Soil and macro-pores under uniaxial compression. II. Morphometric analysis of macro-pore stability in undisturbed and repacked soil

Soil compaction is an increasingly serious problem in intensively managed agricultural land. Damages to the macro-pores are of particular concern, as these structures play a crucial role for many soil processes. Different types of macro-pores may react very differently under mechanical stress. The objective of this study was to determine the mechanical stability of specific types of macro-pores, and to use this approach to compare different types of macro-pores in repacked and undisturbed soil samples taken from a restored soil.Artificial cylindrical macro-pores were created in repacked samples containing also interaggregate pores. The samples then were subjected in uniaxial compression tests to steps of monotonously increasing loads. Keeping the samples in a strain-locked state, the macro-pore structure was assessed after each loading step using micro-computed tomography imaging and quantified by morphometric image analysis methods. As a result we obtained ‘macro-pore compression curves’ that allowed us to separately quantify the stability of the different macro-pore types in an exactly analogous way to the determination of precompression stress from bulk soil compression curves. The cylindrical pores were found to be much more stable and the interaggregate pores much less stable than the bulk soil structure. Reversion of macro-pore deformation upon unloading was negligible compared to the pronounced compression effects. Furthermore, we compared the deformation of the macro-pores in the strain-locked state with results obtained in previous experiments, in which the samples had been unloaded after compression to perform micro-computed tomography imaging, meaning that the load had been increased in a sequence of successive loading–unloading cycles rather than in a monotonous way without unloading between load steps. Deformation was significantly larger under monotonous than under cyclic load increase. This can be attributed to a reduced re-distribution of soil moisture during monotonous load increase compared to cyclic load increase, where the water could re-distribute during the time without load between the loading–unloading cycles. Finally, we found that the deformation of macro-pores in a restored soil during trafficking with heavy agricultural machinery was comparable to the deformation of macro-pores in undisturbed, non-trafficked samples, taken from the same site and subjected to uniaxial compression with strain-locked micro-computed tomography imaging in the laboratory.