Abstract: Different stand ages and rotations polar plantation sites were selected for investigating the dynamics and the distribution characteristics of soil microbial biomass nitrogen(SMBN)from February 2009 to December 2009. The results showed that SMBN annual change dynamics of all experimental sites was similar. The content of SMBN at February were the lowest and the average contents of all sites at different soil profiles were 48.36 mg/kg(0-10 cm),38.74 mg/kg(10-20 cm)and 22.64 mg/kg(20-40 cm), while the content of SMBN at October were the highest and the average contents of all sites at different soil profiles were 23.32 mg/kg(0-10 cm), 17.91 mg/kg(10-20 cm)and 8.18 mg/kg(20-40 cm), respectively. With stand age and rotation of poplar plantation increase, SMBN content decreased. The results also displayed that SMBN content decreased with soil profile depth increase, SMBN average content at 0-10 cm was 11.50mg/kg and 24.83 mg/kg higher than that at 10-20 cm and 20-40 cm for CK, 13.40 mg/kg and 25.30 mg/kg for F1, 9.34 mg/kg and 19.53 mg/kg for F2, 7.17 mg/kg and 19.00 mg/kg for S1, 5.65 mg/kg and 14.48 mg/kg for S2, respectively. The correlation analysis indicated that there existed significant correlations between SMBN and soil total nitrogen and organic matter(correlation coefficient were respectively 0.665 4 and 0.829 6).