Formation and dynamics of organo-mineral associations in redoximorphic soils

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Formation of redoximorphic soils is characterized by a periodical change of oxidizing and reducing conditions. Reductive dissolution and subsequent oxidative re-formation of Fe and Mn oxides causes characteristic accumulation and depletion zones and mineral transformations in these soils.The aim of the project we apply for is to retrace formation and dynamics of organo-mineral associations, i.e., changes of the mineral and organic phase, in redoximorphic soils, which are affected by ground water. Special emphasis is laid on investigating Fe and Mn oxides, which form in the presence of dissolved organic matter. Organic matter affects structural properties of Fe oxides and is stabilised towards decomposition by interactions with the mineral phase. We postulate ageing of Fe oxides during redox cycles to more crystalline phases, the reactivity of which towards organic matter changes. We postulate preferential sorption of aromatic compounds and the spatial differentiation of Fe and Mn oxides together with the associated organic matter. We aim at studying the in-situ formation of the minerals in the presence of dissolved organic substances, i.e., their associations, at high spatial resolution. The chemical composition of these organo-mineral associations will be characterized by nano-scale secondary ion mass spectrometry (NanoSIMS) at the sub-micron scale and the speciation of Fe and Mn by X-ray absorption spectroscopy methods (XANES/EXAFS) at the micron scale. The formation of organo-mineral associations will be investigated in studies with increasing complexity and near-natural conditions: from laboratory studies on formation and properties of organo-mineral associations in mineral mixtures as model substrates over studies with undisturbed Bg horizons of natural soils to field studies in the Bg horizon of a Calcaric Gleysol, where we will collect organo-mineral associations formed in situ. The differentiation of newly formed organo-mineral associations in the experiments from older pedogenic ones will be achieved by using 13C- and 15N-labelled organic model substances and 18O2 in the soil atmosphere.This project will contribute to the understanding of previously unclear correlation between redox-sensitive metal oxides and the C cycle in soils. Given the high reactivity of Fe and Mn oxides, it also accounts for the cycles of further contaminants and nutrients.