Andosol genesis: Transition from silandic to aluandic properties and the related changes for organic carbon storage.

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Andosols are subdivided in silandic and aluandic Andosols, which are often seen as endmembers of the Andosols genesis. Silandic Andosols mainly consist of organic‒mineral associations formed by OM and imogolite-type phases, including allophanes, imogolites, and protoimogolites. Aluandic Andosols consist mainly of aluminium‒OM complexes (Al‒OM complexes). In both subgroups the result is an intimate association of OM and the soil mineral matrix. The current theory is that silandic and aluandic properties are direct results of primary weathering, i.e., result from two different lines of pedogenesis.Our own previous investigations, however, suggest that horizons with silandic properties may transform to horizons with aluandic properties over time. We think that this is caused by acidic dissolved organic matter (DOM) entering the subsoil with the percolating water, causing dissolution of the imogolite-type phases and releasing aluminium. The released aluminium and DOM instantly form into Al‒OM complexes.Thus, the proposed work aims at testing this hypothesis and determining the factors influencing that process. We will study a one-layered Ecuadorian Andosol, with pre-dominating aluandic properties in the topsoil and pre-dominating silandic properties in the subsoil. First, we are going to determine if the sub- and topsoils are saturated with OM by performing sorption batch experiments. Then, the in situ transformation from silandic to aluandic properties will be monitored in a percolation experiment, using an acidic DOM solution (pH at which imogolite-type phases dissolve).We will track changes of the mineral phase with a combination of different techniques. Imogolite-type phases as well as Al‒OM complexes are challenging to characterize and quantify in soil samples due to their short range-ordered and nano- or micro-crystalline nature. Also, they are often closely associated with each other. In order to characterize and quantify them, we will apply a combination of density fractionation, chemical extractions and microscopy (transmission electron microscopy in combination with element detection) as well as modern spectroscopic techniques (Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, synchrotron-based Al K-edge X-ray absorption spectroscopy).We expect to gain new insights into the genesis of Andosols, as well as on the formation and transformation of short range-ordered mineral phases. These mineral phases play a key role for accumulating and storing organic matter in Andosols and other soils.