Groundwater Nitrate Levels under Fertilized Grass and Grass‐Legume Pastures

Abstract High levels of N fertilizer applied to pastures can result in NO 3 ‐N concentrations in groundwater exceeding the USEPA potable water standard of 10 mg N/L. This study was conducted to determine groundwater NO 3 ‐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 beginning of the sixth year, alfalfa ( Medicago sativa L.) was interseeded into the grass pastures and N fertilizer was no longer applied. Groundwater sampies from developed springs and surface runoff samples were collected and analyzed for NH 4 ‐N, NO 3 ‐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 NO 3 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 NO 3 ‐N concentrations in groundwater dropped rapidly during a 2‐yr period. In a tall fescue‐alfalfa area, NO 3 ‐N levels decreased from 17.7 to 9.3 mg N/L. In two orchard‐grass‐alfalfa areas, NO 3 ‐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, NO 3 ‐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.