Micro-scale dry bulk density variation around earthworm ( Lumbricus terrestrisL.) burrows based on X-ray computed tomography

The burrowing activity of earthworms leads to soil structural changes by compaction of surrounded soil matrix, which represents the so-called drilosphere. This is a distinct volume around the resulting macropore, which controls various soil processes. In the aim to detect the bulk density variation around earthworm burrows, two repacked soil columns from a silt loam with defined water ontent and bulk density were created, separately inoculated with the anecic earthworm species Lumbricus terrestris, stored under constant conditions for 7 weeks and finally analyzed by means of X-ray computed tomography. A basic voxel approach was developed to conduct a gapless analysis of the drilosphere and surrounding soil matrix. The measured Hounsfield Units of basic voxels (pixel size of 0.25 mm and slice thickness of 0.625 mm) were converted into the corresponding dry bulk density (BD). The initial mean BD of an entire horizontal soil slice was 1.34 Mg m-3 (core 1) and 1.38 Mg m-3 (core 2). Due to burrowing L. terrestris compacted the inner boundary of the drilosphere (burrow wall) up to a BD of at least 1.75 Mg m-3 which equates to more than 30% compared to the soil matrix. The BD decreased from the inner boundary of the drilosphere to its outer boundary, which is a transition zone to the surrounding soil matrix. However, the BD decrease was not concentrically uniform but revealed heterogeneous patterns of zones with different BD-classes. This locally heterogeneous BD distribution is an evidence of radial earthworm forces acting anisotropically. We conclude that BD heterogeneity in the drilosphere of L. terrestris might have notable implications for the understanding of lateral transfer of water and solutes in the soil profile