LEACHING OF NITRATE FROM SHEEP-GRAZED PASTURES

Abstract

An indirect method has been used to estimate losses of nitrate nitrogen from sheep-grazed pastures on a free-draining soil. The drainage component of a soil water balance was assumed to move nitrate, at the concentration of the soil solution between 30 and 45 cm depth, to below the zone of uptake by plants. Losses of between 60 and 80 kg N/ha were calculated to be leached from control ryegrass-clover pastures in each of the winter drainage seasons studied. More than 85% was lost by early August, even though 40% of drainage occurred after this date. Extra leaching losses in the first season appeared to account for the equivalent of 45% of either the 110 or 450 kg N/ha added in the two fertiliser N treatments. In the second season losses were less than 25%. Removal of clovers from the sward reduced leaching estimates to little more than half those below the control pasture. Estimates confirm the large size of leaching losses under intensive grazing. These have implications for agricultural production and groundwater quality. Keywords: Leaching, nitrate, sheep-grazed, pastures, fertiliser, nitrogen, clover-free, water balance, drainage