Mycorrhizas and Other Specialized Nutrient-Acquisition Strategies: Their Occurrence in Woodland Plants From Kakadu and Their Role in Rehabilitation of Waste Rock Dumps at a Local Uranium Mine

The presence of mycorrhizas, proteoid roots and leguminous nodules was determined in a range of woodland species (from a variety of habitats in soils formed on different parent materials) in the Kakadu area in the monsoonal tropics of northern Australia. In addition, the chemical fertility and the occurrence of mycorrhizal fungi and rhizobia in rudimentary soils ('minesoils') forming in situ on waste rock dumps at a mine site in the region were compared with stockpiled topsoils from the mine area and undisturbed topsoils collected from the surrounding native woodland. A major aim of these investigations was to assess the feasibility of rehabilitating the waste rock dumps without spreading topsoils. More than 90% of the woodland flora examined had one or more specialised nutrient-gathering mechanism. Mycorrhizas were found on 82% of the species, with some 16% of species having both ecto and VA mycorrhizas, often on the same individual plant. Many of these observations are the first records of mycorrhizal infection in the particular genera and species involved. Soil baiting and dilution experiments showed that rhizobia and mycorrhizal fungi were ubiquitous components of the soil biota in all undisturbed woodland soils. However, they were absent or poorly represented in the stockpiled topsoils and in some of the rudimentary soils formed in waste rock at the mine site. The diversity of spore types and/or numbers of infective propagules of VAM fungi was lower in stockpiled topsoils and in minesoils than in the undisturbed woodland soils. Nutrient omission experiments identified that acute deficiency of P, and to a lesser extent N, was a limitation to growth of seedlings on all soils. Zn deficiency was detected in the only soil for which this was examined. A glasshouse experiment, using a young minesoil and application of basal nutrients, demonstrated that inoculation of Acacia holosericea seedlings with rhizobium could completely alleviate the effects of N deficiency. Under conditions of both N and P deficiency, dual inoculation of A. holosericea with rhizobium and spores of the VAM fungus, Glomus, only partly overcame the limitations of P deficiency on seedling growth. Induction of deficiencies of P and Zn in a second minesoil (through application of basal nutrients), demonstrated that inoculation of seedlings of Eucalyptus pellita with spores of the ectomycorrhizal fungus, Scleroderma, partly alleviated the effects of both deficiencies. Rehabilitation strategies implemented at the mine site using either soils forming in situ on the waste rock dumps, or by spreading stockpiled topsoils, will need to ensure deficiencies of P and other nutrients are alleviated and that viable populations of mycorrhizal fungi and rhizobia are introduced and maintained during early phases of vegetation establishment.