NITROGEN-FERTILIZER RESIDUES FOR WHEAT CROPPING IN SUBTROPICAL AUSTRALIA

Applied nitrogen (N) recovered by fertilised wheat and by successive w heat crops in a 4-crop sequence (1987-90) was studied by applying N-15 -depleted ammonium nitrate (0, 2.5, and 7.5 g/m(2)) to a Vertisol in t he summer-dominant rainfall region of northern Australia. Recoveries o f applied N by each of the 4 crops in order of cropping sequence were 60.3+/-4.2, 4.4+/-2.3, 1.3+/-0.49, and 0.8+/-0.56%, there being no eff ect of 2 tillage treatments, conventional tillage (CT) and no till (NT ), on uptake of applied N. There was very low recovery of residual fer tiliser N after the first wheat crop was harvested; usually <10% of th e applied N was recovered. There was evidence of a substantial N carry over benefit where fertiliser N (7.5 g/m(2)) was applied in 1987, but not when applied at the same rate in 1988 or 1989. Carryover effect wa s shown only when fertiliser N was applied after a long fallow when an tecedent NO3--N was already high (100-150 v. 30-55 kg/ha with a normal summer fallow). Carryover of subsoil NO3--N from a single N fertilise r application to the crop, as occurred with application in 1987, will provide useful buffer for declining N supplies of soil N in seasons of good crop response. Routine application of N at moderate rates (<75 k g/ha) provides an effective means of supplementing declining soil N re serves for winter cereals in this region of unreliable rainfall.