SP7: 3D-architecture of soil microaggregates and its control on mechanical stability and on water and oxygen supply to microhabitats
Aim of this subproject (SP7) is to investigate the development and dynamics of 3D architectures of soil microaggregates. The influence of clay content on the microstructure is examined in a toposequence and a central microcosm experiment where the role of soil aging on the initial formation and short term turnover of soil microaggregates is analyzed.
First, the generation of SMA by an alternative dry separation protocol is developed. By exerting a mechanical load in a loading frame macroaggregates crumble into microaggregates. In contrast to the commonly used wet separation approach this concept is considered be better mimic the aggregate separation under field conditions where tillage or mechanical loading with machines lead to the break up of macroaggregates down to microaggregate units. Another advantage of a dry separation technique is that re-aggregation due to sample drying after wet separation is prevented.
By analyzing dry separated microaggregates with X-ray computed tomography (XRCT) information about the 3D architecture of the pore and particle networks are obtained. Structural features, like contact area, coordination number, pore size distribution, channel width and connectivity, to mention a few, are worked out by complex image analysis algorithms.
By combining gas diffusion measurements on SMA and XRCT data transport processes are correlated with the pore network morphology and topology. Quantitative links between the spatial arrangement of pores and solid particles as well as physical processes in intra-aggregate pore networks, like flow and distribution of oxygen and water are determined.
A further research topic is the relationship between the pore network and the resistance against mechanical stress. Studies on the mechanical strength of microaggregates are performed on a loading frame (Zwick/Roell). Information about the tensile strength and fracture energy of the microaggregates are provided in these measurements. In cooperation with other subprojects the fracture surfaces are characterized.
The distribution of the organic material of selected microaggregates is investigated non-invasively by synchrotron-based X-ray computed tomography (SRCT). Usually, water, air and organic material (SOM) could not be sufficiently distinguished in CT images. The visualization of SOM is possible by staining SOM with osmium-tetroxide (OsO4) vapor and using optimized photon energies.
Aim of this project is to link quantitative descriptors of internal microaggregate structures with pore scale processes that determine microaggregate formation and development. Combined with modeling the obtained data will help to clarify the relationship between the composition and structure of microaggregates and transport processes and stabilization mechanisms. Furthermore this studies provide better insights into the evolution of microaggregates as a basis for understanding soil functioning.