Using X-ray microtomography to characterize the burrowing behaviour of earthworms in heterogeneously polluted soils

International audienceLarge amounts of chemicals are released into soil environments due to human activities, thereby harming organisms and their habitats. Earthworms are important bioindicators for chemical risk assessments of soil ecosystems, but the question remains of how they respond to different patterns of pollution. Laboratory experiments in soil mesocosms were carried out to observe changes in the burrowing behaviour of two earthworm species (Aporrectodea caliginosa and Lumbricus terrestris) with regard to different spatial distributions of benzo(a)pyrene, cadmium and copper, using X-ray computed microtomography (μCT). This is, to our knowledge, the first contribution of μCT to study earthworm behaviour in heterogeneously multi-polluted soils. Columns (14 × 30 cm) were filled with a silt loam soil and packed to represent two scenarios of heterogeneous pollution: a) ‘binary’, containing polluted soil in the low half part and unpolluted soil on top; and b) ‘patches’, containing small portions of polluted soil randomly placed into an unpolluted matrix. Earthworms were introduced individually in the columns and left in climatic chambers for 7 weeks. μCT images revealed that earthworms avoided the polluted zones differently (e.g. A. caliginosa>L. terrestris, ‘binary’ configuration). Individuals of L. terrestris that entered polluted areas showed a lower burrowing activity (volume and length of the burrow system) than those that did not enter. The presence of ‘patches’ of polluted soil did not cause a reduction of the burrowing activity such as that found in the ‘binary’ columns, although earthworms showed avoidance. Our results indicate that pollutants had a greater effect on earthworms’ burrowing behaviour in the presence of larger portions of contaminated soil compared with small random ones. The more evident avoidance behaviour of A. caliginosa pinpoints its usefulness in toxicity bioassays based on avoidance patterns, but questions its use in assays based on bioaccumulation of pollutants in tissues, for which L. terrestris may perform better