NITRATE LEACHING FROM A RIVERINE CLAY SOIL UNDER CEREAL ROTATION

Leaching of nitrate from a Red earth (Red Kandasol) at Wagga Wagga was measured during the wheat and lucerne phase of a crop rotation. Wheat was grown in 1993, and the site was sown to lucerne in 1994, a drough t year. The paddock was re-sown to lucerne on 3 June 1995. In 1993, th e soil water content in the 0.25-0.90 m layer had been maintained near its drained limit from previous harvest until the autumn break. With these initial conditions, current season rainfall displaced the reside nt soil water. Drainage measured at 0.25 m was 120 mm and produced 97 mm of drainage at 0.9 m. In contrast, the soil water storage in the 0. 25-0.90 m layer at the autumn break in 1995 was about 70 mm below the drained limit because of the drought in the preceding year. Under thes e conditions, drainage response differed from 1993 in that 112 mm of d rainage at 0.90 m was generated by a much higher drainage of 178 mm at 0.25 m. These differences emphasise a need for appreciable soil water deficits in the subsoil in order to minimise drainage from the root-z one. Infiltration of rainfall, water redistribution, drainage, and tra nsfer of NO3--N within the soil profile were modelled using the Richar d's equation. The model gave acceptable predictions of evapotranspirat ion and its partitioning for transpiration, soil N transformations, an d NO3--N redistribution and leaching under wheat and lucerne. Nitrate leached below 0.25 m was 31 and 77 kg N/ha in 1993 and 1995, but only 4.2 and 12 kg N/ha leached beyond the root-zone, respectively. Root gr owth into the subsoil and uptake was effective in reducing the downwar d movement of NO3--N. Leaching of NO3--N below 0.25 m produced 2.2 kmo l H+/ha in 1993 and 5.5 kmol H+/ha in 1995. Corresponding values for n et acidity production in 0-0.9 m layer were 0.3 and 0.9 kmol H+/ha. It would appear from both measurements and simulations that management s ystems, either current or projected, cannot prevent the movement of NO 3--N from its zone of production. Acidification of upper soil layers i s the inevitable consequence of such export, leading to the need for a dded alkalinity whenever the biomass is not returned to the soil.