Transmission of infective Frankia (actinomycetales) propagules in casts of the endogeic earthworm Pontoscolex corethrurus (Oligochaeta:Glossoscolecidae)

A glasshouse experiment was used to investigate the transfer of infective Frankia propagules in soil by the endogeic earthworm Pontoscolex corethrurus. Seedlings of Casuarina equisetifolia were inoculated, either with a crushed nodule suspension of Frankia, or with casts of P. corethrurus that had been raised in a sterilized soil through which crushed nodules of C. equisetifolia had been thoroughly mixed. Two concentrations of cast material were applied: casts from 3 earthworms and casts from 11 earthworms. Uninoculated plants grew poorly and did not nodulate. All seedlings inoculated directly with the crushed nodule suspension formed nodules; shoot dry weights of these seedlings 10 weeks after inoculation were 70% higher than those of the uninoculated plants. All plants inoculated with casts from 11 earthworms formed nodules. Shoot and nodule dry weights of these seedlings were similar to those of the plants that were directly inoculated with the crushed nodules. Decreasing the number of earthworm casts applied to 3 per plant substantially reduced plant growth compared to the higher cast concentration. Nodulution was erratic in this treatment with only 3 of the 7 plants forming nodules. Shoot dry weights of plants inoculated wih 3 casts were similar to those of the uninoculated treatment. The possible ecological significance of these findings is discussed in relation to the ecology of earthworms in soil.