Abundance of the earthworm Lumbricus terrestris in relation to subsurface drainage pattern on a sandy clay field

Subsurface drainage induces systematic spatial variability in soil properties which may be reflected in the abundance and distribution of soil organisms. We compared the population density of the deep burrowing earthworm Lumbricus terrestris L. above and between tile subdrains in 41 sample pairs on an eight hectare grass field. Above the drains the median number of  individuals was twice as high and their total fresh mass five times as high as between the drains (4.5 vs. 2.1 individuals m^-2  and 9.6 vs. 1.9 g m^-2, respectively). The mean difference (above drain – between drains) was 2.5 individuals m^-2  (95% CI = 1.0 to 4.0) and 6.6 grams m^-2 (95% CI = 3.6 to 9.6). The relatively larger difference in fresh mass was due to a high proportion of adult individuals above drains. One likely explanation for the pattern of abundance is that the lowered water table level near the drains provides an environment beneficial for the population growth of L. terrestris. Due to the role of L. terrestris burrows as flow paths of percolating water the observations may have implications on subdrain function.