Associations of allophanic constituents and organic matter: Formation, properties and stabilization of organic matter

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Weathering of volcanic ashes typically results in the formation of Andosols, the mineral soil of which may contain allophane and/or imogolite. Allophanes contain the largest amounts of organic matter among all terrestrial soils. Allophanic constituents are short-range ordered minerals in the clay fraction with variable Al:Si ratios, which also form in Podzols and Cambisols. Their formation is affected by the soil-moisture regime, the activity of Si in the soil solution, and pH. The interaction between allophanic constituents and soil organic matter is considered an important stabilization process. However, soil organic matter may impede or promote the formation of allophanic constituents. Therefore, the proposed project aims at clarifying the formation of allophanic constituents in the presence of soil organic matter and the fate of associated organic matter. We will check effects of organic matter on the crystallinity of allophanic constituents, the extent of sorption from topsoil and subsoil dissolved organic matter and fractionation of organic matter upon sorption. For that purpose, syntheses of allophanic constituents in the presence of natural dissolved organic matter (obtained in topsoil by a lysimeter, in subsoil by suction cups, or by extraction) at variable Al:Si ratios and pH in adsorption and co-precipitation variants will be performed in a series of laboratory experiments. The reaction products will be characterized by Si-, Al and C-NMR spectroscopy and infrared spectroscopy. In addition, we will clarify in incubation experiments using previously produced mineral-organic associations whether soil microorganisms make use of organic matter associated by co-precipitation, and whether the addition of an easily available organic substrate might stimulate the co-metabolic cleavage of the associated organic matter. The findings of this project will deepen the understanding on the formation, the properties and the dynamics of allophanic constituents and associated organic matter. We will contribute to a more detailed comprehension of the processes of storage and fate of organic matter in soils and the formation and properties of Andosols.