Soil solution Cd, Cu and Zn concentrations as affected by short-time drying or wetting: The role of hydrous oxides of Fe and Mn

Soil solution metal concentrations are affected, among others, by hydrous oxides of iron and manganese. In a greenhouse study, eight selected soils were subjected to different moisture regimes mimicking during 2 weeks (1) strongly oxidizing conditions of a completely dry surface soil; (2) moist, oxic conditions at field capacity; and (3) a short period of flooding and, hence, the establishment of temporarily low oxygen conditions. Hydrous oxides were characterized using selective extractions, X-ray diffraction and Mössbauer spectroscopy. The moisture regimes did not significantly alter the 0.25 M NH₂OH·HCl + 0.25 M HCl-extractable ("amorphous iron oxides") nor the 0.04 M NH₂OH·HCl + 25% HOAc-extractable ("amorphous + crystalline iron oxides") Fe, Mn and Al. X-ray diffraction and Mössbauer spectroscopy revealed that no major conversions or changes in degree of crystallization or redox state had occurred in any of the soils as a result of the 2-week events of flooding or drought. In sandy and acidic or poorly buffered soils that had been dried, soil solution metal concentrations were between 2 and 40 times higher than in the corresponding reference soils that had been kept at field capacity all the time. Soils that had been saturated tended to have slightly lower concentrations of metals in the soil solution. While the importance of iron oxides should not be neglected, it is clear that it is not the single dominant factor. Soil solution metal concentrations at one moment of time may significantly depend on previous soil moisture conditions.