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 (δ 15 N) and carbon (δ 13 C) were measured in plant shoots and in seven earthworm (Lumbricidae) species from a wheat and a wheat-clover cropping system. Variations in earthworm δ 13 C were generally small in these systems containing only C 3 plants. Plant shoot δ 15 N ranged from −2.2‰ to −0.7‰ in white clover and from +0.9‰ to +6.6‰ in winter wheat. Intraspecific variability in earthworm δ 15 N was small. Earthworm δ 15 N was significantly related to the cropping system and to the ecological grouping of the earthworms. Mean δ 15 N 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 δ 15 N values decreasing in the order Allolobophora chlorotica and Aporrectodea caliginosa > Aporrectodea longa > Lumbricus spp. These results demonstrate for the first time that the δ 15 N 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.