest edge; Cl concentrations remained within the range of 8 to 10 mg L−1, suggesting that dilution was not an important factor in NO3 concentration reductions. At a third site, we introduced NO3‐N and a conservative tracer, bromide, into the soil profile at both the grass‐vegetated field border and the forested area, to determine mechanisms responsible for the observed decrease in NO3‐N concentrations. Using ion concentration ratios we determined that nitrate attenuation in the grass‐vegetated field edge was low compared to the forest. Nitrate loss in the forest was almost exclusively through denitrification; plant uptake was insignificant in these experiments. Although grass‐vegetated field borders were less effective than riparian forests at NO3‐N removal, considerable reductions were observed in these areas on the experimental watersheds. Similar reductions would be expected over shorter distances in riparian forests.