The Sanjiang Plain is the largest freshwater wetlands in Northeast China. In order to feed the growing population, about 84 % of the wetlands in this area have been converted to farmland, especially to paddy fields, since the 1950s. However, little is known about the influence of this conversion on soil microbial community composition. In this study, soil samples were collected from two natural wetlands dominated by plant species Carex lasiocarpa and Deyeuxia angustifolia and from a neighboring paddy field that was changed from wetland more than 10 years ago. The composition and diversity of bacterial communities in the soils were estimated by clone library analysis of nearly full length of 16S rDNA sequences. The results revealed that bacterial diversity was higher in paddy fields, and that the composition of bacterial communities differed among the three samples; the difference was more notable between the paddy field and two natural wetlands than between two natural wetlands. The distribution of clones into different bacterial phyla differed among soil samples, and the conversion from natural wetlands to paddy field increased the abundance of Proteobacteria and Firmicutes but decreased the abundance of Chloroflexi. About 63 % and 71 % of clones from two natural wetlands and 49 % of clones from the paddy field had < 93 % similarity with known bacteria, suggesting that the majority of bacteria in natural wetland soils in the Sanjiang Plain are phylogenetically novel. In general, this study demonstrated that long-term conversion from natural wetlands to paddy field changes soil bacterial communities in the Sanjiang Plain.

　　The Sanjiang Plain is the largest freshwater wetlands in Northeast China. In order to feed the growing population, about 84 % of the wetlands in this area have been converted to farmland, especially to paddy fields, since the 1950s. However, little is known about the influence of this conversion on soil microbial community composition. In this study, soil samples were collected from two natural wetlands dominated by plant species Carex lasiocarpa and Deyeuxia angustifolia and from a neighboring paddy field that was changed from wetland more than 10 years ago. The composition and diversity of bacterial communities in the soils were estimated by clone library analysis of nearly full length of 16S rDNA sequences. The results revealed that bacterial diversity was higher in paddy fields, and that the composition of bacterial communities differed among the three samples; the difference was more notable between the paddy field and two natural wetlands than between two natural wetlands. The distribution of clones into different bacterial phyla differed among soil samples, and the conversion from natural wetlands to paddy field increased the abundance of Proteobacteria and Firmicutes but decreased the abundance of Chloroflexi. About 63 % and 71 % of clones from two natural wetlands and 49 % of clones from the paddy field had < 93 % similarity with known bacteria, suggesting that the majority of bacteria in natural wetland soils in the Sanjiang Plain are phylogenetically novel. In general, this study demonstrated that long-term conversion from natural wetlands to paddy field changes soil bacterial communities in the Sanjiang Plain.