Loss of nitrogen from urea applied to rainfed wheat in varying rainfall zones in northern Syria

Abstract

Fertilizer-applied Nitrogen (N) may be lost from the soil by various mechanisms, i.e., runoff, leaching, denitrification, and volatilization. The latter process is of primary concern in calcareous soils of arid and semi-arid regions, especially when urea is used. In this field study from northern Syria, urea alone, urea with either an incorporated urease inhibitor, phenylphosphorodiamidate, or an experimental bran-wax coating were evaluated on wheat for two cropping seasons at two experimental stations with varying average seasonal rainfall (340 mm, 270 mm). Loss of N was assessed with 15N by mass balance, i.e., the amount of N applied minus the crop N uptake and N remaining in the soil. Crop yields and N uptake were related to seasonal rainfall. Losses of N, apparently as volatilized NH3, were relatively low at both sites, i.e., 11–18%. However, compared to the unmodified urea, neither the incorporated urease inhibitor nor the bran-wax coating had any effect on yields, N uptake or N loss. While urea hydrolysis is normally rapid, it may be delayed by dry conditions at the soil surface; similarly, unusually cold periods may delay nitrification following hydrolysis. Thus, under the cool-season conditions of rainfed cropping in the Middle East, efficient use of urea is not likely to be achieved by modification of the urea but by conventional management practices that ensure pre-plant soil incorporation or topdressing during early spring rains.