Nitrate leaching from a Riverine clay soil under cereal rotation

Abstract

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 drought year. The paddock was re-sown to lucerne on 3 June 1995. In 1993, the 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 resident 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 these conditions, drainage response differed from 1993 in that 112 mm of drainage 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-zone.

Infiltration of rainfall, water redistribution, drainage, and transfer of NO^-₃-N within the soil profile were modelled using the Richard"s equation. The model gave acceptable predictions of evapotranspiration and its partitioning for transpiration, soil N transformations, and NO^-₃-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 growth into the subsoil and uptake was effective in reducing the downward movement of NO^-₃-N. Leaching of NO^-₃-N below 0· 25 m produced 2·2 kmol H⁺ /ha in 1993 and 5·5 kmol H⁺ /ha in 1995. Corresponding values for net 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 systems, either current or projected, cannot prevent the movement of NO^-₃-N from its zone of production.

Acidification of upper soil layers is the inevitable consequence of such export, leading to the need for added alkalinity whenever the biomass is not returned to the soil.