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 potential 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 1992/1993) 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 fertilization, with a greater response to N in 1991/1992 and to irrigation in the relatively drier 1992/1993 season. Most fertilizer N was taken up from tillering to anthesis; then it either stabilized or slightly declined, while soil N contributed further to plant N uptake. Compared to rain-fed conditions, more soil and fertilizer N was utilized by the irrigated crop, particularly in 1992/1993. In both years, grain N represented 60% of N accumulated at anthesis under rain-fed conditions, compared to 80% under irrigation. Fertilization and irrigation increased water-use efficiency, particularly under drier conditions. Thus, supplemental irrigation, applied at a sensitive growth stage, would be a valuable management practice for improving yield, water-use efficiency and crop N uptake under the dry conditions of a Mediterranean climate. Given the potential of both irrigation and N to increase output in dry areas, the amounts of both inputs are dictated by rainfall in any one year.