FERTILIZER-NITROGEN USE EFFICIENCY OF IRRIGATED WHEAT .1. UPTAKE EFFICIENCY OF PREPLANT VERSUS LATE-SEASON APPLICATION

The addition of N fertilizer late in the growing season to improve grain quality is a potential management option for production of irrigated hard red spring wheat (Triticum aestivum L.). This study compared the recovery of fertilizer N applied at planting with that of fertilizer N applied at anthesis, and evaluated the effects on soil N uptake. In a 2-yr field study, N fertilizer rates of 120, 180, or 240 kg N ha-1 at planting and 0, 30, or 60 kg N ha-1 at anthesis were applied to 'Yecora Rojo' wheat. The experiment utilized a duplicate plot design such that for every N-rate treatment there were two plots, one plot received N-15 labeled fertilizer at planting, and the other received N-15 labeled fertilizer at anthesis. The recovery of N applied at planting ranged from 30 to 55%, while that of N applied at anthesis ranged from 55 to 80%. The contribution of soil N (non-fertilizer N) to total plant N was not affected by the N rate or timing of application. In another field study 120, 180, or 240 kg N ha-1 was applied at planting and 0 or 45 kg N ha-1 at anthesis the first year, and 85, 140, 195, and 250 kg N ha-1 applied at planting and 0, 25, 45, and 65 kg N ha-1 at anthesis the second year. A small dose (< 1 kg ha-1) of NH4-N-15 was injected 5 cm below the soil surface to follow the effect of N application at anthesis on uptake of existing inorganic soil N. Fertilizer N at anthesis increased the amount of injected N-15 taken up by the plants from 38 to 48% the first year, and from 49 to 61% the second year. In both studies, the amount of fertilizer N applied at anthesis had the greatest influence on postanthesis N uptake, which ranged from 17 to 77 kg N ha-1. Without supplemental N applied at anthesis, postanthesis N uptake only provided from 12 to 18% of the total grain N demand, and postanthesis N uptake was not increased by greater preplant N input levels. Likewise, extractable and mineralizable soil N at anthesis were not affected by preplant N treatments. These results indicate that a late N application can be efficiently taken up by plants, and does not decrease soil N uptake. To achieve acceptable grain protein levels for bread wheat in this irrigated cropping system, N should be supplied late in the season to improve N uptake during grain fill.