A comparison of nitrate leaching under clover-based pastures and nitrogen-fertilized grass grazed by sheep

Nitrate leaching was measured under three pastures - perennial ryegrass and white clover, a herbal ley comprising a mixture of legumes, non-aggressive grasses and deep rooting herbs, and perennial ryegrass fertilized with 400 kg N/ha per year as urea - from 1989 to 1991 at Palmerston North, New Zealand (latitude 40 degrees S). The pastures were regularly mob-grazed by sheep at a stocking rate which varied with the amount of feed available. Drainage was estimated from a daily soil water balance and accumulated over 10 day intervals. Nitrate concentrations in the drainage were measured as the volume-averaged concentrations in the soil solution between 30 and 45 cm depth during these intervals. Although the N applied as fertilizer to the grass was 2.5 times greater than the N estimated to have been fixed by the clover-based pastures, the leaching loss from the former was 6-7 times greater than from the latter. The stocking rate on the grass pasture averaged over each year was only 1.2-1.4 times that on the two clover-based pastures. Leaching from urine patches was estimated to account for 55% of the total N leached from the clover-based pastures, but only 25% of the total leached from the N-fertilized grass. The amount of NO3-N leached should be related to the cumulative drainage to determine whether the average nitrate concentration exceeds the environmental safety limit of 10 mg NO3-N/1. In 1989: when the total drainage was 215 mm, 21.5 kg N/ha would have had to be leached for the concentration to exceed the limit and none of the pastures did so. In 1990, when the total drainage was 270 mm, the critical amount to be leached was 27 kg N/ha which was exceeded by the Grass + N400, but not by either the Grass-clover (5.8 kg N/ha) or the Herbal ley (7.3 kg N/ha). The utilization of N was more conservative in the clover-based pastures than in the N-fertilized grass.