X-ray tomographic and hydraulic characterization of burrowing by three earthworm species in repacked soil cores

To test the assumption that different earthworm species make differently-structured burrow systems that can affect water permeability, the burrow systems made by adults of three earthworms species ( Lumbricus terrestris, Nicodrilus giardi and Octalasion lacteum) were studied in repacked soil cores (20 cm diameter by 40 cm high) under controlled conditions of temperature and humidity. Five core were used for each species and one adult earthworm was incubated per soil core over 21 days. Three-dimensional reconstruction (skeletons and volumetric images) of the burrow systems was performed and characterized after computer analysis of the 2D-slices from X-rays tomography. Total burrow length, number of burrows, branching rate, connectivity, burrow continuity, mean verticality and mean sinuosity of the burrows were measured from the skeletons. Differences were found between species in burrow characteristics: only one vertical and continuous burrow with large diameter for L. terrestris; long and complex burrow systems with branching burrows for N. giardi and O. lacteum. Moreover, 3D volume reconstruction of burrow systems enabled determination of the total volume, the distributed burrow volume as a function of depth and, using the tools of mathematical morphology, the pore size distribution of burrows. Results showed that the burrow volume of N. giardi was the highest; the three species had a unimodal pore size distribution centered on 8, 7 and 5 mm in diameter for L. terrestris, N. giardi and O. lacteum, respectively. Saturated hydraulic permeability was measured for each core and predicted using a water flow model. Significant differences between species were found, the burrow systems made by N. giardi being the most efficient for water flow. Differences in hydraulic properties between species resulted mainly from the difference in burrow continuity, the presence of openings at the soil surface and the burrow depth penetration.