Non-stable Fe minerals in waterlogged soils

Abstract

The hydromorphic soil profile is characterised by the presence of a gleyed horizon with specific features in different soils. The content and composition of non-stable Fe minerals can serve as important criterion for defining the gley type. Iron minerals soluble in the acid ammonium oxalate are assigned to the non-stable type. They can be divided into two groups depending on the moisture content in the analysed hydromorphic soils. The Fe(III) non-stable group includes minerals that are soluble in the oxalate from dry soils. The Fe(III)–Fe(II) non-stable group includes minerals that cannot resist the hydromorphic soil drying in laboratory. Their amount is determined by difference between the content of Fe extracted by the acid ammonium oxalate from the wet soil and the amount of Fe extracted from the dry soil stored for 1 or 36 months. In the hydromorphic Vertisols from the southern European part of Russia, the Fe content accounts for 7–10 and 3–6% of the bulk Fe content, respectively, in the Fe(III)–Fe(II) and Fe(III) non-stable minerals. Thus, a high share of non-stable Fe minerals oxidised during the drying of hydromorphic soils under laboratory conditions is established.To discriminate the hydromorphic soil types, we propose index I that characterises the interaction of Fe(III) non-stable minerals (Fefree) with colloids: I = Fefree/CEC, where CEC is the cation exchange capacity. The ferruginated and reduced gley (Gr) has a high index (I ~ 10). In Gr with high increment of redness (Δа* ≈ 9) under drying the non-stable green rust is converted into brown lepidocrocite in the course of drying. Decrease of the moisture content in this process is also accompanied by change of the green rust composition: fougerite is transformed into tréburdenite, i.e., Fe-ephemer with a higher degree of Fe oxidation, leading to small increment of redness (Δа* ≈ 3) in the greyish matrix. In the oxidised gley (Go) with I = 0.05–0.5, the rate of green rust transformation is retarded by soil colloids. In the deferruginated gley (Gdf) with low I = 0/04–0.17 and zero growth of Δа*, active Fe minerals are not formed in the course of drying.