Interactive effects of nitrogen fertilization and irrigation on grain yield, canopy temperature, and nitrogen use efficiency in overhead sprinkler-irrigated durum wheat

Nitrogen and irrigation management are crucial in the production of high protein irrigated durum wheat (Triticum durum Desf.) in arid regions. However, as the availability of irrigation water decreases and potential costs and regulation of nitrogen (N) increase, there is a need to better understand how irrigation levels interacts with N fertilizer rates. A two-year field experiment was conducted in Maricopa, Arizona USA on a Casa Grande sandy loam to assess effects of N fertilizer and irrigation rates on grain yield, grain N, canopy temperatures yellow berry, and N use efficiency. Five rates of N fertilizer as urea ammonium nitrate (0, 84, 168, 252, and 336 kg N ha−1) were applied in three equal splits at Zadoks stages 30, 32, and 39. Ten un-randomized, sequential rates of irrigation ranging from 0.35 to 1.14 fraction of a non-deficit base irrigation treatment (maintained >45% soil water depletion) were applied by sequentially varying the nozzles in a gradient in an overhead sprinkler system. Irrigation plus rain ranged from 230 to 660 mm in the first season, and 180 to 600 mm in the second season. Grain yield was maximum in 2013 at the 252 kg N ha−1 fertilizer rate and at the 10th water level (1.14 irrigation), and between 168 kg and 252 kg N ha−1 at the 8th water level (1.0 irrigation) in 2014. The maximum grain yield of 7500 kg ha−1 in 2013 was reduced to 5000 kg ha−1 in 2014 due to a warmer, shorter growing season. Economic optimum N rate was at water level 8 both years (196 and 138 kg N ha−1 in 2013, and 2014, respectively). Recovery efficiency of added N was high in this system (i.e., >70%) at N fertilizer and water levels that maximized biomass and grain yields. Grain N was maximum at a lower water level (level 3 or 0.50-0.54 irrigation), was positively affected by N fertilizer rate, and was negatively related to yellow berry incidence. Canopy temperature minus air temperature values decreased linearly with increasing irrigation level. Nitrogen fertilizer applications reduced canopy temperature when water levels >0.54 and 0.69 irrigation fraction in 2013, and 2014, respectively. The study results suggested that canopy temperature and weather data that reflects the grain-filling period could be used to improve irrigation and N management, respectively. In short, irrigated durum wheat growers on this soil would achieve the economically optimum grain yield, with the least risk of yield or protein reduction, by applying 200 kg N ha−1 at the base irrigation level which maintains root zone soil moisture depletion below 45%.