Three-dimensional trajectories of 60Co-labelled earthworms in artificial cores of soil

Information on earthworm burrowing behaviour is scarce and therefore the evolution of the macroporosity related to earthworm activities is still poorly known. We have designed a new apparatus, 'Colonne Gamma', to follow the three-dimensional trajectories of radio-labelled earthworms in artificial cores of soil. Earthworms are radio-labelled by injecting into their coelomic cavity a small source of Co-60 (volume 0.1 mm(3), intensity 13.5 mu Ci). The emission of gamma rays is recorded by three detectors carried by a disc that oscillates vertically around the soil ore where the earthworm is introduced. We have also developed a deterministic model to estimate the positions of the Co-60 source from the number of gamma rays received by each detector during an oscillation. Numerical experiments showed that the uncertainties of estimates were less than 3 mm for each coordinate. To validate the results, we tracked the trajectory (one position every 4 minutes) of a radio-labelled earthworm for 1 week and compared it with the skeleton of the macroporosity obtained by computer assisted tomography of the same soil core. There was a general qualitative agreement between the trajectory and the skeleton. Moreover, based on the precise study of the successive positions of the earthworm we could distinguish two different kinds of activities in the trajectory: displacement and digging events. The 'Colonne Gamma' apparatus therefore has great potential for studies of the ecology and the behaviour of earthworms.