Reconstructing Functional Topsoil

This has been a key focus of research by the PEU since becoming involved with mine rehabilitation in 1996. We have investigated the capacity of remnant forest and pasture soil, as well as spoil, to support plant root-microbe associations that are the natural nutrient providers to plants and obviate the need for excessive use of fertilizers.

Early studies focussed on mycorrhizal fungi and nodulating bacteria. The former exchange phosphorus with plants for carbon, the latter convert nitrogen in the air to organic nitrogen and exchange it with plants for carbon. These associations are essential for sustainable vegetation community development. All native peas, wattles, and she-oaks have nitrogen fixing microbe associations, and about 80% of all native species have fungal associations for nutrient acquisition and other ecosystem services such as nutrient cycling and drought tolerance.

Bacterial associations

The bacteria under study have mostly been from the rhizobia group. These bacteria nodulate native peas and wattles and provide organic nitrogen, in doing so, they help develop nutritional balance in native vegetation communities. For this reason, peas and wattles are commonly used as colonizing species on mine spoil. In our research we cultured many isolates. We are determining how many species are present using DNA sequencing and restriction fragment gel patterns (below).

We have also been testing for specificity in individuals and groups of plant species, and are developing an inoculum to re-establish these microbes where they have been lost due to soil erosion.

Fungal associations

Mycorrhizal fungi fall into two groups, the endomycorrhizae (arbuscular mycorrhizae) and ectomycorrhizae. The ectomycorrhizae form puff balls and other reproductive structures (see above) that release spores into the air. These spores are wind dispersed and when forest soil is available, they readily spread at the Model Site. Endomycorrhizae are transmitted from plant to plant or as spores in the soil. In our studies of stockpiles, pasture soil and soil in the forest remnant, we found many samples with no endomycorrhizae and a few with up to 10% of that which would be expected in an old growth forest. These associations, vital to plant nutrient acquisition, pathogen resistance, drought tolerance, and other ecosystem services, have been largely destroyed by erosion believed to be due to clearing of native vegetation and grazing by hoofed animals.

Our studies show that these microbes can be reintroduced to mine sites and when organic matter is provided (sewage sludge) it is possible, so far, to rebuild levels towards 50% of an old growth forest. Recent studies (in association with the University of Sydney) are also demonstrating the value of these and other free-living microbes in the aggregation of soil, which provides better soil structure and function.