Field experiments with durum wheat were conducted at two locations with different initial soil nitrate contents during two growing seasons, 1994-1995 and 1995-1996, on irrigated Typic Xerofluvent soils of the Ebro Valley (Spain). A total of seven nitrogen treatments with different rates and timing were investigated. In two treatments, 50 kg ha(-1) of nitrogen was applied as a urea foliar spray at the flag leaf stage. The average grain yields ranged from 2422 to 5730 kg ha(-1), depending on the year and location. Grain protein ranged from 14.1 to 16.4%, and other quality parameters such as gluten strength, vitreousness and carotenoids also varied with year and location. The overall results do not suggest that it can be generally recommended to apply N in these Mediterranean conditions. However, in soils with low initial soil NO(3)(-)-N contents, better grain yields and good protein contents, gluten strength, vitreousness and carotenoids could be achieved by applying N fertilizer at about 100 kg N ha(-1). This rate should be raised to 200 kg N ha(-1) when maximum durum wheat quality is needed, although this increases the risks of nitrate leaching. Foliar application of urea had no effects on grain yield and almost negligible effects on grain quality. Soil nitrate residues after harvest increased with increasing nitrogen rates at both locations. This research shows that the quality of the durum wheat produced in the irrigated Mediterranean areas of southern Europe is generally high, even at low rates of nitrogen application. They also suggest that even using high quality durum wheat varieties, the quality can still be increased by nitrogen fertilization, even at low nitrogen rates. The results suggest that it is worthwhile checking the initial nitrate contents of the soil in these areas of low winter rainfall. (C) 2003 Elsevier B.V. All rights reserved.
