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

Information on earthworm burrowing behaviour is scarce and therefore the ev olution of the macroporosity related to earthworm activities is still poorl y known. We have designed a new apparatus, 'Colonne Gamma', to follow the t hree-dimensional trajectories of radio-labelled earthworms in artificial co res 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). T he emission of gamma rays is recorded by three detectors carried by a disc that oscillates vertically around the soil ore where the earthworm is intro duced. We have also developed a deterministic model to estimate the positio ns of the Co-60 source from the number of gamma rays received by each detec tor during an oscillation. Numerical experiments showed that the uncertaint ies of estimates were less than 3 mm for each coordinate. To validate the r esults, 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 mac roporosity obtained by computer assisted tomography of the same soil core. There was a general qualitative agreement between the trajectory and the sk eleton. Moreover, based on the precise study of the successive positions of the earthworm we could distinguish two different kinds of activities in th e trajectory: displacement and digging events. The 'Colonne Gamma' apparatu s therefore has great potential for studies of the ecology and the behaviou r of earthworms.