|abstract:||Improved understanding of nitrogen sources, environmental factors, and nitrification effects on NH3 volatilization is needed for optimal management of nitrogen in crop production systems. In the laboratory, a sponge trapping and KCI-extraction method was modified for measuring NH3 volatilation from different N sources as affected by temperature and soil pH. The kinetics of NH3 volatilization from four N sources surface applied to an Alfisol (a Riviera fine sand, pH 7.9) followed an initial rapid reaction, and then a slow reaction, which was adequately described by a Langmuir kinetic model. The potential maximum NH3 volatilization (qm) under the experimental conditions, as predicted by the Langmiur eqution, decreased in the order: NH4HCO3 (23.2% of applied NH4-N) > (NH4) 2SO4 (21.7%) > CO (NH2)2 (21.4%) > NH4NO3 (17.6%). With an increase in NH4-N application rate, NH3 volatilization increased significantly for (NH4)2SO4, CO (NH2)2, and NH4HCO3 but decreased for NH4NO3. Ammonia volatilization was minimal at the initial soil pH of 3.5 and increased rapidly with increased pH up to 8.5. The potential maximum NH3 volatilization increased by 2- and 30 fold, respectively with an increase in the incubation temperature from 5 to 25 ?C, and from 25 to 45 ?C, respectively. The greatly enhanced NH3 volatilization at 45 ?C, compared with that at 25 ?C, was related to the availability of NH4 for NH3, volatilization over a prolonged period of time.