Introducing deep burrowing earthworms (Lumbricus terrestris L.) into arable heavy clay under boreal conditions

An inoculation of the deep burrowing earthworm Lumbricus terrestris L. was carried out for soil amelioration purposes at an arable field on heavy clay in southern Finland. L. terrestris, whilst entirely absent from the field and its immediate surrounding, was present in many other local soils. Prior to inoculation, field management was changed in favour of L. terrestris by improving the subdrainage and by implementation of reduced tillage. In Autumn 1996, 82 earthworm inoculation units (EIUs) were introduced in three transects at one end of the field, one transect lying within a permanent grass strip outside the cultivated area. In Autumn 1998, L. terrestris had persisted outside the cultivated area with maximum density of 28 ind. m−2 (median: 0 ind. m−2). No individuals were found inside the field. In Autumn 2003, maximum density was 43 ind. m−2 (median: 9 ind. m−2) outside the cultivated area, with evidence for 8 m dispersal from inoculation points (1.1 m year−1). In 2003 individuals were also found in low densities inside the field, close to subdrains, with a maximum density of 4 ind. m−2. Results call for critical evaluation of this inoculation practice.