ASSIMILATION OF LABILE CARBON AND PARTICULATE ORGANIC MATTER BY THE TROPICAL ENDOGEIC EARTHWORM PONTOSCOLEX CORETHRURUS (GLOSSOSCOLECIDAE, OLIGOCHAETA)

Endogeic earthworms belong to a very limited subset of soil fauna members able to utilize stabilized soil organic matter, though the mechanisms remain poorly known. Invasive endogeic earthworms Pontoscolex corethrurus reach remarkably high density in alluvial soil of a gallery forest along Dong Nai River in the Cat Tien National park, southern Vietnam. This soil is extremely poor in organic matter (C-org << 1%), and the population of P. corethrurus should likely rely either on root carbon or on the plant litter, partly comminuted by millipedes, termites and other primary decomposers. In a 28-days long laboratory experiment, we manipulated food availability to P. corethrurus by adding to the C-poor alluvial soil either litter fragments of different size or labile organic carbon (glucose). Both, litter and glucose were enriched in C-13, thus allowing monitoring of C assimilation in earthworm tissues. The assimilation of litter carbon was extremely poor even if very small (<0.25 mm) fragments were added, though earthworms gained ca. 40% of initial biomass when an equivalent amount of glucose-C was provided. Moreover, increased C availability allowed assimilation of indigenous soil carbon and nitrogen. These data suggest that endogeic earthworms can benefit from direct consumption of root-derived carbon, as well as from priming effect of root carbon on soil microorganisms.