GROUNDWATER NITRATE LEVELS UNDER FERTILIZED GRASS AND GRASS-LEGUME PASTURES

High levels of N fertilizer applied to pastures can result in NO3-N co ncentrations 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 appli ed annually to small watersheds with orchardgrass (Dactylis glomerata L.) pastures used for summer-grazing and tall fescue (Festuca arundina cea Schreb.) areas used for winter-grazing-feeding. At the of the sixt h year, alfalfa (Medicago sativa L.) was interseeded into the grass pa stures and N fertilizer was no longer applied. Groundwater samples fro m developed springs and surface runoff samples were collected and anal yzed for NH4-N, NO3-N, and total N for the 5-yr fertilization period a nd for the following 10-yr period without applied N fertilizer. Nitrog en in groundwater was present mainly in the NO3 form, and concentratio ns increased throughout the 5-yr period of fertilizer application and reached levels that were usually in excess of 10 mg N/L. With the chan ge from N fertilizer to legume N, the NO3-N concentrations in groundwa ter dropped rapidly during a 2-yr period. In a tall fescue-alfalfa are a, NO3-N levels decreased from 17.7 to 9.3 mg N/L. In two orchard-gras s-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 concent rations declined to levels similar to those before N fertilization. Al though the amount of N lost via subsurface flow decreased with decreas ing concentrations, subsurface flow remained the main pathway for N lo ss compared with surface runoff or sediment-attached N.