Assessment of soil structural differentiation around earthworm burrows by means of X-ray computed tomography and scanning electron microscopy

The burrowing activity of earthworms creates a distinct area around the resulting macropores called the drilosphere, which controls various soil processes. Density and microstructure of the drilosphere were studied and compared with those of the surrounding soil. For this purpose soil cores were separately inoculated with the vertically burrowing earthworm species Lumbricus terrestris. After 70 days some cores were compacted by a hydraulic press (250 kPa) and all cores were analysed by means of X-ray computed tomography. Mean Hounsfield Units were measured for concentric ROI cylinders (ROI=region of interest) of increasing diameters located around vertical macropore sections within selected horizontal slices. Based on these data we estimated stepwise the distribution of bulk density from the inner boundary of the drilospbere to the surrounding soil. In uncompacted soil the bulk density of the drilosphere was increased by 11% over that of the surrounding soil. In cross section, drilosphere and burrow form a concentric area with a total radius up to 2.2 cm. Soil compaction increased the dry bulk density of soil and decreased the diameter of earthworm burrows. Moreover, we found a less dense part of soil between the dense drilosphere and the remaining soil of the compacted core. Scanning electron microscopy revealed that the coarse silt particles of the bulk soil were rearranged to a parallel orientation due to compaction whereas the microstructure of the drilosphere remained unchanged. In any case, the drilosphere revealed a very homogeneous and dense arrangement of silt particles. (c) 2006 Elsevier B.V. All rights reserved.