3D SKELETON RECONSTRUCTIONS OF NATURAL EARTHWORM BURROW SYSTEMS USINGCAT-SCAN IMAGES OF SOIL CORES

Four soil cores (length, 20 cm; diameter, 16 cm) were sampled in a Swi ss pre-Alpine meadow with high earthworm abundance (>400 individuals/ m(2)); two cores were taken in October 1993 and the other two cores in April 1994. The cores were described using computer assisted tomograp hy which gives a series of section images every 3 mm. A method for rec onstructing the three-dimensional (3D) skeleton of the earthworm burro w system is presented and discussed. This method provides an image of the structural organisation of the burrow system and was found to be a dequately sensitive for use in ecological and functional studies. The seasonal variation of these 3D skeletons was investigated using two ap proaches, i.e, the analysis of: (1) global burrow system characteristi cs, and (2) individual burrow characteristics. At the scale of the glo bal burrow system no difference was found between seasons (same number of burrows and same total burrow length) except for the vertical segm ent distribution, which was homogeneous in spring and decreased with d epth in the fall. The study of individual burrow characteristics revea led that burrows tended to be more vertical in spring and that their b ranching intensity was higher in this season.