GROUNDWATER NITRATE LEVELS UNDER FERTILIZED GRASS AND GRASS-LEGUME PASTURES

High levels of N fertilizer applied to pastures can result in NO3-N concentrations in groundwater exceeding the USEPA potable water standard of 10 mg N/L. This study was conducted to determine groundwater NO3-N levels following a change in N source from fertilizer to a legume in a grass-pasture grazed by beef cattle. For 5 yr, 224 kg N/ha was applied annually to small watersheds with orchardgrass (Dactylis glomerata L.) pastures used for summer-grazing and tall fescue (Festuca arundinacea Schreb.) areas used for winter-grazing-feeding. At the of the sixth year, alfalfa (Medicago sativa L.) was interseeded into the grass pastures and N fertilizer was no longer applied. Groundwater samples from developed springs and surface runoff samples were collected and analyzed for NH4-N, NO3-N, and total N for the 5-yr fertilization period and for the following 10-yr period without applied N fertilizer. Nitrogen in groundwater was present mainly in the NO3 form, and concentrations increased throughout the 5-yr period of fertilizer application and reached levels that were usually in excess of 10 mg N/L. With the change from N fertilizer to legume N, the NO3-N concentrations in groundwater dropped rapidly during a 2-yr period. In a tall fescue-alfalfa area, NO3-N levels decreased from 17.7 to 9.3 mg N/L. In two orchard-grass-alfalfa areas, NO3-N levels decreased from 11.2 to 2.7 and from 8.3 to 3.6 mg N/L. During the remainder of the 10-yr period, NO3-N concentrations declined to levels similar to those before N fertilization. Although the amount of N lost via subsurface flow decreased with decreasing concentrations, subsurface flow remained the main pathway for N loss compared with surface runoff or sediment-attached N.