The effects of three earthworm species on soil macroporosity and hydraulic conductivity

The burrowing characteristics of three common introduced European lumbricid earthworm species were studied in New Zealand agricultural soils. Large (30 cm internal diameter×50 cm high) cylinders were filled with topsoil (0–25 cm) and subsoil (25–50 cm), sieved to remove earthworms and other soil macrofauna. Cylinders were inoculated with either mature Aporrectodea caliginosa, Octolasion cyaneum (both endogeic species) or Lumbricus rubellus (an epigeic species) at the rates of 300 and 600 m−2. Additional control (nil earthworms) and mixed species treatments were also included in the experiment. Cylinders were buried in the field for six months before sampling. After six months, most of the introduced O. Cyaneum and L. rubellus earthworms had died, but significant populations of A. caliginosa were present in all treatments. However, it appears that the different species of inoculated earthworms had created contrasting burrowing patterns. L. rubellus was most active in the top 10 cm of the topsoil, although it did create burrows to at least 21 cm depth. L. rubellus did not create any burrows in the subsoil. Both A. caliginosa and O. cyaneum burrowed relatively evenly throughout the topsoil. Both species also created burrows at all depths sampled in the subsoil. Under saturated conditions, most water flow occurred through earthworm burrows. In the topsoil, burrows of A. caliginosa were more continuous and better connected to the soil surface than burrows of O. cyaneum. In contrast, O. cyaneum created the most continuous burrows through the subsoil.