NITROGEN CYCLING IN PIEDMONT VEGETATED FILTER ZONES .2. SUBSURFACE NITRATE REMOVAL

Subsurface how often constitutes the major pathway for movement of dis solved nutrients such as NO3-N from agricultural fields. The objective s of this study were (i) to determine the changes in shallow groundwat er chemistry along a piezometric gradient from agricultural fields, ac ross grass-vegetated field edges and through adjacent forest on two Pi edmont watersheds and (ii) determine the relative importance of diluti on, denitrification, and plant uptake in subsurface NO3 attenuation. W e monitored changes in groundwater chemistry at three depths along a p iezometric gradient from an agricultural field through a grass field e dge and through a forested filter zone (FFZ). We measured a marked dec rease in nitrate concentrations from 8 to 10 mg L(-1) at the field edg e to almost 0 at the forest edge; CI concentrations remained within th e range of 8 to 10 mg L(-1), suggesting that dilution was not an impor tant factor in NO3 concentration reductions. At a third site, we intro duced NO3-N and a conservative tracer, bromide, into the soil profile at both the grass-vegetated field border and the forested area, to det ermine mechanisms responsible for the observed decrease in NO3-N conce ntrations. Using ion concentration ratios we determined that nitrate a ttenuation in the grass-vegetated field edge was low compared to the f orest. Nitrate loss in the forest was almost exclusively through denit rification; plant uptake was insignificant in these experiments. Altho ugh grass-vegetated field borders were less effective than riparian fo rests at NO3-N removal, considerable reductions were observed in these areas on the experimental watersheds. Similar reductions would be exp ected over shorter distances in riparian forests.