NITROGEN FERTILITY INFLUENCE ON WATER-STRESS AND YIELD OF WINTER-WHEAT

Available soil water and N fertility are the primary factors limiting dryland winter wheat (Triticum aestivum L.) yields in the Central Great Plains. The objective of this field study was to determine how the level of N fertilization influences water use, water stress, and yield of dryland winter wheat grown in this area. The study was conducted during the 1988 and 1990 growing seasons on a Platner loam (fine, montmorillonitic, mesic Aridic Paleustoll) near Akron, CO. Nitrogen fertilizer was broadcast as NH4NO3 at 0, 28, 56, 84, and 112 kg N ha-1. Canopy temperatures were measured with an infrared thermometer and used to compute the crop water stress index (CWSI). Evapotranspiration was computed and rooting depth inferred from weekly neutron probe readings of soil water content. In general, plant height, above-ground biomass, leaf area index, rooting depth, water use, and grain yield increased with increasing N. Values of CWSI declined after rain and increased as soil water again became limiting. Nitrogen treatment effects on CWSI varied with the severity of water stress. When CWSI < 0.38, increasing N rate decreased water stress because a slight increase in rooting volume resulted. When CWSI > 0.38, increasing N rate increased water stress because the excessive transpirational demand of the resulting larger leaf area and vegetative mass was not fully compensated by the increased rooting volume. Grain yield averaged over the 2 yr increased with increasing N up through the 56 kg ha-1 rate. The effect of N fertility on water use efficiency (WUE) was significantly different between years, with WUE increasing with increasing N up through the 56 and 84 kg ha-1 rates in 1988 and 1990, respectively. Grain yield was linearly correlated with cumulative evapotranspiration. Increasing levels of N fertility can be detrimental to winter wheat yields when water-limiting conditions reduce evapotranspiration rates to less than 62% of potential evapotranspiration.