3D characterisation of earthworm burrow systems in natural soil cores collected from a 12-year-old pasture

To link earthworm burrow distribution with the spatial variability of soil functions, accurate mapping of their spatial burrowing patterns is required. Eight natural soil cores (25 cm in diameter; up to 60 cm long) were collected from a pasture in spring 2001-2003. Earthworm populations were sampled on the first date and the anecie species Nicodrilus giardi was found to be dominant (55% of the average density of 101 worms m(-2)). Soil cores were imaged using medical X-ray tomography equipment. These tomography images served as a basis for 3D reconstructions generated using specifically written software. Finally, the reconstructed burrow systems were measured and analysed using mathematical morphological approaches. The 3D reconstructions thus derived showed dense systems of interconnected burrows. A number of burrows extending vertically from the top to the bottom of the cores, but most were short, disconnected burrows. These two classes of burrows could not be linked with the activity of a particular species. In addition to the visual appraisal of the burrow system shape, structural parameters such as burrow volume, burrow wall surface area, burrow length density, topology and burrow angles were computed from the 3D reconstructions. Total burrow length density ranged from 687 to 1212 m m(-3). Burrow volume density represented less than 2.5% of total soil volume and ranged from 13.3 x 10(3) to 24.2 x 10(3) cm(3) m(-3). Inspire of the apparently high continuity of burrows, only 9-43% of the volume was connected to the soil surface. Total burrow wall area ranged from 7721 to 12764 cm(2) m-3 while surface-connected burrow wall surface area ranged from 1069 to 7237 cm(2) m(-3). The drilosphere volumes (i.e. a 2 turn thick sheath around burrows) were estimated to range from 44.9 x 10(3) to 52.9 x 10(3) cm(3) m(-3). Earthworm activity was found to vary throughout the year as revealed by changes in burrowing patterns. The burrow systems in spring 2001 were denser than that in others years, and the burrow systems in spring 2003 appeared to be partially re-filled close to the surface. This temporal variability demonstrates that it is virtually impossible to obtain true replicates of burrow systems of a given earthworm community without knowing (1) which burrows were created by which species and (2) the burrows age. However, the accurate description and quantification of earthworm burrow systems using powerful image processing tools allows a detailed discussion of the potential impact of earthworms on soil functions under natural conditions. (c) 2005 Elsevier B.V. All rights reserved.