The ecological dose value (ED50) for assessing Cd toxicity on ATP content and dehydrogenase and urease activities of soil

Two soils of contrasting texture, organic matter content and pH were treated with CdSO4 solutions to give a Cd concentration range of 0–4000 mg kg−1 soil. The content of ATP and dehydrogenase and urease activities of soils were assayed after 3 h, and 7 and 28 days of Cd contamination. The relative ED50 values were calculated by two kinetic models (model 1 and model 2) used by Speir et al. (1995) and by the sigmoidal dose–response model (model 3) employed by Haanstra et al. (1985). Model 1 was the most successful in calculating the ED50 values for the ATP content, urease and dehydrogenase activities when both soils were contaminated by Cd. Similar ED50 values were predicted by model 1 (describing the full inhibition) and model 3 only when the correlation coefficients r2 were higher than 0.9. The ED50 values of ATP calculated by model 1 were markedly higher than those calculated by model 2 (describing partial inhibition) when both models gave correlation coefficients higher than 0.9. This behavior was due to the high asymptote values obtained using model 2. According to model 2, some of the enzyme activities responsible for the ATP synthesis were probably not inhibited at the highest Cd concentrations. The inhibitory effect of Cd on the ATP content and both enzymatic activities was lower in the Castelporziano soil, which had the highest total organic carbon content.