Is it possible to delineate aggregates in X-ray images of intact soil samples?

Soil structure refers to the spatial arrangement of primary soil particles, their cohesion and the pores in between them. It has a fundamental impact on a variety of soil functions including carbon sequestration and water holding capacity. Researchers in this field either approach the topic by investigating the geometry of pore networks in undisturbed soil; or they instead evaluate properties of aggregates obtained from disassembling soil clods. Which of the two approaches is chosen depends on the requirements and traditions in the respective soil science discipline. There have been surprisingly little efforts undertaken to relate both viewpoints on soil structure quantitatively. In this study, we present and evaluate methods to delineate soil aggregates in eight X-ray images of undisturbed soil samples. The approaches exploit crack formation upon shrinkage in drying soil. Comparing the image-derived aggregates to results from drop-shatter tests, we observed promising trends but overall, the results remained inconclusive. On the one hand, this was due to the very small number of studied samples. On the other hand, the presented aggregate delineation approaches have potential for improvement. We suggest to develop this line of research and apply it to larger numbers of samples, different scales and different physical aggregate isolation approaches, like dry and wet sieving. For example, it may be evaluated whether microaggregates are identifiable in still intact macroaggregates.