SP4

Aggregation in soil is a very dynamic, multi‐cause process, with changes in aggregate architecture having major implications for many soil functions, e.g., water storage and transport, aeration, biological activity, and the storage and cycling of organic carbon. In the project we relate the abundance and interactions of the various aggregate‐forming agents (AFA) present in smaller building units (BU) and soil microaggregates (SMA) to the overall structure, mineralogical composition, and pore size distribution of microaggregates. Our basic assumption is that microaggregation is controlled by surface properties of participating AFA such as clay minerals, Fe‐ and Al‐oxides and organic matter. The role of surface charge on the formation of BU and SMA is determined in model experiments. Furthermore we assume that the composition of organic matter at outer particle and aggregate surfaces determines the ability to form mineral‐organic‐mineral bonds and thus influence the stability of microaggregates. Here our central approach is to utilize X‐ray photoelectron spectroscopy (XPS) to characterize the surface chemical composition of model BU and SMA within a 2‐10 nm thin surface layer. Changing soil environmental conditions (pH, Eh, ionic strength) might vary in space and in time and can make AFA to lose their aggregating function. Organic matter initiating aggregate formation can be removed by desorption and microbial consumption, resulting in destabilization of aggregates. In summary, our work program addresses four different issues: (1) overall structure, mineralogical composition, and pore size distribution of microaggregates, (2) temporal change and spatial distribution of elements within microaggregates, (3) quality and stability of microaggregate‐entrained OM, and (4) stability of microaggregates upon selective removal of AFA.