Natural abundance of 15N and 13C in earthworms from a wheat and a wheat-clover field

The natural abundances of the stable isotopes of nitrogen (δ15N) and carbon (δ13C) were measured in plant shoots and in seven earthworm (Lumbricidae) species from a wheat and a wheat-clover cropping system. Variations in earthworm δ13C were generally small in these systems containing only C3 plants. Plant shoot δ15N ranged from −2.2‰ to −0.7‰ in white clover and from +0.9‰ to +6.6‰ in winter wheat. Intraspecific variability in earthworm δ15N was small. Earthworm δ15N was significantly related to the cropping system and to the ecological grouping of the earthworms. Mean δ15N values of all earthworm groups were significantly lower (by 3.5‰ to 4.4‰) in the wheat-clover field than in the wheat field. Within each of the two cropping systems, earthworm nitrogen isotope ratios differed significantly between ecological earthworm groups, with δ15N values decreasing in the order Allolobophora chlorotica and Aporrectodea caliginosa > Aporrectodea longa > Lumbricus spp. These results demonstrate for the first time that the δ15N signature of legumes can influence those of soil invertebrates. Because isotope measurements reflect assimilated tissue nitrogen, they offer novel insights into the feeding ecology and trophic positions of earthworms.