Microaggregate development and turnover and its linkage to the function of soils is a major research field in soil science. Soil microaggregates are all compound soil structures – including the organo-mineral associations – smaller 250 µm (Totsche et al., 2018). They are composed of mineral, organic and biotic components, arranged in an eventually deterministic, but still vastly unknown spatial pattern. Conceptually they are considered as the fundamental composite building units of structured soil. During pedogenesis, microaggregates are formed by an interplay of physical, chemical and biological aggregation mechanisms, the quantitative role of which, although progressively more investigated, is still poorly understood. Work on microaggregate structure and development has focused on the stability and turnover of organic matter with the target elements carbon and nitrogen, while the role of microaggregates for the fate and biogeochemical cycling of other crucial elements, e.g. silicon, iron, or phosphorus, is largely unclear. Essentially no established knowledge exits on the flow of fluids and transport of solutes within the microaggregates and thus on the water retention characteristics and role for nutrient storage and supply. Only limited information is available on the stability of microaggregates against mechanical, hydraulic and osmotic stresses. Similarly, data on the rates and underlying deterministic or stochastic controls on microaggregate formation, stability and decay in space and time are scarce. These knowledge gaps oppose our attempts to mechanistically understand the dynamics and functioning of the soil structure at the scale of microaggregates. In line with this, only few quantitative approaches for the modelling of the formation, the structure, the properties and functions of microaggregates are available.
The symposium responds to the growing awareness of the importance of the micro-architecture for the functioning of natural porous media. We aim to compile and discuss the current knowledge on aggregation, the composition and turnover of microaggregates as well as their controlling role for the different environments they form in.