Dynamics of Soil Pore Networks Investigated by X-Ray Microtomography.

Soils are complex three phase media governing numerous physical, chemical and biological processes which control ecosystem functions. Transport and transformation of solid, liquid and gaseous soil compounds are intimately connected and strongly dependent on the state of soil structure and pore space geometries. Soil structure in turn is inherently dynamic due to changes in pore water pressure resulting in shrinking and swelling and/or soil compaction and shear deformation by applying external loads and last but not least also continuously modified by biological factors (root growth and soil faunal activities). To appreciate the dynamic states of soils and functions employed measurements are required to have minimum effect on the evolvement of soil structure with changing boundary conditions. Non-invasive techniques such as X-ray microtomography are capable to resolve soil structure and associated pore architectures on various scales and at the same time allow for non-destructive observations of structural dynamics upon changes in boundary conditions. We used synchrotron- and lab-based microtomography (MCT) systems to investigate soil structural dynamics of soil aggregates and typical core-size soil samples. The effects of shrinking/swelling and compaction on pore space architectures were studied under controlled boundary conditions (matric potential, mechanical loads). Three-dimensional reconstructions of the pore space were analyzed with 3D image analysis tools to quantify changes in morphological characteristics of soil structure and related pore networks upon changes in boundary conditions, respectively. The potential of such quantitative data to further develop modeling approaches of transport and deformation processes in soils will be discussed. Further improvements in MCT imaging approaches and 3D image analysis of soil structure dynamics coupled with physical measurements of transport functions are viewed a key to achieve a more comprehensive understanding of ecosystem fluxes in soils.