Nitrogen Recovery Efficiency of Durum Wheat Under Variable Irrigation.

Durum wheat (Triticum durum Desf.) is an important, high-yielding winter crop in Arizona. One hundred percent of this crop is irrigated in South Western USA, mainly by surface irrigation. Nitrogen fertilizer management is crucial in durum wheat, as grain N must be > 22.8 g kg^-1. Few studies have been conducted to assess the optimal N and water application in sprinkler-irrigated durum wheat. We conducted a 2-year field study in Maricopa, Arizona on a Casa Grande sandy loam to determine optimal N and irrigation rates on grain yield, N uptake, and recovery efficiency (RE) of N fertilizer. The cultivar Orita was drill-planted at 134 kg seed ha^-1 in 15-cm rows in early December in 2012 and 2013. Five N rates (0, 84, 168, 252, and 336 kg N ha^-1) as urea ammonium nitrate were applied in three equal splits at Feekes 5, 7, and 10, by a high clearance vehicle fitted with fertilizer nozzles every 30 cm. Ten un-replicated, rates of irrigation from 40 to 120 % reference evapotranspiration (ET) replacement were applied by varying the nozzles in a gradient across one, 55 m-wide span of an overhead sprinkler system. Irrigation plus rain ranged from 230 to 660 mm in season one, and 180 to 600 mm in season two. Grain yields in both years were maximum at the 252 kg N ha^-1 fertilizer rate and at the 120 % ET irrigation in 2013 and at the 100 % ET irrigation in 2014. This maximum grain yield of 7500 kgha^-1 in 2013 was reduced to 5000 kg ha^-1 in 2014 due to a hotter, and therefore shorter growing season.  Recovery efficiency of added N was high in this system, ranging from 30 to 90 %.  At the maximum yielding levels of N and irrigation RE was ~ 70 %.