A dynamic study of earthworm feeding ecology ­using stable isotopes

Changes in the specific diet of earthworms with time in relation to landuse changes and two different climates were studied by analysing ¹³C and ¹⁵N natural abundance in soils and animals. Soil samples from three depths (0-10, 10-20 and 20-30 cm) and earthworms were collected from two sites: Santiago (Northwest Spain) and North Wyke (Southwest England) both consisting of replicated long-term grasslands and recently converted to maize plots. Earthworms were hand-sorted in the field at the peak of the maize growth and after harvesting at both sites. In the Spanish plots, nine and eight earthworm species, all belonging to the three ecological categories (epigeic, anecic and endogeic), were found under maize and permanent pasture, whereas at the English site five and seven different species were, respectively, identified. At both sites ¹³C isotopic values of the earthworm tissues reflected changes in diet from C ₃ to C ₄ with epigeic and epi/anecic worms in the maize plots showing one delta unit difference in relation to the ones found in the grassland plots. Anecic worms seemed to be less responsive to landuse changes. The higher ¹³C values of the Spanish soils were also reflected in the earthworm tissues when compared with the English samples. ¹⁵N values showed no clear relationship with the cropping treatments but were clearly related to the ecological grouping, with endogeic worms reaching the highest values whereas for the epigeic and epi/anecic species the lowest values were obtained. This finding was also previously recorded by other authors and suggests that, in the future, stable isotope techniques could also be a useful tool in taxonomic studies.