Burrow systems of endogeic earthworms: Effects of earthworm abundance and consequences for soil water infiltration.

By creating burrows, earthworms influence the transfer properties of soils. The effects of endogeic species on soil transfer properties, however, are not yet well understood because these earthworms generally create burrows that are refilled by casts and have no preferential vertical orientation. Thirty soil cores were incubated for various periods (1–3 or 4 weeks) at different earthworm densities (70, 210, 345 or 480 individuals m −2 ). The cores were then scanned using X-ray tomography and the burrow systems were characterised by measuring the total burrow volume, bioturbation volume (refilled burrows and lateral compaction around the burrows), the number of branches, tortuosity and continuity (assessed by computing the number of burrows with a vertical extension greater than 15, 20 and 25% of the core). We also computed the mean geodesic distance, i.e. the mean distance from the bottom to the top of the core assuming that distances inside burrows are null. Rainfall simulations were carried out on 17 cores chosen to encompass the variations observed in the burrow systems. The water transfer efficiency of each core was estimated by measuring two parameters: breakthrough volume and the percentage of water transmitted after 1 h of rain. Burrow and bioturbation volume increased significantly and steadily with time and earthworm density. We estimated that on average Allolobophora chlorotica burrowed 22 cm per week. All other burrow system characteristics also increased with time and earthworm density except the mean geodesic distance, which decreased significantly. This suggests that intraspecific interactions had no significant effect on burrow system geometry. Univariate PLS regressions were used to understand which burrow system characteristics had the strongest influence on water transfer. These regressions showed that the mean geodesic distance was the most important parameter. This means that in addition to individual burrow characteristics, the spatial arrangement of the whole burrow system also had a major effect on transfer properties. [ABSTRACT FROM AUTHOR]