Fate of urea nitrogen applied to irrigated wheat at heading

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Abstract
This paper describes a field experiment designed to determine the fate of urea applied to a semi-dwarf wheat (cv. Matong) at heading. The urea (50 kg N ha-1) was broadcast onto the soil surface and washed into the soil with 22 mm of water applied by sprinkler irrigation. Ammonia (NH3) volatilization was determined by a micro-meteorological method and total nitrogen (N) loss was measured by the 15N balance technique. The effect of the fertilizer application on grain yield and protein content was also assessed.With this method of application the pH of the surface soil remained below 7.2 for the period of the study. The low pH, together with the dense wheat canopy, ensured that no NH3 was volatilized after urea addition.The 15N balance study showed that no N was lost from the plant-soil system between the time of the urea application (127 days after sowing; DAS 127) and the first flood irrigation 5 days later (DAS 132). However, between the first (DAS 132) and second flood irrigations (DAS 141) after fertilization, 12% of the applied 15N was lost, presumably by denitrification. Recoveries of labelled N in the soil plus plants were not significantly different between DAS 141 and harvest (DAS 193); the mean recovery being 84% of the applied N. Neither biomass nor grain yield was significantly increased by the application of urea, but the N concentration of the grain was increased from 2.03 to 2.46%.Applying fertilizer N at heading increased grain protein percentage of this semi-dwarf cultivar, thus allowing the option of using management to improve the marketability of the grain.