Speciation of Cadmium and Changes in Bacterial Communities in Red Soil Following Application of Cadmium-Polluted Compost

Lack of legal limits for heavy metal content in compost in China necessitates studying effects of heavy metal distribution on resulting toxicity to soil microbes, after the application of heavy metal–contaminated compost. A sequential extraction procedure was used to study the dynamic speciation of cadmium (Cd), and polymerase chain reaction–denaturing gradient gel electrophoresis (DGGE) analysis was applied to observe the changes in bacterial communities in red soil that were stressed by Cd-polluted compost. Results showed that the Cd fractions of exchangeable, carbonate, and Fe-Mn oxides increased (range: 0.88–10.12, 0.19–2.58, and 0.47–1.9 mg/kg, respectively), organic fraction of Cd decreased (0.05–0.34 mg/kg), and residual fraction of Cd did not regularly change with increased Cd content in compost during simultaneous incubation. DGGE profiles indicated that bacterial communities were affected by Cd-polluted compost to some extent. New bands emerged in the early incubation stage, and some bands disappeared in the late incubation stage. Similarity coefficients of DGGE profiles showed that genetic similarity increased at 14 days before incubation (highest, 93.8%) and decreased after 28 days (lowest, 74.6%). High Shannon's diversity index (H) revealed an activation effect of Cd on bacterial communities, which was mainly attributed to the increase in the carbonate fraction with 12.8 mg/kg Cd treatment. Low H values implied an inhibition of bacterial communities by 107.8 mg/kg Cd compost, which was possibly owing to the increases in exchangeable and carbonate fractions.