Effect of nitrogen fertiliser placement on grain protein concentration of wheat under different water regimes

Two experiments were conducted in pots 105 cm deep and 11 cm in diameter to determine the effects of subsoil nitrogen (N) on grain yield and grain protein concentration (GPC) of wheat (Triticum aestivum L. cv. Molineux). In both experiments, KNO3 was applied in solution at different times and depths in the profile. In the first experiment, in which a sandy soil low in available N was used, application of 150 mg N at 60 cm, 2 weeks after anthesis, significantly increased grain yield and GPC. The N was taken up gradually by the plant after N was applied. Adding N to the subsoil increased root growth and this resulted in increased water use and water use efficiency. Although there was an increase in the rate of N uptake by the roots, the main factor that influenced the utilisation of subsoil N was the root length density. In the second experiment, the effects of depth and time of N application, and of a reduction in post-anthesis water supply, were determined. A more fertile soil was used than the one in the first experiment. There were 5 KNO3 treatments: nil N; 150 mg N applied to the topsoil at sowing; 75 mg N to the topsoil and 75 mg N to the subsoil (60 cm depth) at sowing; 150 mg N to the subsoil at sowing; 75 mg N to the topsoil at sowing and 75 mg N to the subsoil 1 week after anthesis. The effect of post-anthesis water stress was assessed by allowing the topsoil to dry and then supplying half the amount of water used by the well-watered control treatment at 60 cm in half of the pots. Adding N increased yield and GPC but there was no significant difference in yield and GPC between the different N treatments. When N was applied to the topsoil only, most of it was used by the wheat plants or leached to the subsoil by anthesis; post-anthesis uptake of N depended on the amount of N in the subsoil. Adding N, irrespective of the depth of placement or time of application, increased water use and water use efficiency. In both experiments, increasing the availability of N in the soil after anthesis reduced the amount of N that was remobilised from the roots and stem to the grain. The recovery of applied N in both experiments was high (about 80%). These experiments have shown that N available in the subsoil after anthesis can be used very efficiently and can contribute to both grain yield and GPC. A critical factor in the efficient use of this N appears to be root length density in the subsoil.