Distribution of Copper and Cadmium Fractions in Two Forest Soils

Pollution of soil by Cu and Cd seems to be increasing in Switzerland. To know the background concentration in low polluted soil we have measured their concentrations in every horizon and calculated the total amounts. The concentrations were low: the largest concentrations of Cu and Cd occurred in the topsoil of a Typic Dystrochrept (acid brown soil) (13.5 mg kg^-1 Cu and 437 μg kg^-1 Cd) and in the Oi-Oa-Al and Bh horizons of a Typic Haplorthod (podzol) (up to 16 mg kg^-1 Cu and 460 μg kg^-1 Cd in Oi), though when corrected by density, concentrations were higher in Al horizon of the Haplorthod and Al and BC of the Dystrochrept. The Dystrochrept was found to contain more of the metals than the Haplorthod. The soil is the main storage compartment of both ecosystems. A six step sequential extraction was applied to evaluate the affinity of metals for the soil constituents and the binding strength to the solid matrix. Water, 0.1 M NaNO₃ , 0.1 M hydroxylamine, 1 M hydroxylamine plus acetic acid, H₂ O₂ and aqua regia were used as successive reagents. Patterns of Cu and Cd distribution in a given soil are different and, to a lesser extent, for a given element the distribution patterns depend on the soil type. Organic matter or Fe and Mn oxides seem to immobilize Cu whereas Cd is associated mainly with Fe and Mn oxides. If the sequential extraction is considered as succession of reagents of increasing strength, then Cd seems to be less strongly bound to soil than Cu and thus more readily solubilized and translocated within soil. The results show the limitations of multistep sequential extraction as a technique for studying metal fractions.