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 applicat ions and the N use efficiency of wheat, three field experiments with N-15 w ere set up on winter wheat, on three different soils in France. Different c rop 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 re covered from plant and soil at different dates after N-15 addition and at m aturity of wheat was measured. The fate of fertiliser N was rapidly determi ned, most of the fertiliser N accumulated in the wheat at maturity having b een 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 dens ity, soil type and date of application. It was linearly related to the inst antaneous crop growth rate calculated at the day of N-15 application. The a mount 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 lev el of total N-15 measured in the different N pools (soil+plant) reflected t he% N-15 uptake by the plant. There was consequently a negative linear rela tionship between the percentage of N-15 not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. Th ese results suggest that crop N demand at the time of N application determi nes 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.