Nitrogen fertilizer responses of a Heteropogon contortus and a Paspalum plicatulum pasture in relation to rainfall in central coastal Queensland

Abstract

An indigenous pasture in which Heteropogan contortus, and a sown pasture in which Paspalam plicatulum was dominant were fertilized with standard applications of superphosphate and potassium chloride. During five years the pastures were treated with 0,56, 112, 168 and 336 kg N ha-1 per year as urea in three equal dressings. Both pastures responded similarly in dry matter yield, nitrogen recovery and nitrogen concentration. The yield responses were linear and had a mean of 23 kg ha-l year-1 for every kg of applied nitrogen ; the mean recovery in plant tops was 44 per cent for Heteropogon and 50 per cent for Paspalum ; the nitrogen concentration of both pastures was increased from about 1 per cent without N to a maximum of 2 per cent with 336 kg N. The response in yield and the nitrogen recovery were linearly related to summer rainfall. In the indigenous pasture 336 kg N ha-1 reduced the proportion of H. contortus by more than half and plants died at the two highest levels of N. Resulting bare ground was first filled by weeds and later by introduced grasses from adjacent areas. In the Paspalum pasture P. plicatulum was partly replaced by H. contortus at low levels of nitrogen. Soil nitrogen increased, while pH and exchangeable potassium decreased with increasing amounts of nitrogen.