Abstract: Based on 15N isotope tracer and a pot experiment, the dynamics of soil microbial biomass N during different rice growing stages under different fertilizer treatments was investigated, and the microbial mechanism on the enhancement of inorganic fertilizer-N use efficiency was discussed. Treatments included (15NH4)2SO4 (F) and (15NH4)2SO4 plus either chicken manure (FC), pig manure (FP), or vinasse (FV). The results showed that in the early growth stage of rice, the combined use of (15NH4)2SO4 and chicken manure, pig manure, or vinasse increased the microbial immobilization rate of the applied (15NH4)2SO4-15N and reduced soil mineral15N content. However, releasing rates of immobilized15N during high N demand period of rice were 87%、81%,and 81%, corresponding to treatment FC, FP, and FV, respectively. Values of use efficiency of the applied (15NH4)2SO4-N under FC, FP, and FV management were all over 60% except (15NH4)2SO4 alone treatment which was only 39%. Thus, combined applying mineral and organic N fertilizer is a promising practice to enhance N use efficiency in agricultural systems due to the reduction in N loss and synchronizing N assimilation with crop requirement.