Making Soil Transparent – Recent Developments in x-Ray Tomography.

During the last two decades our capabilities to explore the 3D microstructure of soil structure have tremendously increased. This was mainly accomplished by tomographic techniques and especially the availability of x-ray CT systems. Today it is standard to get reliable quantitative information on the size distribution of, their shape, surface density and connectivity.

This is an excellent development since it is widely acknowledged that soil structure is a highly meaningful indicator for soil functions. This is not only true for water and matter fluxes, but also for the turnover of soil organic matter and the diversity of soil biota.

Today, there is a huge variety of methods to quantify the structural complexity based on 3D CT images. We identified a reduced set of measurable quantities, which may serve as some standard procedure to characterize pore structure in soil. Beyond such a static analysis of soil structure it is time now to do the next step towards exploring its temporal dynamics. Variations of soil structure are the consequence of two different classes of processes, both relevant for soil functioning. First, there are directed changes of soil structure during soil formation and after disturbance as e.g. induced by tillage. This dynamics is expected to be an important aspect for the stability and resilience of soil systems. Second, there are small scale changes in soil structure acting continuously with different intensities but without changing macroscopic quantities as porosity and pore size distribution. This can be seen as a permanent reorganization of the inner soil architecture. It is responsible for a temporal physical protection and access of organic matter with respect to microbial decomposition. Both, the directed and non-directed processes are mainly driven by soil biota including soil fauna, microbes and plants but also by swell/shrinkage and freeze/thawing cycles.

Our tools to visualize soil structure typically provide the situation at single moments in time while repeated analysis of the same location is difficult. This is especially true when considering natural boundary conditions. We present two new techniques allowing for the detection of structure dynamics with respect to the two classes mentioned above. First, the deformation of soil samples is mapped during compression by analyzing the movement of structural patterns. Second, the actual state of soil structure is traced using introduced particles having an increased iron content so that they can be localized during repeated records using CT imaging. This approach has the potential to provide rate parameters for the turnover of soil structure as a new highly valuable characteristics of soil structure formation and matter turnover.