Carbon food resources of earthworms of different ecological groups as indicated by 13C compound-specific stable isotope analysis

One of the challenges in soil ecology is to determine which organisms utilise stable forms of carbon in soil. Recent studies have indicated that endogeic earthworms are able to mobilise such stable carbon compounds. However, it remains unclear which particular compounds of stabilised carbon in soil are utilised by earthworms. Furthermore, current knowledge on ecological groups and food resources of earthworms is mainly based on direct observations, hence only reflecting what was ingested but not what was actually assimilated. We analysed seven earthworm species in beech and spruce forests and the associated litter, earthworm middens and soils, and employed compound-specific 13C stable isotope analysis of fatty acids (FAs) to identify the origins of carbon resources of the earthworms. To relate food resources to carbon compounds of different stability, we analysed 13C signatures of FAs of different particle size fractions. FA δ13C profiles of epigeic and endogeic earthworm species indicated assimilation of recently-fixed and recalcitrant carbon resources, respectively, whereas anecic earthworms assimilated a mixture of resources of different stability. Utilisation of carbon resources did not differ between beech and spruce forests. Endogeic species were associated with neutral lipids of soil particle size fractions with δ13C signatures of the bacterial marker a15:0 in earthworms resembling those of the clay fraction. This suggests that they assimilated carbon associated with small particle size fractions attached to clay-humus complexes. The results showed that earthworms of different ecological groups utilise carbon pools of different origin and stability. As indicated by neutral lipids of bacterial origin, physically-stabilised organic matter appears to contribute to the nutrition of endogeic earthworms in forest systems.