Nitrogen balance in crop systems in tropical Australia

Abstract

The nitrogen balance of native and applied nitrogen is discussed for three regions in northern Australia. The problems of estimating the nitrogen addition of legumes to the soil-plant system at Katherine (14°28'S) are discussed in terms of changes in nitrate concentration in the subsoil, and the contribution of nitrogen in rainwater to the system. Legumes like Townsville stylo and guar can contribute up to 90 kg/ha of nitrogen per season. Only under grazing conditions is the soil organic nitrogen increased with Townsville stylo. On the sub-coastal plains near Darwin (12°33'S) labelled fertilizer nitrogen losses under flooded rice conditions vary from 37 to 94 per cent depending on fertilizer type and mode of application. Considerable amounts of fertilizer nitrogen were found in the flood-water up to 2 weeks after application. Surface broadcasting increased immobilization of fertilizer as compared with deep placement. At the Ord River valley (15°39'S) with cotton crops, high apparent recoveries of urea and other nitrogen fertilizers are obtained (up to 64 per cent) with high nitrogen applications (225 kg/ha) by banding the fertilizer at sowing at 1 m spacing at 15 cm depth.