MYCORRHIZA INOCULUM POTENTIALS IN TROPICAL SECONDARY SUCCESSION

Vesicular-arbuscular mycorrhizae can be indispensable for establishment and growth of tree seedlings in infertile lowland wet tropical soils. Persistence of mycorrhizal fungi after disturbance, however, is problematic to assess. We used a greenhouse bioassay employing Psidium guajava L. and Allium cepa L. to estimate the most probable number of mycorrhizal fungus propagules in two Costa Rican soils (Oxic Dystropepts) with different vegetation histories. We collected soils at the La Selva Biological Station from sites in secondary forest, abandoned pasture, and plots kept bare of vegetation for four and six years. Both bioassay hosts yielded positively correlated estimates of mycorrhiza propagule numbers. Median propagule estimates per 100 gm dry soil for the pasture site are 57 and 63; these estimates significantly exceed those for the other sites which range from 0.2 to 10 for bare plots and were estimated to be 0.6 and 10 for secondary forest. These bioassay estimates are positively correlated (Pearson r = 0.66 and 0.72) with counts of whole spores in these soils, but not with counts of sporocarps, spore clusters, or the most numerous, spores apparently empty of cytoplasmic contents or parasitized. Growth of both bioassay hosts in pasture soil significantly exceeded growth in soils from the other three sites in accord with the bioassays and whole spore counts.