6533b837fe1ef96bd12a30c1

RESEARCH PRODUCT

Transformation of clay-sized minerals in soils exposed to prolonged regular alternation of redox conditions

subject

description

Abstract The direction of the transformation of Fe-bearing minerals under harshly changing redox conditions is still under debate. Some studies suggest preferential accumulation of weakly crystalline Fe oxides while other studies showed that repeated redox cycles favour the presence of crystalline phases. Since characterized by distinct redox cycles, paddy soils are ideal for studying redox-related transformations of Fe oxides and Fe-bearing clay minerals. We analysed changes in the Fe mineral assemblage upon long-term reduction–oxidation cycles along a chronosequence of 100, 700, and 2000-year-old paddy soils developed on comparable parent material relative to two non-irrigated counterparts. The clay mineral assemblage, as indicated by X-ray diffraction, showed no significant differences between paddy and non-paddy soils as well as no changes with time under paddy management. Speciation of Fe mineral phases in top- and subsoil was accomplished by Mossbauer spectroscopy at room temperature and 4.2 K, in combination with X-ray diffraction and selective chemical extractions (dithionite–citrate–bicarbonate, NH 4 oxalate). We found ferrous and ferric Fe in silicates, nano- to micro-crystalline goethite and haematite, as well as a ferrihydrite-type phase. In topsoils, prolonged paddy cultivation resulted in general loss of silicate- and oxide-bound Fe in the topsoils and the remaining Fe oxide assemblage became more and more dominated by organic-rich weakly crystalline phases. Total contents of Fe oxides in subsoils increased due precipitation of Fe leached from the topsoils. Micro-crystalline goethite was the predominating Fe oxide phase in the upper subsoil, likely due to Fe 2 + -catalysed solid-state transformation of freshly precipitated hydrous Fe oxide phases. Lepidocrocite formed in the deeper subsoil, probably promoted by less CO 2 in the soil water. Thus, paddy cultivation causes different Fe mineral weathering, forming, and transforming environments, with the depth differentiation of these environments becoming increasingly distinct with time under paddy management, i.e., the number of redox cycles the soils underwent.