NITROGEN AND WATER EFFECTS ON WHEAT YIELD IN A MEDITERRANEAN-TYPE CLIMATE - II - FERTILIZER-USE EFFICIENCY WITH LABELED NITROGEN

Under the Mediterranean farming conditions of Syria, rain-fed cropping predominates, but irrigation is increasing where water sources are av ailable. In both rain-fed or irrigated systems, it is important to und erstand N use by crops and its behavior in the soil. In this paper, we report on nitrogen fertilizer-use efficiency (NFUE) by wheat (Triticu m aestivum L.) under 1/3, 2/3 and full irrigation with N-15-labeled fe rtilizer 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 va lues in the above-ground crop varied with measurement date, reaching a maximum before anthesis, and then, during the grain-filling period, e ither remaining constant under irrigation or decreasing, particularly under the rain-fed conditions. Irrigation increased the recovery of ap plied N in grain and straw at harvest from 10% in the wetter year to o ver 60% in the drier year. Nitrogen at 100 kg ha(-1) level increased r ecovery by >45% in the wetter year, while fertilizer recovery improved in the drier year only with enhanced water availability from irrigati on. 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 studie s. With irrigation, the crop removed significantly more fertilizer N t han under rain-fed conditions, with less remaining in the soil; over 4 0% of the fertilizer N remained in the top 20-cm sail as organic N. Ir rigation had no effect on the N-15 recovery at depth, with no signific ant re-mineralization being detected. While NFUE is increased by highe r rainfall and irrigation, fertilizer N losses under the Mediterranean climatic conditions of Syria are low. The apparent inefficiency induc ed by organic immobilization adds to total soil N, which can potential ly be used by future crops. (C) 1998 Elsevier Science B.V. AU rights r eserved.