In order to provide a theoretical basis for improving crop nitrogen(N)use efficiency，the effect of soil microorganisms on the behaviors of ammonium and nitrate in soil was studied through the method of soil irradiationsterilization. Two factors were involved in this study，including different soil treatments(sterilization and non -sterilization)and different N forms( 15N labeled ammonium and 15N labeled nitrate). After cultivated in a controlled environment for30days，the contents of total ammonium and nitrate in soil along with the contents of ammonium and nitrate derived fromfertilizers and soil were determined in sterilized and unsterilized soil. The N residue，fixation and loss of ammonium and nitrate fertilizers were also evaluated. The results showed that soil sterilization inhibited the transformation of ammonium tonitrate，stimulated the release of soilammonium，and had no significant effects on the soilresidual，soil fixation and loss ofammonium fertilizer. Soil sterilization had no effect on the conversion of nitrate intoammonium，but reduced the soil fixation of nitrate fertilizer and caused an increase loss of nitrate. The addition of ammonium fertilizer promoted the mineralization of soil organic N compared with nitrate fertilizer. The ammonium fertilizer had higher fixation and higher loss than nitrate.Overall，Soil sterilization was beneficial to the accumulation of ammonium in the soil，but reduced the accumulation of nitrate. Although the application of ammonium could stimulate the release of soil mineral N and more ammonium N was fixedinsoil，the loss of ammonium fertilizer was higher compared with nitrate fertilizer.