NITROGEN DYNAMICS OF OATS, SORGHUM, BLACK GRAM, GREEN PANIC AND LUCERNE ON A CLAY SOIL IN SOUTH-EASTERN QUEENSLAND

Changes in the distribution of nitrate-nitrogen (N) in a clay soil (Pellustert) under oats (Avena sativa cv. Minhafer), sorghum (Sorghum bicolor cv. E57), black gram (Vigna mungo cv. Regur), green panic (Panicum maximum cv. Petrie), and lucerne (Medicago sativa cv. Hunter River), and the uptake of N into plant shoots, were measured at Narayen on the brigalow (Acacia harpophylla) lands of south-eastern Queensland over each cropping season in 1975-85. Nitrate-N accumulated in the subsoil (30-150 cm) under sorghum and black gram, but not under oats. Green panic depleted nitrate-N after 2 years, and lucerne after 1 year. Losses of nitrate-N during 2 wet years reached 300 kg/ha under sorghum and black gram, and 57 kg/ha under oats, but were negligible under green panic and lucerne. Leaching to below 150 cm in the soil was the probable cause. The supply of soil N to oats, sorghum, and black gram was adequate during the 10 years, but the N yield of green panic decreased from 239 kg/ha to less-than-or-equal-to 50 kg/ha after 5 years. Accumulation of nitrate-N under sorghum and black gram could be utilised by rotating these crops with green panic or lucerne. This would also improve the productivity of green panic pastures. Rotating the summer crops with oats (winter crop) or with deep-rooted crops (e.g. sunflowers) should also be tested. Alternatively, reduction of production of nitrate-N in the soil could be attempted. Zero or reduced tillage could do this, but it may also increase leaching by increasing the entry and movement of water in the soil.