Mycorrhizas in the Kakadu region of tropical Australia .2. Propagules of mycorrhizal fungi in disturbed habitats

Bioassays were used to provide estimates of both the distribution and abundance of mycorrhizal fungus propagules in soils from mine sites and natural habitats in tropical Australia. These bioassays measured mycorrhizal formation by bait plants that were grown in intact cores of soil collected along a transect at each site. The mine sites included, had sparse, patchy or dense vegetation cover on waste-rock materials that would initially have been devoid of mycorrhizal fungus propagules. The natural habitats included eucalypt savanna sites which had been subjected to hot annual fires for 4 years or remained unburnt. Propagules of vesicular-arbuscular mycorrhizal (VAM) and ectomycorrhizal (ECM) fungi occurred in all sites, but were sporadically distributed in highly disturbed areas, where they were associated with patches of vegetation. Both the relative abundance and frequency of occurrence of inoculum of VAM and ECM fungi increased with vegetation cover in older disturbed sites. Propagules of VAM fungi were substantially more numerous in some mine site habitats with dense vegetation cover, than in adjacent natural habitats. The presence of substantial amounts of phosphorus, released from weathering rock, in mine waste rock materials may have reduced the requirement of plants for mycorrhizal associations during vegetation establishment in mine sites. In woodland sites, hot annual fires appeared to restrict ECM fungi to infrequent patches in the surface horizon, while ECM fungus inoculum was much more frequently detected in soil from the unburnt site. These fire regimes apparently had less effect on the distribution or quantity of VAM fungus inoculum.