N-APPLICATION ENHANCES REMOBILIZATION AND REDUCES LOSSES OF PREANTHESIS-N IN WHEAT GROWN ON A DUPLEX SOIL

The effect of fertilizer N on the remobilization of N to the grain in wheat was evaluated on a duplex soil at East Beverley, Western Australia. Remobilization of N to the grain was determined using the stable isotope, N-15, which was fed to wheat plants during the vegetative growth phase. Nitrogen was applied at 15, 30 and 60 kg N ha(-1) with half being applied at seeding and the remainder at 35 days after sowing (DAS), before the onset of tillering. The high N treatment increased N uptake between stem elongation and anthesis and grain protein concentration by 2.9% relative to the low N treatment. Both the nitrogen harvest index (NHI) and the nitrogen utilization efficiency (NUE) declined as the rate of N application was increased; the decline was more pronounced when the rate of N application was increased from 30 to 60 kg N ha(-1). Total grain N at 60 kg N ha(-1) was increased by 54%, relative to that at 15 kg N ha(-1), due to a 2.3 fold increase in remobilization of pre-anthesis accumulated N and despite a 5 fold reduction in post-anthesis uptake. The application of 60 kg N ha(-1) reduced the losses of pre-anthesis N from 19% to 6%. The absolute contribution of the N taken up early in the growth of the crop (before the second node stage) to grain N was 0.9, 1.4 and 3.2 g m(-2) at 15, 30 and 60 kg N ha(-1) respectively. The contribution of pre-anthesis N to the grain was 2.2, 3.7 and 5.1 g m(-2) when 15, 30 and 60 kg N ha(-1) were applied respectively. This contribution accounted for 60, 82 and 95% of the total grain N respectively. Results indicated that with greater amounts of N accumulated before anthesis, there was higher remobilization to the grain and less risk of net N losses at maturity, because post-anthesis water deficits developed more rapidly.