Decreases over seven years in subsoil nitrate in a vertisol with grain sorghum and grass

Subsoil chemical properties and root distributions were measured over seven years under annual grain sorghum (Sorghum bicolor) cropping with crop residue retention and zero tillage and under perennial Rhodes grass (Chloris gayana). The site, in the semi-arid subtropics of eastern central Queensland, was a Vertisol with alkaline topsoil and acid, strongly sodic subsoil. Nitrate had accumulated between 0.6 and 2.8 m during cropping for the preceding 16 years. The hypothesis that some of this residual nitrate would be depleted by the grass but not by the sorghum was tested during the experiment. Of the initial nitrate (153 kg N ha⁻¹ between 0.6 and 1.6 m in June 1978), 106 and 23 kg N ha⁻¹ remained under sorghum and grass respectively in June 1981, 67 and 7 kg N ha⁻¹ in December 1983 and 24 and 7 kg N ha⁻¹ in June 1985. Between 1.6 and 2.8 m in June 1981, 132 and 73 kg N ha⁻¹ were found under sorghum and grass respectively and there were minor reductions between 1.6 and 1.8 m subsequently. Loss of nitrogen from the soil profile under sorghum was almost accounted for by grain nitrogen. Implications for crop rotations of nitrate accumulation in the subsoil and slow removal by sorghum are discussed.