This thesis documents a study dealing with the rehabilitation of land disturbed by mining at the Curragh open-cu t coal mine, in central Queensland. The aim of the study was to investigate the feasibility of using direct seeding to establish simultaneously a quick-growing cover crop, for erosion control, and a mixture of native tree and shrub species, for long-term stability.
The main materials available for growth media following recontouring at the mine are in situ overburden spoil and the local brown clay, either freshly- stripped or sourced from stockpiles. Chemical analysis of these materials indicated that improved pasture species could be expected to respond to the addition of N, P, Zn and S on the freshly- stripped brown clay and to the addition of N, P and Zn on the stockpiled brown clay. The overburden spoil was predicted to be deficient in N, P and K.
To further investigate the nutrient status of these materials, a nutrient omission trial and a rate trial were conducted under glasshouse conditions. In the nutrient omission trial, large increases in the yield of Rhodes grass (Chloris gayana) were obtained with the addition of N, P, S and Mo on the freshly- stripped brown clay. On the overburden spoil, large increases were obtained with the addition of N and P. In the rate trial, on both these media, large increases in the yield of both Rhodes grass and Japanese millet (Echinochloa crus-galli) were obtained with both N and P addition. Yield plateaus were obtained for both these nutrients, and at high levels of N and P, the yields on the two media were very similar.
The findings of the glasshouse trials were used in the design of the Broadcast Seeding trial and the Tree Planting Trial, both large-scale field trials located at the mine. The Broadcast Seeding Trial included the following treatments combined factorially: Seven growth media (including fresh and stockpiled soil at different depths overburden spoil with and without gypsum and coal reject), two cover crops (Rhodes grass and Japanese millet) at five seeding rates, four fertiliser rates, and flat and sloping sites. A standard application of native tree seed mix was common to all plots, except several control plots.
Twelve weeks after establishment of the Broadcast Seeding Trial, the soil media bad markedly greater cover and total herbaceous yield than did the non-soil media, reflecting vigorous weed growth on the former. At 30 weeks, Rhodes grass had become well established on the non-soil media, and differences in yield and cover were therefore substantially reduced. On both occasions, the non-soil media supported much greater tree seedling densities than the soil media, which showed very low rates.
Ground cover increased significantly with increasing fertiliser rate on the overburden spoil and coal reject media in the Broadcast Seeding Trial. Ground cover was not significantly related to fertiliser rate on any of the soil media. Total herbaceous yield increased significantly with increasing fertiliser rate on the overburden spoil, but not on any other media. A response in tree seedling height to fertiliser addition was obtained on all the non-soil media, but not on any of the soil media.
Increased Rhodes grass seeding rate resulted in increased per cent cover and Rhodes grass yield on all the non-soil media in the Broadcast Seeding Trial. This result was attributed to the low level of competition from other species on these media. On the 15 cm stockpiled soil, both per cent cover and Rhodes grass yield increased significantly with increasing seeding rates. No response to seeding rate was obtained on the other soil media. This difference was attributed to the occasional patches of exposed overburden spoil present on the 15 cm treatment. Total tree seedling density decreased significantly with increasing Rhodes grass seeding rate on all the non-soil media, but not on any of the soil media (density being uniformly very low). Japanese millet establishment was extremely low for all treatments, most likely the result of an unfavourable water regime during the germination phase.
The results of the Broadcast Seeding Trial showed that the use of direct seeding is a practical cost-effective option in the rehabilitation of Curragh coal mine. The very poor tree establishment rates on the soil media indicate that areas of bare overburden spoil must be provided in site rehabilitation, or else weed control must be implemented.
The aim of the Tree Planting Trial was to screen a range of native tree and shrub species for use in site rehabilitation at Curragh mine. Plant establishment was by hand-planting of young seedlings, rather than by direct seeding, to ensure more uniform growing conditions and adequate experimental replication. Treatments included two growth media (soil and overburden spoil), 24 tree and shrub species, and three fertiliser rates.
Survival and growth rates of trees on replaced brown clay were markedly less than on bare overburden spoil. This difference was attributed to the much greater site competition on the brown clay from heavy weed growth. The higher salinity and sodicity on the overburden spoil did not appear to prevent good growth rates for many species.
Important differences in growth and survival rate were observed between species, and allowed the categorisation of species into two groups, viz. (1) good survival and growth on overburden spoil and (2) good survival on brown clay (tolerance of weed competition). The trial showed that a wide range of species is available for rehabilitation of the Curragh mine.
The research program identified the major factors that need to be addressed in achieving a cost-effective rehabilitation strategy for the Curragh coal mine. Additionally, it provided information applicable to the rehabilitation of other coal mines in central Queensland and identified areas for additional research.