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

Jia-li SHENTU1,Zhen-li HE1,2,Xiao-e YANG1,Ting-qiang LI1(1MOE Key Laboratory of Environmental Remediation and Ecosystem Health,College of Environmental and Resource Sciences,Zhejiang University,Hangzhou 310029,China)(2University of Florida,Institute of Food and Agricultural Sciences,Indian River Research and Education Center,Fort Pierce,FL 34945,USA)  
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.
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