Release, fixation, and distribution of ammonium (NH4+) as a source of nitrogen can play an important role in soil fertility and plant nutrition. In this study, ten surface soils, after addition of 1,000 mg NH4+ kg-1, were incubated for 1 week at the field capacity moisture and 25 ± 2 _C temperature, and then NH4+ release kinetic was investigated by sequential extractions with 10 mMCaCl2. Furthermore, NH4+ distribution among three fractions, including water-soluble, exchangeable, and non-exchangeable, was determined in all soil samples. NH4+ release was initially rapid followed by a slower reaction, and this was described well with the Elovich equation as an empirical model. The cumulative NH4+concentrationn released in spiked soil samples had a positive significant correlation with sand content and negative ones with pH, exchangeable Ca2+m and K+, cation exchange capacity (CEC), equivalent calcium carbonate (ECC), and clay content. The cation exchange model in the PHREEQC program was successful in mechanistic simulation of the release trend of native and added NH4+ in all control and spiked soil samples. The results of fractionation experiments showed that the non-exchangeable fraction in control and spiked soil samples was greater than that in water-soluble and exchangeable fractions. Soil properties, such as pH, exchangeable Ca2+ and K+, CEC, ECC, and contents of sand and clay, had significant influences on the distribution of NH4+ among three measured fractions. This study indicated that both native and recently fixed NH4+, added to soil through the application of fertilizers,were readily available for plant roots during 1 week after exposure.
