The effects of strategic nitrogen fertiliser application during the cool season on perennial ryegrass-white clover pastures in the Western Cape Province 1. Soil nitrogen dynamics
The dynamics of soil inorganic-N in reaction to a single, strategic application of fertiliser N were studied during the period 2000 to 2002 in an Oakleaf soil with a clay content increasing from 10 to 30.8% with depth. Soil inorganic-N was monitored to a soil depth of 500 mm (0-100, 200-300 and 400-500 mm layers) over a seven-week period (7, 21 and 49 days) following the application of 0, 50 and 150 kg N ha-1 in the form of LAN (28) applied as a one-off application to a perennial ryegrass-white clover pasture during five different seasons (autumn, early winter, late winter, early spring or late spring). Inorganic-N content increased from 2000 to 2002 possibly as a result of biological N fixation by the clover. The application of 50 kg N ha-1 did not result in significant differences in inorganic-N content compared to the 0 kg N ha-1 treatments during almost the entire study and can therefore be regarded as a low risk treatment with regard to environmental pollution. The increasing effect of 150 kg N ha-1 on inorganic-N lasted for at least 3 weeks in the 0-100 mm layer and 7 weeks in the 200-300 mm layer, with the highest inorganic-N content measured in autumn and early winter during years of average rainfall (2000 and 2002). Leaching of fertiliser N within one week after application to the 400-500 mm soil layers during 2000 and 2001 emphasised the possible negative effect of application rates exceeding the absorption capacity of the pasture. Nitrogen application rates exceeding the N absorption capacity of the pasture may therefore result in reduced nitrogen response efficiencies (kg additional dry matter per kg N) and inorganic-N contamination of natural resources especially in winter when the inorganic N-uptake capacity of the pasture is low as a result of low pasture productivity.