The rhizosphere is a micro-zone near the plant root surfaces where the microbial activity is much higher than non-rhizospheric soil. In this paper, the Rhizosbox technology was employed to study the effect of application of nitrogen fertilizers (urea and (NH₄)₂SO₄) on the variation of microbial biomass C and N, and the chronic changes of population size of nitrifiers and denitrifiers in soil. The rkey esult indicated that the microbial biomass C and N in rhizosphere were greater than those in non-rhizospheric soil and declined gradually with the experiment lasting. The urea treatment resulted in a higher microbial biomass compared to ammonia sulfate treatment. In general, a similar trend of the change of microbial biomass N was observed: the biomass N began to rise from the 3rd day after the addition of nitrogen fertilizers and got the peak value at the 7th day and then decreased gradually while the biomass N in non-rhizospheric soil did not change significantly. A larger population of nitrifiers and denitrifiers was detected in the rhizosphere than non-rhizosphere. The population of nitrifier achieved the peak value at the 7th and 11th day with urea or (NH₄)₂SO₄ addition respectively and then declined rapidly. The denitrifier propagated chronically with the application of (NH₄)₂SO₄. In the contrast, the largest population of denitrifier appeared at the 11th day. The results manifested that the rice root exhibited an obvious rhizospheric effect on the growth of nitrifer and denitrifiers in soil, which might be regarded as a great potential for resulting in nitrogen loss from soil, especially during fertilization period. Urea showed a hysteresis effect anseemed to be able to supply nitrogen to plant constantly and lastingly comparing to the (NH₄)₂SO₄. It was suggested that the application of urea lead to a higher fertilizer utilization ratio and a smaller risk to the ambient environment.