Bioavailability of Cu, Zn and Mn in contaminated soils and speciation in soil solution

Greenhouse experiments were conducted in which five native Australian tree species were grown in soil spiked with copper, zinc, and manganese. The pots were equipped with samplers that allowed regular collection of the soil solution. The primary aim was to determine the critical concentration of each metal in soil solution that caused symptoms of toxicity in the trees. Preliminary experiments had shown the combination of added metal salt and lime that was required to achieve a given metal concentration in soil solution in the range 0.1-4 mM and a pH in the range 5–5.5. The Cu treatments at all concentrations caused the death of the plants. The critical concentration was therefore 0.1 mM. Zinc and Mn were less toxic, and the treatments caused a reduction in growth with increasing solution concentration. The critical concentrations were at approximately 0.2 mM for Zn and 1 mM for Mn. Because solution concentrations were declining with time and were variable, there was considerable uncertainty in these values, and they should be used with caution. A secondary aim of the experiments was to assess the relative importance of inorganic and organically complexed forms of the metals by carrying out speciation of the collected solutions. In all cases it was found that the metals were only present in inorganic forms; possible explanations for this are given