Assimilation of labile carbon and particulate organic matter by tropical endogeic earthworms Pontoscolex corethrurus (Glossoscolecidae, Oligochaeta)

Endogeic earthworms often dominate in biomass in the soil macrofauna of temperate and tropical zones. Representatives of this morphoecological group play a unique role in ecosystems due to their ability to utilize stabilized organic matter from mineral soil horizons, but the mechanisms underlying this ability are still poorly understood. Endogeic earthworms Pontoscolex corethrurus reach a remarkably high density in the alluvial sandy soil of a gallery forest along the Dong Nai River in the Cat Tien National Park, southern Vietnam. This soil is extremely poor in organic matter, and the earthworms appear to depend on labile carbon from plant roots and detritus. In a 28-day laboratory experiment, food availability to P. corethrurus has been manipulated by supplementing the soil either with different size fractions of plant detritus or with labile organic carbon (glucose), both enriched in C-13 in order to monitor carbon assimilation in earthworm tissues. The results have shown that P. corethrurus earthworms fail to utilize carbon from plant debris but effectively assimilate glucose carbon even when it is added at a very low concentration. Thus, the high abundance of these earthworms in poor sandy soils is apparently explained by their ability to assimilate root-derived labile carbon. Moreover, the presence of carbon available to soil microorganisms allows the earthworms to utilize native soil organic carbon and nitrogen.