Soil solution Cd, Cu and Zn concentrations as affected by short-time drying or wetting: The role of hydrous oxides of Fe and Mn F.M.G. Tack a, ⁎ , E. Van Ranst b , C. Lievens c , R.E. Vandenberghe d a Laboratory of Analytical Chemistry and Applied Ecochemistry, Department of Applied Analytical and Physical Chemistry, Ghent University, Coupure Links 265, B-9000 Ghent, Belgium b Laboratory of Soil Science, Department Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium c Laboratory of Applied Chemistry, Department SBG, Universiteit Hasselt, B-3590 Diepenbeek, Belgium d Department of Subatomic and Radiation Physics, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium Received 4 March 2005; received in revised form 29 May 2006; accepted 18 July 2006 Available online 6 September 2006 Abstract 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 NH 2 OH·HCl + 0.25 M HCl-extractable (“amorphous iron oxides”) nor the 0.04 M NH 2 OH·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. © 2006 Elsevier B.V. All rights reserved. Keywords: Heavy metals; Mössbauer spectroscopy; X-ray diffraction; Selective dissolution 1. Introduction Trace metals in soils may influence biological activity when they become available for uptake by organisms. In soils and sediments, elements exist in several different forms and are associated with a range of components (Cottenie et al., 1979). In the water phase, the chemical form of a metal determines the biological availability and chemical reactivity (sorption/ desorption, precipitation/dissolution) towards other compo- nents of the system. The binding form in the solid phase is related to the kinetics and equilibria of metal release to the liquid phase and, hence, the likelihood of remobilization and bioavailability (Tack and Verloo, 1995). Hydrous oxides of manganese, iron and aluminium are known as an important soil component controlling the retention of trace metals in soils (Jenne, 1968; Shuman, 1976; Kinniburgh et al., 1976). These amorphous and microcrystalline structures have a large sorption capacity for trace metals. They contribute to a “dynamic” trace element fraction in the soil. In an oxidizing, dry environment, they can evolve to more crystalline and stable structures, which can immobilize trace metals. A very important effect of organic material in soils in relation to metal binding exists in their interaction with aluminium, iron and manganese oxides (King, 1988). Biological and microbial activity associated with the presence of organic matter contributes to the existence of a periodic reducing environment after rain, irrigation or flooding, necessary to maintain these Geoderma 137 (2006) 83 – 89 www.elsevier.com/locate/geoderma ⁎ Corresponding author. E-mail address: filip.tack@ugent.be (F.M.G. Tack). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.07.003