Abstract: As an important mode of nitrogen (N) loss, ammonia volatilization in farmlands causes air pollution and water eutrophication. Increased N application dose should be accompanied with reduced ammonia volatilization to improve N fertilizer efficiency. This could lay a strong theoretical basis for optimal fertilizer use. To that end, a double-layer-sponge-trapping method was used to measure ammonia volatilization following the application of different N fertilizers in the growing season of wheat-maize double cropping system and in an incubation experiment with different soil moistures or amended with wheat or corn straw. The experiments were conducted at the Luancheng Agro-Ecosystem Experimental Station (LAES) of Chinese Academy of Sciences from October 2002 through June 2004. Urea N fertilizer was applied at the rates of 200, 400 and 600 kg(N)·hm^-2·a^-1, respectively, and with no N fertilizer as the control. The study showed that ammonia volatilization was significantly affected by the fertilization dose and time, soil moisture and irrigation mode. For winter wheat, ammonia volatilization occurred immediately after N fertilizer application．It gradually increased, apparently reaching the maximum in the first 1~5 days after fertilization. Ammonia volatilization was high during summer maize growing season, increased sharply after (N fertilizer) dressing, and hit the highest rate on the 1st day after fertilization. N loss via accumulated ammonia volatilization was 0.66~35.00 kg(N)·hm^-2·d^-1, accounting for 0.09%~14.90% of the applied fertilizer. Ammonia volatilization largely occurred in summer maize season, accounting for 80% of N loss in the double cropping system. The high ammonia volatilization in the summer maize season was due mainly to the high temperature and overland broadcast mode of fertilization. Irrigation reduced ammonia volatilization. The earlier the irrigation after fertilization was conducted, the less was ammonia volatilization. Ammonia volatilization also increased with lower initial water content. Amendment with wheat or maize straw increased the rate of urea hydrolysis in loam soils, triggering peak NH₃ loss to occur one day earlier and much lower than non-amended urea. With urea alone, cumulative NH₃ loss was 7.2%~9.7% of applied urea. When urea was amended with wheat or maize straw, cumulative NH₃ loss was only 1.1%~2.1% or 2.2%~7.2% of applied urea, respectively. Thus the characteristics of ammonia loss were dependent on the modes of application of urea and soil moisture conditions.