Soil microorganisms represent a significant component of terrestrial biodiversity and play a critical role in myriad ecological processes. The conversion of native forests to tree plantations is a major contributor to biodiversity loss on a global scale; however, our understanding of the influences of forest type conversion on soil microbial diversity and community composition remains unclear. Here we sampled twelve spatially interspersed stands of native mixed broadleaf forests, broadleaf-dominated (50–70% broadleaf trees), bamboo-dominated (20–40% broadleaf trees), and pure bamboo forests in subtropical China, to examine the effects of forest type conversion on soil bacterial and fungal diversity and composition. We found that the tree species richness, stand basal area, and soil carbon to nitrogen ratio decreased, while the total soil phosphorus increased from the native broadleaf forests to pure bamboo forests. Bacterial and fungal operational taxonomic units (OTUs) across the three soil depths increased significantly from 760 and 243 in native broadleaf to 795 and 336 in pure bamboo forests, respectively. However, neither bacterial nor fungal richness differed significantly among forest types at the phylum level. Across forest types, both bacterial and fungal OTU richness increased with the bamboo proportion and decreased with the soil carbon and nitrogen ratio. The fungal OTU richness also increased with the soil phosphorus content; however, neither bacterial nor fungal richness was affected by the tree species richness or stand basal area. Both bacterial and fungal community compositions changed with forest type and soil depth, with their compositional overlap from native broadleaf forests decreasing consistently with the bamboo proportion and soil phosphorus. Overall, our results provide insights into the importance of overstory tree composition and the availability of soil nutrients in affecting soil microbial diversity and composition, while providing management guidance for the inclusion of soil microbial diversity in forest conservation and management.