Evolution of burrow systems after the accidental introduction of a new earthworm species into a Swiss pre-alpine meadow

 The unintentional introduction of a new earthworm species (Aporrectodea nocturna) into a Swiss pre-alpine meadow resulted in a great increase in earthworm density in the newly colonized area (386 m^–2) compared with the density observed in the natural area (273 m^–2) where an earthworm community was already present. To investigate the impact of this introduction on the burrow systems, eight soil cores (length 25 cm, diameter 16 cm) were taken (four in the colonized area and four in the natural area) and analysed with computer-assisted X-ray tomography. The resulting images were processed to obtain the 3D-skeleton reconstructions of the earthworm burrow systems. Due to high variability in these burrow systems, only slight differences were observed between the two areas. The total burrow length and the mean burrow lengths tended to be greater in the colonized area. Moreover, the distribution of pore numbers with depth showed different patterns with a maximum for depths between 10 cm and 15 cm in the colonized area and a maximum for depths between 20 cm and 25 cm in the natural area. These differences may have been related to: (1) the particular behaviour of A. nocturna, which was observed to cast at the surface in this site, and (2) the predominance of juvenile earthworms around the colonization front. These differences were sufficient to create significant effects on the continuity of the burrow systems (assessed by the number of different pathways between virtual horizontal planes) for the two areas. The colonized area was characterized by a greater pore continuity, which could have resulted in enhanced transfer properties. Received: 2 July 1999     

Quantitative analysis of earthworm burrow systems with respect to biological soil-structure regeneration after soil compaction

 On arable land, tilled with conventional tillage (CT) and conservation tillage (CS) respectively, plots were compacted by wheeling them 6 times with a 5 Mg wheel load in spring 1995. Immediately after compaction, undisturbed soil monoliths were excavated from the compacted and uncompacted plots. The monoliths were defaunated and inoculated with either Lumbricus terrestris or Aporrectodea caliginosa. One monolith from each plot remained uninoculated as a control. After 6 months the monoliths were defaunated again and then scanned with X-ray helical computed tomography. The data were transformed, the void systems inside the monoliths were reconstructed and visualised, and the parameters total void length, total void volume, tortuosity and continuity were quantified. The parameters' values were generally lower in the controls than in the inoculated monoliths. Differences in burrow construction could be explained by the different life strategies of the two earthworm species. Changes in burrow morphology due to tillage system and soil compaction were minor. Only the continuity of the burrow systems clearly changed: decreasing for L. terrestris and increasing for A. caliginosa. This can be explained by a change in the earthworms' burrowing activity to minimise energy expenditure in compacted soil. By extrapolating field data, we concluded that earthworms have great potential for biologically regenerating the soil structure after a single compaction event. Due to higher earthworm abundances in soil managed by CS the regeneration of the soil structure is assumed to be better in these plots than those tilled by CT. Received: 17 December 1997     

Earthworm activity and soil structure changes due to organic enrichments in vineyard systems

 The effect of organic enrichment on earthworm activity and soil structure was studied in two French vineyards, by comparing control and test plots. In each vineyard the organic matter quantitatively increased the abundance and biomass of the earthworm community. These increases were associated with a higher level of species diversity and a higher evenness corresponding to the development of endogeic community. These earthworm community changes were associated with an increase in granular bioturbated areas and in macroporosity in the top soil layer. The micromorphological approach incorporated an original process of image analysis which appeared to be an appropriate method for characterizing pore morphology in this study. The pores when characterized by their size and shape could be related to ecological groups and growth stages of earthworms. Received: 4 August 1997     

Sublethal effects of imidacloprid on the burrowing behaviour of two earthworm species: Modifications of the 3D burrow systems in artificial cores and consequences on gas diffusion in soil

Earthworms have been termed ‘ecosystem engineers’ (sensu [Jones, C.G., Lawton, J.H., Shachak, M., 1994. Organisms as ecosysem engineers. Oikos 69, 373-386.]) because of the important roles they play in the soil. As a consequence, it is assumed that if earthworms change their behaviour following exposure to pesticides or pollutants this could have a drastic impact on soil functioning. To test this assumption under laboratory conditions, we studied the burrow systems made by two earthworm species (the anecic Aporrectodea nocturna and the endogeic Allolobophora icterica) in artificial soil cores containing imidacloprid, a widely used neonicotinoid insecticide. After 1-month incubation period, the macropores created in the soil core were analyzed by tomography. In order to further characterize transfer properties associated with burrow systems gas diffusion measurements were also carried out. The burrow systems made by the two earthworm species were very different: A. nocturna made more continuous, less branched, more vertical and wider burrows than A. icterica. Some changes to A. nocturna burrow systems were observed after exposure to imidacloprid (they made a smaller burrow system and burrows were more narrow), but only at the highest concentration of imidacloprid used (0.5 mg kg⁻¹). A. icterica worms were more sensitive to imidacloprid and many differences in their burrow systems (length, sinuosity, branching rate and number of burrows) were observed at both concentrations tested (0.1 and 0. 5 mg kg⁻¹). As a consequence, the continuity of the burrow systems made by both species was altered following imidacloprid treatment. Gas diffusion through the A. nocturna soil cores was reduced but no difference in gas diffusion was observed in the A. icterica soil cores.     

Characterization of the burrow system of the earthworms Lumbricus terrestris and Aporrectodea giardi using X-ray computed tomography and image analysis

 The burrow systems of two earthworm species (Lumbricus terrestris and Aporrectodea giardi) were studied in artificially packed soil columns placed in controlled conditions in the laboratory. At the end of the incubation, which lasted 246 days, the burrow systems were characterized on the undisturbed columns using X-ray computed tomography. This method provided a set of digitized images corresponding to horizontal 3-mm-thick sections. The following parameters were measured using image analysis on each section: number of biopores, their volume, and their individual orientation calculated using an elliptical model of the earthworm channel. The profiles of these parameters through the columns showed that the burrow systems of L. terrestris and A. giardi, which are both anecic species, were very different in terms of total volume, number of burrows, burrow orientation and extension with soil depth. These results led us to conclude that the burrow system of L. terrestris can be considered as a permanent structure whereas that of A. giardi is closer to the burrow system of endogeic species. Received: 12 June 1998