Shallow ground water nitrate-N and ammonium-N in cropland and riparian buffers

The extent of nutrient reduction in shallow ground water flow between cropland and riparian buffers in the Northeast is not well established, yet there is an increasing need to quantify such reductions. A four-year project was initiated in 2002 to determine the relative effectiveness of riparian buffers on reducing nutrients in soil water and shallow ground water flow from adjacent cropland. The main objective of this study was to determine if shallow ground water nitrate nitrogen (NO₃-N) and ammonium nitrogen (NH ₄-N) concentrations differed among cropland (hay or corn), restored riparian buffers (grass and Salix-grass), and established forested riparian buffers. Sixteen paired ground water monitoring wells were established in cropland and riparian buffers at two agricultural research sites during July 2002 and July 2003. Samples of ground water, tile drainage water, and stream water were collected approximately monthly over the 2003 field season and analyzed for NO₃-N and NH₄-N concentration. Average NO₃-N concentration across sites was significantly lower in buffers for each sampling. Average NH₄-N concentration was consistently higher beneath buffers, and decreased markedly over the sampling period. Soil drainage, as indexed by depth to water table, was significantly correlated with NO₃-N concentrations among cropland and buffer wells at individual sites. We hypothesize that this reflects the relationship between soil drainage and its direct impact on both NO₃-N leaching and denitrification potentials across the landscape. Forested buffers had the lowest average NO ₃-N, highest NH₄-N, and the highest water table. Cropland soils with appreciable NO₃-N in ground water adjoining riparian buffers on outwash deposits were ineffective at reducing NO₃-N. Consistent NO₃-N reductions occurred between cropland and buffers where ground water flowed from moderately well and well drained cropland to poorly drained riparian buffer soils.