The immobilization of fertilizer N by soil microbes and its changes in brown soils of different fertilization treatments were studied by a 14-year field location experiment using the ¹⁵N isotope tracer technique. The results of pot experiment showed that the immobilization of fertilizer N by microbes in various treatments reached a maximum after 5 days of fertilization, making up 13.3%-15.4% of the nitrogen applied except single application of nitrogen. In single N treatment the immobilization of fertilizer N by microbes accounted for 5.4% of the nitrogen applied. The increase in microbial biomass N after fertilization originated mainly from fertilizer N. Fertilizer N in the biomass accounted for 64.1%-87.3% of the total microbial biomass N. The biomass N was gradually released during crop growth. At the tillering stage of wheat the immobilization of fertilizer N by microbes decreased markedly, making up only 1.72%-3.29% of nitrogen applied. After the tillering stage the biomass ¹⁵N tended towards stable. ¹⁵N released by biomass accounted for 34.3%-54.3% of ¹⁵N absorbed by crop in different fertilization treatments except the application of alone fertilizer N. There existed a close relationship between the ¹⁵N released by biomass and the ¹⁵N absorbed by wheat(r=0.860).