Elucidating the role of surface alteration and dynamic relocation for the formation and maturation of microaggregates in the soil-parent rock continuum

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Aggregation and the build-up of aggregates is one of the unique features of soils. It results in a non-random spatial arrangement of the solid phase already at the submicron scale. Within this system, microaggregates are considered the fundamental building blocks for the aggregated structure in almost all soils, including those with an aggregate hierarchy. Soil microaggregates are defined as compound structures smaller <0.25mm, comprising the colloidal-sized and nanoparticulate composite building units and the organo-mineral composites. Noteworthy, microaggregates, may be present as suspended or colloidally-dispersed components of the mobile phase. As such, they are prone to transport with the seepage and may affect the surface and pore-space properties. Surface alteration by interactions of seepage components with immobile surfaces is likely an important, yet essentially unexplored pathway triggering formation of microaggregates in the soil-parent rock continuum. In matured soils, the commonly found associations of clays with other, often poorly crystallized but highly reactive minerals and organic matter is the consequence of nucleation in the chemically heterogeneous soil suspension. Both pathways coexist and can be studied in the soil-parent-rock transition zone were weathering and formation/alteration of secondary mineral phases are still in the early stage. The stability of microaggregates and their interactions are dependent on wetting-drying and in turn by hydration-dehydration cycles. Such moisture-related dynamics regularly take place in soils of the temperate regions even down to the soil-parent-rock transition zone and suggests that the hydraulic and osmotic stress and their history results in attachment, detachment, translocation and accumulation. With this project we intend to close fundamental knowledge gaps in the understanding of the formation and stability of microaggregates in the soil-parent rock continuum. The research program comprises studies in-situ (profile analysis loess pedosequence in a space-for-time approach), in-vitro (dynamic relocation experiments) and in-silico (numerical simulations of hetero-aggregation). We focus on two so far ignored formation pathways of microaggregates and composite building units, i.e. the "geochemical inheritance" and "heteroaggregation from Suspension", thereby considering the role of dynamic relocation of composite building units and microaggregate forming materials from upstream compartments. With the research in this subproject, we aim for a quantitative understanding of the interplay and interdependence of the formation pathways in space and time, the controlling processes and underlying mechanism by the exploration of the resulting phenomena. Our improved understanding will ultimately fuel our endeavor for the development of a quantitative theoretical framework of microaggregate formation and function in soils guided and realized within the coordination project.