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To date,research on the adverse effects of continuous cropping has focused on field crops;forest plantations,despite their very significant economic and ecological value,have received less attention.The evolution of microbial community in the rhizosphere of forest plantations,in particular,has rarely been examined.In this study,changes in the size,composition,and structure of bacterial and fungal communities in the rhizosphere of different generations of poplar (Populus deltoides) plantations were studied using real-time polymerase chain reaction (PCR) and PCR-denaturing gradient gel electrophoresis and compared with the composition of the microbial community in a bare land (control) adjacent to the plantation sites through sequencing analysis and by constructing phylogenetic trees.The numbers of bacteria and fungi increased significantly with successive poplar generations,although the increases in the two groups were not parallel.When compared with the control,the bacterial community increased greatly in the second generation,and the most significant increase occurred in the third generation.In contrast,the most significant increase in the fungal community occurred in the first generation,and the increase in the third generation was insignificant.In terms of community composition,the first generation showed little change in either community;however,the second generation showed remarkable changes in the bacterial community,and the third in the fungal community.The numbers of Gammaproteobacteria,Alphaproteobacteria,and Actinobacteria increased by 10.92％,7.38％,and 5.46％,respectively,whereas those of Acidobacteria decreased by 18.38％ in the second generation.These changes in the number and composition of microbial communities in the rhizosphere could be one of the reasons for the decline in yield and quality associated with long-term monoculture.