Relationship between rate of crop growth at date of fertiliser N application and fate of fertiliser N applied to winter wheat

To investigate the relationship between the timing of fertiliser N applications and the N use efficiency of wheat, three field experiments with N-15 were set up on winter wheat, on three different soils in France. Different crop N demands on the day of fertiliser application were obtained by varying either crop densities or date of fertiliser application. Labelled (NH4NO3)-N-15-N-15 was applied at tillering and during stem elongation. The N-15 recovered from plant and soil at different dates after N-15 addition and at maturity of wheat was measured. The fate of fertiliser N was rapidly determined, most of the fertiliser N accumulated in the wheat at maturity having been taken up within a few days of application. N-15 recovery by the crop at final harvest (%) varied greatly (19-55% N applied) according to crop density, soil type and date of application. It was linearly related to the instantaneous crop growth rate calculated at the day of N-15 application. The amount of fertiliser N immobilised in the soil was constant at 20 kg N ha(-1), for all soil types and crop densities. Because residual mineral N-15 in the soil at harvest was negligible and immobilisation was constant, the level of total N-15 measured in the different N pools (soil+plant) reflected the% N-15 uptake by the plant. There was consequently a negative linear relationship between the percentage of N-15 not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. These results suggest that crop N demand at the time of N application determines the ability of the crop to compete for N with other processes, and may be a major factor determining the division of N between soil and crop.