At present there are 441 operating mines in Australia mining commodities such as coal, mineral sands, iron ore, base, light and precious metals, uranium, and diamonds (Geoscience Australia, 2013). A significant number of these have an open cut or surface component to them. Open cut mines are usually easier, cheaper, and quicker to bring into production than underground mines, but often have a relatively short life-span, after which it may become necessary to move to underground mining to access deeper commodities.
Government authorization must be obtained to mine Crown or private land in Australia and land disturbed by mining activities must be returned to a sustainable post-mining land use. In more recent times, miners usually undertake progressive rehabilitation programs where mine closure planning is developed within the initial stages of mine operations rather than only being considered after mining ceases; however, Australia also has a legacy of unplanned mine closures, unsafe workings, hazardous mine sites, and unreclaimed lands, resulting from previously inadequate or non-existent mine closure practices and legislation (Smith, 2007). For example, in New South Wales alone there were more than 550 derelict mine sites requiring rehabilitation in 2008 (Nolan, 2008) where no individual or company is held responsible for their management or rehabilitation and no particular government agency has statutory responsibility for their rehabilitation (New South Wales Government, 2013). To minimize the risk of future derelict mines occurring, current mines are strictly regulated and must lodge a security deposit to cover rehabilitation costs if the mine becomes insolvent.
Land preparation prior to mining depends upon the mining technique to be adopted, the vegetation, and terrain. Surface or open cut mining involves a general process of removal of the topsoil layer and any other soil layers necessary to get to the substance that is being mined. The first stage of the mining process involves the removal of the existing trees and vegetation. Cleared timber and vegetation may be mulched and stockpiled to be re-used in rehabilitation. The topsoil is then removed and stockpiled and the soil layer below, usually referred to as overburden, is also removed and stockpiled separately. Once established, topsoil stockpiles may be revegetated until reused. When mining operations are complete, the overburden material is reapplied and contoured and then topsoil is reapplied, spread, and incorporated with the overburden to provide a planting medium. As post-mined soils often have a less developed structure, reduced organic matter, and lower nutrient contents than original soils, amendments are often made to change their physical and chemical properties. In particular, fertilizer addition is common practice and the addition of sewage sludge/biosolids and compost can also occur.
A major challenge in rehabilitating land that has been subjected to mining is re-establishment of a self-sustaining vegetative cover. This can be difficult because of the post-mining soils aforementioned reduced structure, organic matter, and nutrient content. As decomposers, soil microbial communities mediate critical ecosystem processes, and microorganisms are an important element for successful reclamation because of their role in nutrient cycling, plant establishment, geochemical transformations and soil formation. In addition, symbiotic nitrogen-fixing bacteria and mycorrhizal fungi are important microbial groups intimately involved in plant establishment on soils. Yet microorganisms generally remain an undervalued asset on mine sites. For example, prior to mine closure, companies need to demonstrate that they have rehabilitated the site to meet a number of predetermined criteria that usually fall into some general categories such as landform stability, topsoil, vegetation, fauna, water and safety, with the aim being to return disturbed land to a stable, productive, and self-sustaining condition and taking future land use into account. Microorganisms such as bacteria, actinomycetes, and fungi have an effect on most of these criteria, yet in Australia for example, they are not specifically included in any of them (Australian Government, 2006a). Furthermore, the mining industry’s governance in Australia is administered by the State and Territories and all State and Territories have their own mine closure policies which require mining companies to develop site-specific post-mining rehabilitation plans for approval by the relevant State authority as part of the development assessment process. Under Australia’s federal system, Commonwealth legislation via the Environment Protection and Biodiversity Conservation Act 1999, which came into effect in 2000, is only applicable in areas under national jurisdiction, on projects where matters of national environmental significance are concerned, such as mining operations in national parks, or which may have impacts on threatened or migratory species.
For the agencies with responsibility for sign-off, assessing the attainment of a self-sustaining restoration which emulates the structure, diversity, function, and dynamics of a specific ecosystem is more complicated than simply having an area which appears stabilized, contains growing plants, and a number of types of animals. Over time, for example, some of the fast-growing but short-lived plants may die, trees may cease to grow as the effects of initial fertilizer treatments decline, and the soil may not develop the desired texture or structure of the original substrate. Furthermore, a significant number of mines have unexpected or unplanned closures and a more integrated approach to mine closure planning can help achieve effective and earlier mine closure and completion (Australian Government, 2006a). Success criteria for rehabilitation need to be based on ecological principles and those based on a narrow set of vegetation indices or single chemical parameters have generally been found to be inadequate. A combination of attributes at both the landscape level and addressing more specific ecosystem properties are thought to be necessary (Australian Government, 2006b).
This paper will outline the importance of microorganisms to soil health and the rehabilitation of disturbed soils and how microbial community metrics can be used to monitor and quantify the soil microbial community status to demonstrate their recovery in rehabilitated soils. The focus is on decomposer soil microorganisms rather than symbiotic microorganisms, as the role of the latter in the rehabilitation of degraded and mined soils is better recognized in the industry and has been covered by other authors (e.g., Barea, Requena & Jiménez, 1996; Corbett, 1999).
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