Nitrate leaching from a drained, sheep-grazed pasture .1. Experimental results and environmental implications

Nitrate-nitrogen (N) and chloride concentrations were measured in the drainage from two hydrologically similar mole- and pipe-drained paddocks over 3 years: 1989, 1990 and 1991. Urea (50 kg N/ha) that was applied to both paddocks in early spring 1989 had no effect on the nitrate concentrations in the drainage water. In late autumn 1990, 120 kg N/ha as urea was applied to one of the paddocks, but the nitrate leached in the following winter was equivalent to only 8% Of the fertilizer N. The nitrate concentrations in the drainage were greater than 10 g N/m(3) for most of the season, with peak concentrations up to 35 g N/m(3), even for the paddock where urea was not applied. The paddocks were periodically mob-grazed by sheep during the monitoring period, but this had no measurable effect on the nitrate and chloride concentrations in the drainage collected during or immediately after grazing.