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Characteristics of stockpiled topsoil at iron ore mines in the Pilbara Region of Western Australia

Skalski, Casey (2008) Characteristics of stockpiled topsoil at iron ore mines in the Pilbara Region of Western Australia. Honours thesis, Murdoch University.

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Topsoil is a critical component of the rehabilitation process as a source of seeds and other propagules, beneficial micro-organisms and nutrients which assist in the success of the rehabilitation project. To enhance rehabilitation success and maximum species return to a site, using fresh topsoil is preferable. However, this is not always feasible due to many reasons such as long distances between donor and recipient sites or a disturbed area not being ready for rehabilitation at the time of topsoil collection (as in open cut mines). Therefore, in iron ore mining in the Pilbara soil must be stockpiled, sometimes for extended periods of time. The process of stockpiling has been reported previously to generally cause a decline in the quality of the topsoil which has the potential to adversely affect the success of rehabilitation projects. However, there is little information on the impact of topsoil storage on the soil's physical, chemical, biological and seed bank properties in arid regions, specifically that of the Pilbara region of Western Australia. Based on this lack of knowledge, the objectives of this study were to:
1. Investigate the quality of topsoil from iron ore mines in the Pilbara in existing storage stockpiles with particular focus on the depth of material within the stockpile.

2. Investigate the effect of topsoil recovery and handling on its chemical, physical, biological and seed bank properties for revegetation.

The above mentioned objectives were achieved through four experiments:
Experiment one investigated the physical, chemical and biological properties of topsoil via laboratory analysis of stocked topsoil of different ages and of soil located at different depths within the stockpile.

Experiment two investigated the differences in soil seed bank of stockpiled topsoil of different ages and different depths within the stockpile through a germination study in a constant temperature room.
Experiment three investigated the impact of topsoil and subsoil mixtures on the germination and early growth of two test species, Acacia ancistrocarpa and Senna glutinosa in a glasshouse pot trial.

Experiment four investigated the physical, chemical and seed bank impacts of the current Rio Tinto topsoil recovery and stockpiling procedure on topsoil using laboratory analysis and a seed bank study of samples collected during the recovery and windrowing stages of the recovery process.

The major findings of this study were:
Stockpiling does produce measurable changes in soil physical, chemical and biological properties, but the impact appears to not be as detrimental to plant growth as has been postulated previously.

Depth of topsoil within the stockpile had minimal effects on the soil's physical (particle size) and chemical properties (pH, electrical conductivity, organic matter, Colwell extractable P, KCl-40 extractable S, exchangeable cations and extractable micronutrients).

Low levels of P, Sand Zn in stockpiled topsoil, and in subsoil from reference sites suggest that these nutrients may restrict plant growth in revegetation.

In both undisturbed topsoil and stockpiled topsoil there was a low mean seedling emergence in comparison to carrot seeds grown in the same medium. The very low levels of germinable seed suggests a limited soil seedbank is present in topsoil. However, it is possible that germination cues need to be applied to achieve seed germination.

Dilution of topsoil with different proportions of subsoil (up to 95 %) produced no specific effects on shoot biomass of Senna glutinosa, while > 20 % subsoil only weakly decreased shoot growth of Acacia ancistrocarpa. However, neither legume formed effective nodules during the experiment and this may have limited the plant responses to soil mixes.

- Windrowing of topsoil did not significnatly affect topsoil physical, chemical and seed bank properties.
Based upon the above mentioned findings, the following courses of action are recommended to enhance the success of topsoil re-use in environmental rehabilitation:

- Ensure that procedure is followed. It was noted that at one of the mine sites from which soil was sampled, two of the topsoil stockpiles were higher than the recommended height (as mentioned in both DMP and Rio Tinto guidelines) and not sign posted indicating that some aspects of the Rio Tinto topsoil handling procedure had not been followed. Therefore, closer monitoring of stockpiling activities should be in place to ensure that the stockpiles do not exceed the height limit and are signposted to assist with record keeping.

- Encourage seed growth on stockpiles. Plant growth on the stockpiles will assist to replenish soil seed bank, encourage soil microbial activity and assist in the cycling of nutrients in the soil. All these factors are important to ensure the topsoil remains viable during its storage period. The more viable the topsoil is when it is re-used in rehabilitation, the more likely plant growth on the rehabilitated site will be successful.

- Further research on topsoil/ subsoil mixing. This study indicated that a mixture of topsoil and subsoil at Marandoo mine did not adversely affect the emergence and early growth of the selected test species. As a result, further research should be conducted to determine if this result can be replicated in other species. If so, and if longer term plant growth is similarly unaffected by subsoil mixtures with topsoil, such mixtures could be used in rehabilitation of areas where topsoil is limited.

- Seedbanks in soils and cues for seedling germination. The low levels of seedling emergence from topsoils, even the reference site at Marandoo and Tom Price mines, suggests that little seed is stored in topsoil of the vegetation complexes subject to mining. This needs to be verified by physical recovery of seed from topsoil across a wider range of sites that are likely to be mined in future. If seed exists in the topsoil, but does not germinate, the main cues for germination need to be determined. If limited seed can be recovered from topsoil, then the use of topsoil in rehabilitation switches from it being a source of seed to being a medium for plant growth. Seed collection and broadcasting technologies would then be much more important considerations for Rio Tinto in its rehabilitation planning.

- Nutrient management. Low levels of P, Sand Zn were extracted by standard soil tests on topsoil in stockpiles and windrows. Even though the topsoil from reference sites was also low in these nutrients, and subsoil even lower, further consideration of these nutrients seems warranted. Leaf and soil sampling of revegetation areas previously completed on the iron ore mines in the Pilbara would be useful to ascertain whether these nutrients are limiting plant growth. From this survey, decisions could be made about the necessity of adding fertiliser in rehabilitation to ensure adequate supply of limiting nutrients. However, it is possible that the native plants are well adapted to acquisition of nutrients from soils with low availability of nutrients.

Item Type: Thesis (Honours)
Murdoch Affiliation: School of Environmental Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bell, Richard
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