Grazing Impacts on Nitrate Leaching from Fertilized Grasslands L.B. Owens, M.J. Shipitalo, and J.V. Bonta USDA-Agricultural Research Service, North Appalachian Experimental Watershed Coshocton, Ohio 43812 (USA) Introduction Recent lysimeter studies from New Zealand (Silva et al., 1999) and the US (Stout et al., 1997) indicate that intensive grazing has the potential to substantially increase nutrient leaching compared to ungrazed grassland. Cuttle et al. (2001) suggested that urine patches may contribute to increased NO3-N leaching. Our objective was to do a field-scale assessment of the impact of grazing on nitrate leaching from grasslands using small watersheds with underlying clay layers. Materials and Methods Two small watersheds (<1 ha), WS 103 & 110 (Figure 1) in a 13.8 ha area (Figure 2) at the North Appalachian Experimental Watershed near Coshocton, OH (USA) have been used in grazing/grassland studies for 30 years. In these unglaciated soils, nearly impermeable, geologic clay layers intercept infiltrating water and Figure 1. (L) Diagram showing the pasture area and the study watersheds and Figure 2. (R) Grazing area. create perched water tables, which are sampled at developed springs, i.e. water is intercepted at the Lower Kittanning clay layer by tile lines and brought to a samping outlet (Figure 3). A spring site in each watershed was sampled weekly for several years then monthly to measure impacts of surface management on the quality of water moving to groundwater. These orchardgrass (Dactylis glomerata L.) pastures received Figure 3. (L) Spring sampling site and Figure 4. (R) Cattle grazing the grass-legume pasture. 224 kg N ha -1 as NH 4 NO 3 annually for 5 years and were rotationally grazed by a spring calving beef cow- calf herd. Then alfalfa (Medicago sativa L.) was seeded into the orchardgrass (Figure 4) and no N fertilizer was applied. After 19 years of no N fertilizer applications, the orchardgrass stand was reestablished; cattle were removed; 224 kg N ha -1 as NH 4 NO 3 was applied annually; forage was removed as hay. After 4 years, rotational grazing was resumed (Table 1). Table 1. Time periods and management practices for WS 103 and WS 110 pastures. _________________________________________________ Time Period # of Management (May – April) Yrs _________________________________________________ 1975 – 1980 5 224 kg N ha -1 annually; rotational grazing 1980 – 1981 1 Seeding legumes in grass; limited late season grazing 1981 – 1999 18 Grass-legume pastures (No N added); rotational grazing 1999 – 2003 4 224 kg N ha -1 annually; no grazing; hay made 2003 - 2+ 224 kg N ha -1 annually; rotational ______________________grazing_____________________ Results and Discussion The major pathway of N transport from pasture systems is via subsurface flow (Owens et al., 1994). Flow-weighted seasonal average concentrations of NO 3 -N increased during the initial period of grazing on fertilized grass (224 kg N ha -1 ) to near 10 mg N L -1 or above (Figure 5). With grazing on a grass- legume mixture and no N Figure 5. Seasonal flow-weighted NO 3 -N concentrations (1975-1991) for WS 103 and WS 110. Gr = Growing Season (May through October); Dor = Dormant Season (November through April). fertilizer, NO 3 -N concentrations decreased sharply at first and then gradually to around 2 mg L -1 . Nitrate-N concentrations remained low when N fertilizer was applied to grass again in these watersheds, but with haying instead of grazing (Figure 6). A second spring was developed in association with WS 110 in 1997. It captured a larger amount of the groundwater from the clay layer than the first spring but had greater variability in NO 3 -N concentrations. This variability, as well as higher NO 3 -N concentrations is shown graphically (Figure 6) and with standard deviations (Table 2). The NO 3 -N concentrations in each of the 3 springs showed no increasing trend prior to resumption of grazing with the possible exception of the final dormant season (Dor 2002-03). Dormant 0 2 4 6 8 10 12 14 1975 Gr 1975-76 Dor 1976 Gr 1976-77Dor 1977 Gr 1977-78 Dor 1978 Gr 1978-79 Dor 1979 Gr !979-80 Dor 1980 Gr 1980-81Dor 1981 Gr 1981-82 Dor 1982 Gr 1982-83 Dor 1983 Gr 1983-84 Dor 1984 Gr 1984-85 Dor 1985 Gr 1985-86 Dor 1986 Gr 1986-87 Dor 1987 Gr 1987-88 Dor 1988 Gr 1988-89 Dor 1989 Gr 1989-90 Dor 1990 Gr 1990-91 Dor NO 3 -N Conc (mg L -1 ) WS 103 WS 110 224 kg N ha -1 - Grazing Grass-legume Grazing