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Spoil Amelioration Experiment

Funding
Thiess Pty. Ltd.

Background
Open-cut coal mine spoil is essentially devoid of biological organisms, including symbiotic microbes involved in sustainable nutrition. It is also often deficient in available plant nutrients, can have extremes of pH and can exhibit surface crusting. In order to reconstruct a self-sustainable native forest or woodland ecosystem on spoil requires some amelioration strategies to produce more favourable plant growth conditions.

Aim
To determine whether inoculation of spoil with symbiotic microbes, and the use of fresh forest topsoil, fertilizer, gypsum or biosolids (sewage sludge) would overcome adverse spoil properties, to support the establishment of a selection of native forest species.

Summary of Results
Mount Owen mine spoil (Hunter Valley, NSW, Australia) was found to be primarily deficient in nitrogen and to a lesser extent phosphorus in supporting seedling growth of endemic species. These nutrients can be supplied either through biological interactions with soil microbes, or by applying them to the spoil surface eg. in the form of fertilizer.

Two important soil microbes are rhizobia bacteria and mycorrhizal fungi.  These microbes form symbiotic relationships with certain plants, helping them to access primarily nitrogen and phosphorus respectively, in exchange for photosynthetic carbon. Inoculating Acacia species growing in spoil with nitrogen-fixing rhizobia bacteria increased plant growth. A strain of rhizobia was isolated which successfully grows in spoil and forms symbiotic relationships with at least two different Acacia species. Inoculation of Corymbia maculata with mycorrhizal fungi increased root infection levels but did not result in increased plant growth. This may have been because nitrogen was more growth limiting or some level of fungi/plant specificity, which was not investigated.

Fresh forest topsoil was the most successful spoil ameliorant, in terms of native species richness and vegetation structure. This outcome was primarily due to the provenance seed bank contributing 61% of final plant numbers and 46% of species richness. The topsoil did need to be supplementary seeded with canopy, sub-shrub and herb species, as these were not very prevalent in the topsoil seed bank after years of extensive grazing in the forest.

The relative conditional probability  method was used to identify that application of 400 kg/ha fertilizer was the most successful of the other spoil amelioration treatments. It supported the highest rates of seedling emergence, establishment and survival over four years; compared to spoil with only 50 kg/ha of fertilizer added. It was also the only treatment with significantly higher species richness during the first three years of rehabilitation. Gypsum tended to increase establishment and survival overall, whilst a combination of gypsum and 400 kg/ha fertilizer application generally produced an intermediate response between the gypsum plus 50 kg/ha fertilizer and the 400 kg/ha fertilizer treatments. Biosolids plots had high numbers of grasses and weeds, which may have resulted in excessive competition. The canopy heavily dominated the vegetation structure of biosolids plots and this may have shaded lower storeys.

This experiment has been monitored for 8.5 years to date, and in recent years a number of species have successfully reproduced and progeny recruited. This is a good sign for the sustainability of these systems over time. As a result, the plant density of seeded species has increased in all treatments since the seven-year survey. The biosolids plots had the least number of new seedlings at the most recent survey, as there were fewer reproducing parent plants than in other treatments.

Conclusions
If forest topsoil is available, spreading it over the spoil is the best way of reconstructing a forest ecosystem. Using 400 kg/ha fertilizer had an initially positive response on native plant density and species richness, but the effect dwindled over time. Restoring soil microbes to the system would be a long-term solution to overcome poor soil nutrient content, rather than regularly re-applying fertilizer. Biosolids has potential as a longer-term source of nutrients, but needs to be managed in a way that does not allow excessive grass and weed competition with native species.