Rising levels of atmospheric $CO_2$ may stimulate forest productivity in the future, resulting in increased carbon storage in terrestrial ecosystems. However, heavy metal contamination may interfere with this, though the response is not yet known. In this study, we investigated the effect of elevated $CO_2$ and Pb contamination on microorganisms and decomposition in pine tree forest soil. Three-year old pine trees (Pinus densiflora) were planted in Pb contaminated soils (500 mg/kg-soil) and uncontaminated soils and cultivated for three months in a growth chamber where the $CO_2$ concentration was controlled at 380 or 760 mg/kg. Structures of the microbial community were comparatively analyzed in bulk and in rhizosphere soil samples using community-level physiological profiling (CLPP) and 16S rRNA gene PCR-DGGE (denaturing gradient gel electrophoresis). Additionally, microbial activity in rhizospheric soil, growth and the C/N ratio of the pine trees were measured. Elevated $CO_2$ significantly increased microbial activities and diversity in Pb contaminated soils due to the increase in carbon sources, and this increase was more distinctive in rhizospheric soil than in bulk soils. In addition, increased plant growth and C/N ratios of pine needles at elevated $CO_2$ resulted in an increase in cation exchange capacity (CEC) and dissolved organic carbon (DOC) of the rhizosphere in Pb contaminated soil. Taken together, these findings indicate that elevated $CO_2$ levels and heavy metals can affect the soil carbon cycle by changing the microbial community and plant metabolism.