EFFECT OF FERTILIZER RATE AND FORM ON THE RECOVERY OF N-15-LABELED FERTILIZER APPLIED TO WHEAT IN SYRIA

N-15-labelled fertilizer was applied at different rates (0, 30, 60, 90 kg N ha(-1)) and in different forms (urea or ammonium sulphate) to wh eat grown in Syria in three seasons (1991/92, 1992/93 and 1994/95). Re covery of N-15-labelled fertilizer in the above-ground crop at harvest was low (8-22 %), with the amount of N-15-labelled fertilizer recover ed in the crop increasing as the rate of application increased. Fertil izer application caused a significant increase in the amount of unlabe lled soil N in the crop, suggesting that the application of N fertiliz er caused a 'real' added nitrogen interaction. Recovery of N-15-labell ed fertilizer in the crop was unaffected by the form of the fertilizer . On average 31 % (14-54 %) of the N-15-labelled fertilizer remained i n the soil at harvest, mostly in the 0-20 cm layer. At the lowest appl ication rate (30 kg N ha(-1)) most of the residual fertilizer was as o rganic N, but at the higher application rates (60 and 90 kg N ha(-1)), a greater proportion of the N-15-labelled fertilizer was recovered as inorganic N, presumably as the result of top-dressing N in dry condit ions in the spring. The amount of N-15-labelled fertilizer remaining i n the soil increased as the fertilizer rate increased, but was unaffec ted by the form of fertilizer applied. Losses of N-15-labelled fertili zer were large (> 35%), probably caused by gaseous losses, either thro ugh volatilization of N from the calcareous soil, or through denitrifi cation from wet soils rich in organic residues. N fertilization strate gies in the West Asia/North Africa (WANA) region should take note of t he low recovery of N fertilizer by the crop in the season of applicati on, and the resultant large quantities of residual fertilizer.