EARTHWORMS AS VECTORS OF VIABLE PROPAGULES OF MYCORRHIZAL FUNGI

Casts of 13 earthworm species with differing ecological strategies were collected from more than 60 sites and examined for the presence of propagules of vesicular arbuscular mycorrhizal (VAM) fungi. Intact spores of VAM fungi were found in all but one collection. VAM root fragments were also present in some cast samples. The diversity of VAM spore types found in casts was similar to that in adjacent, uningested soils. However, the relative concentration of spores (No. g-1 dry wt) in casts was higher than in the general soil for the two earthworm species (Pontoscolex corethrurus and Diplotrema heteropora) for which this was assessed. There was a strong seasonality in the number of spores recovered from casts of D. heteropora and this reflected seasonal patterns of spore production in soil. Glasshouse experiments demonstrated that spores recovered from casts of P. corethrurus and D. heteropora maintained viability and initiated mycorrhizal infection on Sorghum bicolor. Similarly, some VAM root fragments found in casts were also able to infect S. bicolor. Spores of ectomycorrhizal fungi and fragments of ectomycorrhizal roots were observed at low frequencies in some cast samples from sites dominated by sclerophyllous vegetation. The infectivity of these propagules was not determined. The ubiquity and ecological significance of short distance transport of VAM propagules by earthworms is discussed in relation to the ecological strategies of earthworms.