Ammonia volatilization from urea reduces nitrogen fertilizer efficiency by crops and pollutes the environment. Pot experiment was carried out in two rice successive seasons to compare the effect of the three N fertilizers and two methods of fertilizer addition on decreasing NH_3 volatilization. Plastic pots contain 10 kg soil of Nile Delta clay soil of Egypt belonged to vertisol were fertilized with 220 kg N/ha of regular urea (U), sulfur coated urea (SCU) and urea phosphate (UP). Hydroquinone (HQ) and neutral salts (KCl and CaCl_2) also were added with regular urea by two methods of addition: broadcasting on soil surface and incorporation with top soil, both before flooding. The main results can be summarized as follows: 1. The concentration of ammoniac-N (AN), NH_3 aq. + NH_4, in the floodwater decreased when urea was added with different materials at the first day after fertilization compared to urea lonely and consequently reduced ammonia volatilization either with surface or incorporated addition (from 62 to 49% and 20 to 11%, respectively). 2. SCU and UP treatments decreased the concentration of AN in the floodwater and hence decreased NH_3 volatilization either with surface or incorporated addition compared to regular urea (up to 15 and 10%, respectively). 3. Method of incorporated fertilizer into the soil delayed urea hydrolysis and reduced AN concentration in the floodwater which reduce NH_3 volatilization more than surface application (from 62 to 20% with regular urea treatment). 4. Rice grain yield and its content of N were enhanced in all treatments compared to urea alone either in broadcasting or incorporated addition.
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