It is well understood that gold mine tailings are commonly characterised by acidic conditions with pH levels below 5, due to the presence and oxidation of iron pyrites (Cooke & Johnson, 2002:49). Elevated concentrations of metals such as Manganese, Copper, Zinc, Cadmium, Nickel, Iron, Mercury and Arsenic are also commonly associated with the aforementioned tailings (Mulugisi et al. 2009:512). Platinum tailings have been reported to be characterised by increased salinity and alkalinity (van der Walt et al. 2012:103). Both Mendez & Maier (2008:278) and Cooke & Johnson (2002:49) explain that mine tailings in general are practically void of macronutrients and organic matter, that it supports a seriously strained microbial community and does not have a sufficient soil structure. According to Cooke & Johnson (2002:49) the absence of organic matter in fine textured soils, as is regularly the case with mine tailings, has the potential to cause severe soil compaction, high bulk densities, reduced water infiltration rates and waterlogging at the soil surface. Nel et al. (2014:97) elaborates that the adverse chemical and physical properties regularly encountered with gold and other tailings, inhibits natural vegetation colonisation and increases the difficulty of successfully establishing sustainable vegetation communities on mine tailings if it is used as a growth medium, this is also supported by Cooke & Johnson (2002:49).
Degradation of the environment caused by gold tailing storage facilities (TSF) commonly spreads much further than the waste site boundaries. This principally occurs by means of wind and water erosion that then leads to the pollution of surrounding air and soils as well as the contamination of water sources such as dams, streams, rivers and underlying sediments. Environmental aspects that are greatly influenced by TSF erosion and gold mining acid mine drainage include nutrient cycling of soils, regeneration of vegetation and the biogeochemical cycling of toxic elements (Weiersbye et al. 2006:101).
Erosion and dust control at a mining site is a critical output of the rehabilitation process that the contracted rehabilitation company must ensure; it is also one that is required by law to be carried out by the most practical means necessary (Weiersbye et al., 2006:101). The establishment of grass mixtures on mine tailings is considered in the industry as the best method, being practical and effective, to accomplish erosion and dust control; this practice is also referred to as “grassing” or as “phytostabilisation” (Weiersbye, 2006:101; Mendez & Maier. 2008:279 ).
Phytostabilisation entails the use of stress tolerant plants to stabilize tailings in the plant rhizosphere, reducing erosion and immobilizing heavy metals hosted within specific tailings. Subsequently the heterotrophic microbial community will be improved, that will likely result in improved vegetation establishment and growth on tailing materials (Cooke & Johnson, 2002:49; Mendez & Maier. 2008:279). Grassing implies the use of grass species for phytostabilisation (Weiersbye, 2006:101) Mendez & Maier (2008:279) also state “The ultimate objective for successful; phytostabilisation is the long-term succession of the plant community in mine tailings to promote soil development processes, microbial diversity, and finally, to restore soil ecosystem functions to a state of self-sustainability.”
Considering that mine rehabilitation is a major industry in South-Africa. An article in the Mining Weekly magazine published on the 6th of June 2014 cites the views of environmental advisor Dr. Anthony Turton. He predicts that according to statistical and environmental data, major environmental rehabilitation business opportunities are set to arise during the next decade in the Witwatersrand mining complexes due to gold mines being pushed towards insolvency (Mavuso, Z. 2014). Hence a need for improved vegetation seeding technologies arises.
Turner et al. (2006) as cited by Nel et al. (2014:97) elaborates that seed coating technologies are able to offer assistance in the establishment of vegetation on mine tailings, by improving the micro environment surrounding the seed. Tow & Lazenby (2001:1) also explain that a proper understanding of plant competition and succession is required for effective rehabilitation and appropriate management as these principles are directly relevant for maintaining a desired vegetation composition. Bradshaw (2002:5) supports Tow & Lazenby (2001) and explains that we are required to understand more accurately how the ecosystem, which we wish to create works by not only studying critical aspects of its structure, but to also strive to understand its function.
This study focuses on the establishment and succession of three perennial grasses Cynodon dactylon, Digitaria eriantha, Eragrostis curvula, one annual grass Eragrostis tef, a nurse crop Sorghum bicolor and a perennial legume Medicago sativa. These species are used together in five different seed mixtures, three coated and two uncoated that are sown on artificial plots at two gold mine sites and in platinum tailings. Supporting pot trials were conducted with the same growth mediums in order to determine the growth potential of the species within the different tailing growth mediums.