Nitrogen and water effects on wheat yield in a Mediterranean-type climate. II. Fertilizer-use efficiency with labelled nitrogen

Under the Mediterranean farming conditions of Syria, rain-fed cropping predominates, but irrigation is increasing where water sources are available. In both rain-fed or irrigated systems, it is important to understand N use by crops and its behavior in the soil. In this paper, we report on nitrogen fertilizer-use efficiency (NFUE) by wheat (Triticum aestivum L.) under 1/3, 2/3 and full irrigation with N-15-labeled fertilizer at different application rates (0, 50, 100 and 150 kg N ha(-1)) for two seasons with varying rainfall, i.e. 323 and 275 mm. NFUE values in the above-ground crop varied with measurement date, reaching a maximum before anthesis, and then, during the grain-filling period, either remaining constant under irrigation or decreasing, particularly under the rain-fed conditions. Irrigation increased the recovery of applied N in grain and straw at harvest from 10% in the wetter year to over 60% in the drier year. Nitrogen at 100 kg ha(-1) level increased recovery by >45% in the wetter year, while fertilizer recovery improved in the drier year only with enhanced water availability from irrigation. The Difference method (28-95%) for estimating N recovery diverged from the N-15 Direct method (21-63%), emphasizing the need to examine both labeled, and unlabeled, N pools for interpretation of N-15 studies. With irrigation, the crop removed significantly more fertilizer N than under rain-fed conditions, with less remaining in the soil; over 40% of the fertilizer N remained in the top 20-cm sail as organic N. Irrigation had no effect on the N-15 recovery at depth, with no significant re-mineralization being detected. While NFUE is increased by higher rainfall and irrigation, fertilizer N losses under the Mediterranean climatic conditions of Syria are low. The apparent inefficiency induced by organic immobilization adds to total soil N, which can potentially be used by future crops. (C) 1998 Elsevier Science B.V. AU rights reserved.