Abstract:Pot experiments were carried out on the 15N-lablled soil after winter wheat to study the changes of soil microbial biomass C,N and soil fixed ammonium during rice growth. The results showed that soil microbial biomass C increased with the rice growing and reached the highest at the harvesting stage. The soil microbial biomass N decreased a little bit at the beginning of experiment and then rose to the highest at the tillering stage, after which it decreased again with the increase of N absorption by rice. The content of soil fixed ammonium changed little during the rice growing.some of the 15N fixed by soil clay minerals exchanged with the soil mineral N, showing that the fixed ammonium in last crop was also effective to rice at the second cropping.