15N-labelled fertilizer was applied at different rates
(0, 30, 60, 90 kg N ha−1) and in different forms
(urea or ammonium sulphate) to wheat grown in Syria in three seasons
(1991/92, 1992/93 and 1994/95).
Recovery of 15N-labelled fertilizer in the above-ground crop at
harvest was low (8–22%), with the amount of
15N-labelled fertilizer recovered in the crop increasing as the
rate of application increased.
Fertilizer application caused a significant increase in the amount of unlabelled
soil N in the crop,
suggesting that the application of N fertilizer caused a ‘real’ added
nitrogen interaction. Recovery of
15N-labelled fertilizer in the crop was unaffected by the
form of the fertilizer.
On average 31% (14–54%) of the
15N-labelled fertilizer remained in the soil at harvest, mostly in
the 0–20 cm layer. At the lowest application rate (30 kg N
ha−1) most of the residual fertilizer was as
organic N, but at the higher application rates (60 and 90 kg N
ha−1), a greater proportion of the 15N-labelled
recovered as inorganic N, presumably as the result of top-dressing N in dry
conditions in the spring. The amount of
15N-labelled fertilizer remaining in the soil increased as the
fertilizer rate increased, but was unaffected by the form of fertilizer applied.
Losses of 15N-labelled fertilizer were large (>35%), probably
caused by gaseous losses, either
through volatilization of N from the calcareous soil, or through denitrification
from wet soils rich in organic residues.
N fertilization strategies in the West Asia/North Africa (WANA) region
should take note of the
low recovery of N fertilizer by the crop in the season of application, and
the resultant large quantities of residual fertilizer.