Fractionation of copper and manganese in agricultural soils near an abandoned copper mine

Experiments were conducted to examine the depthwise distribution of total contents of Cu, Zn, Fe, and Mn in selected agricultural soils and in a natural soil, near an abandoned Cu mine. The chemical forms of Cu and Mn in the profiles were evaluated by a sequential extraction method. The respective fractions of Cu and Mn were designated as exchangeable (Ex-), Ph-displaceable (Pb-), acid soluble (Aci-), Mn oxide-occluded (MnO-), organically bound (OM-), amorphous Fe oxide-occluded (AFeO-), crystalline Fe-oxide occluded (CFeO-), and residual (Res-) fractions. In the natural soil, Y-N, the total content of Cu, Zn, Fe, and Mn increased with depth and the poorly weathered basalt appeared in the C horizon. Throughout the profile of Y-N, Cu and Mn were primarily found in the Res-fraction, except for the Mn in the C horizon where the Mn was considered to occur mainly in the form of Mn oxides. A remarkable accumulation of Cu in the surface layers was observed in the paddy field, Y-R, and a large part of the accumulated Cu was assumed to be adsorbed onto the organic matter or occluded by the oxides and hydroxides of Fe and Mn. In the surface layers of the reclaimed orange orchard, Y-O, gradual accumulation of Cu was observed in the Aci-, MnO-, OM-, AFeO-Cu fractions although the increase in the total content of Cu was not conspicuous at present. In addition, the accumulation of Mn through the adsorption onto or the occlusion by poorly crystallized hydroxides was also indicated. In the upland fields, Y-P and Y-W, pollution with Cu was not detected. However, the total content of Mn in the surface layers was higher than that of Y-O and Y-R, and an increase of the Mn content in the Ex-, Aci-, and MnO-Mn fractions was observed.