Quantitative estimates of burrow construction and destruction, by anecic and endogeic earthworms in repacked soil cores

Abstract

Although the role of earthworms in soil functioning is often emphasised, many important aspects of earthworm behaviour are still poorly understood. In this study we propose a simple and cost-effective method for estimating burrow system area and continuity, as well as a new and often neglected parameter, the percentage of burrow refilling by the earthworms own casts. This novel parameter is likely to have a huge influence on the transfer properties of the burrow system. The method uses standard repacked soil cores in PVC cylinders and takes advantages of clay shrinkage and the fact that earthworms were previously shown to prefer to burrow at the PVC/soil interface. In this way, after removing the PVC cylinders off dry cores, the external section of the burrow system made by earthworms along the soil walls could be easily described. We applied this method to characterise the burrow systems of four earthworms species: two anecics (Aporrectodea caliginosa nocturna and Aporrectodea caliginosa meridionalis) and two endogeics (Aporrectodea caliginosa icaliginosa and Allolobophora chlorotica). After one month the burrow's area generated by both anecic species were much larger (about 40cm2) than the endogeic burrow's area (about 15cm2). A. nocturna burrow system continuity was higher than that of A. meridionalis and both anecic burrow systems were more continuous than those made by the endogeic earthworms. This was partly explained by the far larger proportion of the burrow area that was refilled with casts: approximately 40% and 50% for Al. chlorotica and A. caliginosa, respectively compared with approximately 20% for the anecic burrows. We discuss whether these estimates could be used in future models simulating the dynamics of earthworm burrow systems by taking into account both burrow creation and destruction by earthworms.