Toxicity interactions of cadmium, copper, and lead on soil urease and dehydrogenase activity in relation to chemical speciation

The toxicity of the trace metals Cd, Cu, and Pb alone and in combination was assessed by measuring the activity of the soil enzymes dehydrogenase and urease. We assayed the enzymatic response of a forest soil exposed to single metals and metal mixtures in a factorial design. The chemical speciation of the metals was measured in 0.01 M KNO3 solution extracts using an ion-selective electrode for Cu2+ and estimated from voltammetric determinations of labile metals for Cd2+ and Pb2+. The toxicity interaction of the metal mixtures was predicted according to the classical Bliss independence and Loewe additivity models. Significant antagonistic effects were observed for the majority of the combinations for both the dehydrogenase and the urease assays. Given the strong interaction upon the chemistry, the toxicity interaction data were analyzed based on resulting free metal activities to tease out the effect of chemistry changes from that of toxicological responses. As a result, a stimulation of enzymatic activities was observed in the mixtures in comparison to the enzymatic activities obtained with the individual metal.