Effect of heavy metal contamination on the population of sulfate-reducing bacteria and the sulfate-reducing activity in paddy rice soils

The responses of microbial to soil heavy metal-contamination were studied by comparing the changes in the population of sulfate-reducing bacteria (SRB) and the sulfate-reducing activity (SRA) in three types of paddy soils under laboratory condition. The experimental soils were Huangsong paddy soil, Red paddy soil and Purple paddy soil. Huangsong paddy soil developed from shallow-sea deposit of intermediate lake deposit, Red paddy soil developed from alluvial deposit, and Purple paddy soil developed from parent material of neuter purple sandstone and shale. Soils were treated with different concentration of Cd2+, As5+, Cu2+, Pb2+ and Cr3+. The results showed that the population of SRB and the SRA were slightly increased by adding Pb2+(200 (mg/kg) dried soil), while the population of SRB and SRA were prominently inhibited by adding Cd2+ (more than 1.0 (mg/kg) dried soil), As5+ (more than 30 mg /kg dried soil), Cu2+(more than 500 (mg/kg) dried soil), Pb2+(more than 400 (mg/kg) dried soil) and Cr3+(more than 200 (mg/kg) dried soil). The lowest metal concentration (LOMC) of different metals at which the population of SRB and SRA were influenced was different. The responses of population of SRB and SRA in different soils to same metal were different. The population of SRB and SRA was increased by adding Pb2+(200 (mg/kg) dried soil) in Huangsong paddy soil, Red paddy soil, while there was no effect on it in Purple paddy soil. The recovery times of the population of SRB and SRA were different for different types soils as well as levels of heavy metal contamination. Generally, the higher the contamination of heavy metal applied, the longer the time needed. The biological activities of the population of SRB and SRA were so low in Red paddy soil and Huangsong paddy soil due to adding of Cr3+(1600 (mg/kg) dried soil) that the recovery of population of SRB and SRA by soils themselves was almost impossible. Thebiological activity of the Purple paddy soil were also low but it could recovery partially itself. The contaminatedcritical point for same heavy mental, which inhibited the population of SRB and the SRA, also varied a lot for different types of paddy soils.