Microbial toxicity of Cd and Hg in different soils related to total and water-soluble contents

Microbial toxicity tests with Cd and Hg and up to 18 soil samples were combined with sorption and solubility measurements. The data ought to indicate to what extent toxic effects of the metals are influenced by sorption and the other factors that may alter their biocidal action. The microbial reduction of Fe(III) oxides to Fe2+ ions was used as a parameter of microbial activity to calculate the total amounts of Cd and Hg that cause 10, 50, and 90% inhibition [effective doses (ED)10, ED50, ED90]. The corresponding solution concentrations [effective concentrations: (EC)10, EC50, EC90] were derived from Freundlich adsorption isotherms. The very large variability in ED values in different soils (ED10: Cd, 5-95 mg/kg; Hg, 0.125-125 mg/kg) can be related to the sorption and solubility behavior of the metals. Nevertheless, the EC values of both metals also indicate a considerable variation (EC10: Cd, 0.01-1.13 mg/liter; Hg, < 0.001-0.041 mg/liter). The influence of soluble soilborne substances on metal speciation is the main reason for their varying toxic potential in different soil solutions. The strong and complex influence of soil properties on the toxicity of heavy metals in soils indicates that both chemical and biological methods of soil analysis are needed to assess the risks of soil contamination adequately.