Use of Legumes in the Revegetation of Mine Wastes
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Introduction Mine waste heaps and overburden materials are usually barren, and not colonised by natural revegetation. These materials are thus susceptible to leaching, wind and water erosion, resulting in pollution of the environment. Vegetation establishment on these wastes is the most environmentally appropriate method of controlling all of the above pollution problems (Bradshaw and Chadwick, 1980). In addition to this, the establishment of a mixture of indigenous plant species on such wastes could be considered as being most appropriate. However, most mine wastes contain both nutrient deficiencies and mineral toxicities which inhibit the growth of plants. A combination of addition of essential nutrients with selection of species tolerant to toxicities offers a logical approach to revegetation problems. Establishment of a self-sustaining permanent plant cover, which would require little aftercare is a practical necessity in such re-vegetation efforts. For nutrients which are adsorbed by the wastes (e.g. phosphorus), it is easy to envisage that a single large application would suffice for numerous years of continued plant growth. For nutrients which are easily leached (e.g. nitrogen), it will be necessary to make regular small applications for a number of years (i.e. until organic N recycling is adequate). Because mine wastes contain no organic matter, they are often totally devoid of nitrogen (Bradshaw et al., 1982). The continuous long-term addition of fertilizer N that would be required for sustained vegetative growth over numerous years is a serious management problem. The inclusion of leguminous nitrogen-fixing species in re-vegetation attempts offers a solution to this nitrogen problem (Jefferies etal., 1981). Such legumes could survive without nitrogen fertilization, and would eventually contribute N to the entire ecosystem. In fact, the inclusion of N fixing species is the only reliable way of ensuring the establishment of a self-sustaining ecosystem in a situation where organic and mineral N are non-existent. Low input approaches to re-vegetation of mine wastes include: direct planting onto the waste substrate, the use of hardy, climatically adapted species having low nutritional requirements; the use of species tolerant of any toxic substances present; and the inclusion of nitrogen-fixing plants to provide a sustainable source of nitrogen.