The burrowing characteristics of three common earthworm species

The burrowing characteristics of 3 common earthworm species were studied using X-ray computed tomography (CT) scanning in large cylinders (24.1 cm diam.) packed with topsoil (0-25 cm) and subsoil (25-50 cm) to representative field bulk density values and sown with ryegrass. Replicated cylinders (n=3), kept under constant moisture and temperature conditions, were inoculated with mature species of Lumbricus rubellus, Aporrectodea caliginosa, or Octolasion cyaneum earthworms at rates similar to their population density in the field. A non-inoculated, unreplicated control was also included. The number, biomass, and activity of the 3 species were then examined. CT scans were taken every 5-10 cm through the soil cylinders 20, 40, and 60 days after inoculation to measure burrow parameters. Mean pore area was greatest for O. cyaneum and L. rubellus, and least for A. caliginosa. Porosity produced by both L. rubellus and A. caliginosa declined with depth. L. rubellus was most active in the top 5 cm, whereas A. caliginosa was most active in the top 10 cm. O. cyaneum created its burrows relatively uniformly throughout the top 20 cm. No species created significant porosity below 20 cm. The greatest amount of porosity and number of pores were created in the cylinders inoculated with A. caliginosa. However, porosity created per earthworm was least for A. caliginosa and L. rubellus and greatest for O. cyaneum. Porosity per biomass was least for L. rubellus and greatest for A. caliginosa and O. cyaneum. A. caliginosa created mainly temporary burrows, with 72-85% of its burrows backfilled between scans. L. rubellus burrows lasted longer (56-64% backfilled between successive scans) and hydraulic conductivity measurements suggested that L. rubellus burrows were surface-connected and more continuous than those created by A. caliginosa. It appears that, of the 3 species studied, L. rubellus has the most beneficial effects on the measured soil physical properties.