Microbial activity and community diversity in a variable charge soil as affected by cadmium exposure levels and time

Effects of cadmium(Cd) on microbial biomass,activity and community diversity were assessed in a representative variable charge soil(Typic Aquult) using an incubation study.Cadmium was added as Cd(NO3)2 to reach a concentration range of 0～16 mg Cd/kg soil.Soil extractable Cd generally increased with Cd loading rate,but decreased with incubation time.Soil microbial biomass was enhanced at low Cd levels(0.5～1 mg/kg),but was inhibited consistently with increasing Cd rate.The ratio of microbial biomass C/N varied with Cd treatment levels,decreasing at low Cd rate(<0.7 mg/kg available Cd),but increasing progressively with Cd loading.Soil respiration was restrained at low Cd loading(<1 mg/kg),and enhanced at higher Cd levels.Soil microbial metabolic quotient(MMQ) was generally greater at high Cd loading(1～16 mg/kg).However,the MMQ is also affected by other factors.Cd contamination reduces species diversity of soil microbial communities and their ability to metabolize different C substrates.Soils with higher levels of Cd contamination showed decreases in indicator phospholipids fatty acids(PLFAs) for Gram-negative bacteria and actinomycetes,while the indicator PLFAs for Gram-positive bacteria and fungi increased with increasing levels of Cd contamination.