Mine tailings contain a significant amount of heavy metals/toxic waste and poses a risk to adjacent vegetation and water bodies (Fellet et al. 2011). A long-term and cost-efficient method called phytoremediation is used to employ flora to decrease the detrimental effects of toxic waste found in mine tailings (Fellet et al. 2011).
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The use of vegetation to restore mine tailings will be explored through two scientific journal articles below. Article 1: Wu et al. (2011). Mycorrhizo- Remediation of Lead/Zinc Mine Tailings using Vetiver: A Field Study. This study was taken place in China and Wu et al. (2011) conducted a field study using Vetiver grass (Chrysopogon zizanioides) in attempts to remediate mine tailings. Vetiver grass is known to be a perennial grass originating from hotter climates. Although it was introduced in China, some argue that vetiver grass contains many uses such as soil stabilization and protection against soil erosion since its root system is usually more than 3 meters deep (Greenfield 1993 and Truong 2002 in Wu et al. 2011). But more importantly, recent studies state that vetiver has great potential to recover mine tailings. Wu et al. (2011) hypothesized that arbuscular mycorrhizal fungi (AMF) would promote plant growth under “heavy metal stress” due to its ability to provide better nutritional requirements in the roots as well as decrease metal concentrations. Secondly, the establishment of mycorrhiza- associated vetiver would hinder a significant concentration of heavy metals in the harvestable portion (phytoextraction). Therefore, the study aims to complete a field study on how strong AMF can influence the growth of vetiver and the “uptake accumulation of Pb and Zinc in plant tissue” (Wu et al. 2011). An additional study examined if refuse compost would aid in an increased growth rate in mycorrhizal vetiver (Wu et al. 2011). AMF is a beneficial fungus that helps sustain plant development by maintaining a symbiotic relationship and modifying the soil microbial community (Wu et al. 2011 and Solis-Dominguez et al. 2011). Another key feature of AMF is that they can shield plants against heavy metal toxicity levels and binds heavy metals by sorting them into vacuoles (Solis- Dominguez et al. 2011). This study was taken place in a lead and zinc mine tailings site in Shaoguan, China.
The field site was tilled to 20cm in depth and fenced with iron wire to hinder animal presence during the experiment (Wu et al. 2011). There were three blocks comprised of 6 plots each (with dimensions 1 x 2m) and contained refuse compost while, the control block did not contain any refuse compost (Wu et al. 2011) Then two blocks were set for vetiver plantings with the addition of 100kg refuse compost for each plot as well as 10cm of tailings (Wu et al. 2011). From each block, three plots were inoculated with “sand-based inocula mixture of Glomus intraradices and G. mosseae” (Wu et al. 2011). To specify, the mixture had high contents of spores, root particles and AMF hyphae and then about 50g of the mixed inocula was mixed in prior to the vetiver planting (Wu et al. 2011). Then 16 individual vetiver slips were inserted into the plots; meanwhile, the other three plots did not contain mycorrhizal inoculation (Wu et al. 2011). The tailing samples were collected and air-dried for a week then crushed and filtered with a 200 µm mesh. Soil characteristics such as pH, electrical conductivity, total nitrogen, phosphorous, total cadmium, copper, zinc and lead were analyzed (Wu et al. 2011). After four months, the plants were harvested and concluded that the highest colonization was discovered in tailings with the treatment of mycorrhizal inoculum (55. 3%), then by mycorrhizae alongside refuse compost (41. 2%) (Wu et al. 2011). Regarding metal concentrations in plant tissues, they were found to be in lower concentrations in vetiver alongside treatments with refuse compost versus treatments with mycorrhizal inoculation (Wu et al. 2011). Nutrient contents were also analyzed and found that total nitrogen and phosphorous concentrations were substantially higher alongside mycorrhizal inoculation treatments as well as tailings with refuse compost inoculated by means of AMF. To apply this to ecological restoration of mine tailings, vetiver is better suited for phytostabilization than phytoextraction because the metal removal capacity is low (Wu et al. 2011). Metal tailings ponds are filled with metal contamination and consequently lack essential plant nutrients including nitrogen, phosphorous and organic matter (Wu et al. 2011). With the addition of mycorrhizal fungi, vetiver is a good plausible source for revegetation and this would directly be applied to ecological restoration of mine tailings. Article 2: Solis- Dominguez et al. (2011).
