Effect of timing and nitrogen fertilizer application on winter oilseed rape (Brassica napus L.).: II.: Nitrogen uptake dynamics and fertilizer efficiency

Field experiments were carried out on grey-brown podzolic soil in the four consecutive growing seasons (1998-2001) at Krzeslice Farm, central-western Poland. The effect of seven N fertilization treatments (in kg N ha(-1)): 80(NF) + 80(CAN); 80(NF) + 50(CAN) + 30(CN); 80(CAN) + 80(CAN); 80(CAN) + 80(CAN) + 30(CN); 80(AN) + 80(AN); 80(AN) + 50(AN) + 30(CN), where, NF - nitrofos NPK, CAN - calcium-ammonium nitrate, AN - ammonium nitrate, CN - calcium nitrate and control (without N) on N uptake dynamics and N efficiency was studied. Mineral fertilizers were applied at the start of spring regrowth, beginning of stem elongation and at the flower-bud-visibility stage. The study revealed two distinct strategies of oilseed rape plants' adaptation to timing and N fertilizer application sequences. Both strategies based on nitrogen uptake rate (NUR), were analysed at different plant growth stages. Ammonium nitrate (AN) applied in the two-split system gave the highest NUR (387 mg m(-2) day(-1)) during stem elongation (for comparison, a value of 166 mg m(-2) day(-1) was obtained in the control). In the case of calcium-ammonium nitrate (CAN), a moderate level of NUR was obtained (304 mg m(-2) day(-1)) but N uptake lasted 12 days longer compared with the AN treatment. Hence, N accumulation in leaves at the end of flowering explained about 81 % of yield variability. The second adaptation strategy was attributed to the three-split N treatment. Plants fertilized with AN and CAN fertilizers showed an inconsistent pattern of NUR with time. Nitrogen accumulation in stems at the beginning of maturity, explained 69 % of yield variability. Nitrogen-use efficiency did not show any response to N treatments.