Uptake of nitrate (NO3-) by wheat (cv. Condor) grown on a heavy clay soil in the Goulburn-Murray Irrigation Region of south-eastern Australia was investigated. 15N-labelled NO3-was applied at 2 gNm -2 on 7 September, 5 October and 5 November 1987 to microplots in a larger field experiment involving two irrigation treatments. The irrigation treatments were either complete irrigation until maturity or partial irrigation. Plant tops were harvested from three microplots selected at random from each treatment on 4 occasions; 7, 14, 21 and 28 days after N application. Irrigation had no effect on the rate of uptake of lSN. Recovery of lSN-labelled NO 3 -in the plant tops increased from 50% for the September ~5N addition to 70% for the October and November additions. The low recovery of ~SN in September occurred when soil NO3-values were relatively high. Daily uptake of ~SN-labelled NO3-ranged from 0.08 to 0.15 g N m-2 between September and November, whereas total N accumulation data indicated a mean N uptake of 0.17, 0.18 and 0.20gNm-2d -1 during September, October and November, respectively. Knowledge of the daily rate of uptake of nitrogen (N) applied to irrigated wheat is necessary in order to match the supply of fertilizer N with the demand for N by the crop. By matching the availability of NO~ to crop demand, the recovery of N in the crop has been increased and losses of NO~-by leaching or denitrification has been reduced (Myers 1987; McGill and Myers 1987).
Uptake of fertilizer N depends on the balance between crop demand for N and the amount of N available from the soil (van Keulen and Seligman 1987). Nitrogen is required for the synthesis of protein, so the demand for N increases as the crop grows. The availability of N to the crop is a function of the amount present in the soil and the size and extent of the root system of the crop.