NITROGEN AND WATER EFFECTS ON WHEAT YIELD IN A MEDITERRANEAN-TYPE CLIMATE - I - GROWTH, WATER-USE AND NITROGEN ACCUMULATION

Available soil water is the principal factor that limits the yield pot ential of wheat (Triticum aestivum L.) and its response to nitrogen (N ) under Mediterranean climatic conditions. We examined wheat yield, N fertilizer uptake, and water use for two seasons (1991/1992 and 1992/1 993) with total seasonal rainfall of 323 and 275 mm, respectively. In addition to natural rainfall, supplemental irrigation at various rates (1/3, 2/3 and full irrigation) was also considered in assessing these parameters and their interactive effects. Grain yield and dry matter accumulation and N uptake varied between years, mainly due to rainfall and sowing date. Yields were increased by irrigation and N fertilizat ion, with a greater response to N in 1991/1992 and to irrigation in th e relatively drier 1992/1993 season. Most fertilizer N was taken up fr om tillering to anthesis; then it either stabilized or slightly declin ed, while soil N contributed further to plant N uptake. Compared to ra in-fed conditions, more soil and fertilizer N was utilized by the irri gated crop, particularly in 1992/1993. In both years, grain N represen ted 60% of N accumulated at anthesis under rain-fed conditions, compar ed to 80% under irrigation. Fertilization and irrigation increased wat er-use efficiency, particularly under drier conditions. Thus, suppleme ntal irrigation, applied at a sensitive growth stage, would be a valua ble management practice for improving yield, water-use efficiency and crop N uptake under the dry conditions of a Mediterranean climate. Giv en the potential of both irrigation and N to increase output in dry ar eas, the amounts of both inputs are dictated by rainfall in any one ye ar. (C) 1998 Elsevier Science B.V. All rights reserved.