Effects of Arbuscular Mycorrhizal Fungi on Plant Biomass and Rhizosphere Microbial Community Structure of Mesquite Grown in Acidic Lead/Zinc Mine Tailings. The focus of this study was to observe how the ability of AMF inocula would help aid in the establishment of mesquite (Prosopis juliflora) in an acidic mine metalliferous tailing. Mesquite is a “salt and rough tolerant deciduous, large crowned and deep-rooted bush or tree with the ability to establish symbiosis with nitrogen-fixing bacteria and to form AMF associations” (Solis-Dominguez et al. 2011). Mesquite was chosen for this study because as stated in previous literature, there is evidence that mesquite can grow in soils contaminated by metal elements (Solis-Dominguez et al. 2011). For this reason, mesquite has shown tolerance to metal elements including lead, arsenal and chromium under special conditions (Solis-Dominguez et al. 2011). The study hypothesized that utilizing arbuscular mycorrhizal fungi plays a significant role in establishing plant growth in tailings with a compost amendment of 10%. On top of that, there were 3 goals Solis- Dominguez et al. (2011) accomplished and they were:
Furthermore, the commercial desert inoculum was determined as the most effective type for AMF (Solis-Dominguez et al. 2011). AMF inoculation played a significant role in the development of beneficial soil bacteria in the soil community and therefore increased plant growth from 44% to 76% increase after 60 days due to increased root length (Solis-Dominguez et al. 2011). The impact of AMF on plant growth is significant and would be highly encouraged for establishing vegetation in mine tailings. CritiqueIn this paper, the use of flora including vetiver and mesquite are analyzed to evaluate the enhancement of mine tailings. The study performed by Solis-Dominguez et al. (2011) was done thoroughly regarding the need and the importance of their research towards supporting plant growth in mine tailings. Furthermore, Solis- Dominguez et al. (2011) were very detailed regarding which size pots they used for their experiment and characterized where everything was from which is crucial for future studies to replicate if they wish to do so. However, there would be a great potential for the experiment to be based on a long-term study of three to five years instead of 2 months to truly see changes in the microbial soil community as well as metal concentrations under limited soil conditions like mine tailings. Another key point to realize is that it would be beneficial if Solis-Dominguez et al. (2011) could translate their greenhouse results successfully into the field. Field based experiments would account external environmental factors that may affect the response of the vegetation growing on the mine tailings that would not have been otherwise included in the greenhouse experiment. Whereas Wu et al. , (2011) established their study based on the field which is more practical than in the greenhouse. On another note, they don’t compare mesquite with other plants (other than its tolerance) and it would be more effective to see a comparison among a variety of plants. This way, perhaps a community of vegetation can be grown to restore mine tailings instead of one species of plant in case of insect infestations and soil degradation.
The experiment component from Wu et al. 2011 could have been improved as it was unclear and confusing. Perhaps an image of the experimental design could be inserted so it can help the reader understand more thoroughly. And in practice, if someone wanted to repeat this experiment, it would be difficult to do so and would increase the number of mistakes and therefore, lack accuracy in the findings/results.
Mine tailings are greatly disturbed sites and the following papers have done a thorough examination of how particular species (vetiver and mesquite) can be utilized to revive mine tailings with the aid of refuse compost, mycorrhizal inoculation and AMF. AMF plays a huge role in plant growth and rhizosphere microbial populations via root exudation (Graham et al. , 1981 in Solis- Dominguez et al. , 2011) especially under limiting soil conditions.
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