Ecological Restoration Of Dabaoshan Mine Overburden In Guangdong Province

The extreme acidity and heavy metal pollution are two of major environmental problems associtated with mine wastelands around the world. The excess acidity tends to deteriorate soil properties (substrate structure, nutrient deficiency and so on), and enhance heavy metal toxicity, which greatly inhibit plant establishment, growth and colonization. These wastelands are therefore often absence of vegetation cover and readily become a source of pollution posing potential hazards to waters, soils and human health of the surrounding areas. A good vegetation cover can fulfill the objectives of stabilization, pollution control, visual improvement and removal of threats to human beings. However, establishing a healthy, long-standing and self-sustainable vegetation cover on those mine soils with extreme acidity and high heavy metal toxicity remains a great challenge.In the present study, a full ecological survey was conducted at an extremely acid mine wastelands_Dabaoshan mine overburden, in Guangdong Province. Physico- chemical analysis and net acid generation (NAG) test were employed to identify the major limiting factors to re-vegetation. The results showed that the extreme acidity and high heavy metal toxicity were the two major constraints to ecological restoration of the mine soils. Then, we established vegetations on the mine spoils according to the following procedures: (i) planting strips on the surface of the abandoned mine lands were constructed by digging ditches of 30×30 cm (50-cm intervals) and holes of 50×50×50 cm (2-m intervals); (ii) lime and chicken manure were added to the ditches at the rate of 15 t ha-1 (dry weight), respectively; (iii) the ditches were filled back with mine soil and allowed to equilibrate for one month prior to planting during the rainy season; (iv) The seedlings of different plants, having grown for five months in plastic containers, were planted in planting strips; (v) Five different vegetation composition patterns were built as follows: Cynodon dactylon (Plot I), Pennisetum purpureum (Plot II), Eucalyptus robusta (Plot III), Neyraudia reynaudiana + Panicum repens (Plot IV) and E. robusta + N. reynaudiana + C. dactylon (Plot V). During the follwing two years, the vegetation development on the mine spoils were investigated, meanwhile, the soil characteristics including acidification, fertility parameters, and the accumulation of heavy metals in the soil-plant systems were monitored Furthemore, a greenhouse experiment was conducted to reveal the interactions among soil acidification, vegetation, and heaby metal mobility, etc. The main findings were as follows:1. The mine soil pH values and acid neutralization capacity (ANC) increased gradually with time after the establishment of vegetation. The net acid generation (NAG) significantly decreased as the remediation time progressed. All the NAG values were above zero despite a general reduction trend, indicating that the mine soils still had acid-forming and re-acidification potential. The results of the greenhouse experiment suggested that there were high re-acidification risks in the mine soils of L1, L1+NPK treatments. However, the further additions of phosphate or river sediment (L1+P, L1+RS, and L1+NPK+RS) could prevent re-acidification effectively, which indicated that phosphate and river sediment played a quite specific role in preventing the re-acidification in the mine soils.2. The total heavy metal concentrations in the mine soil remained stable with the remediation time indicating the decreasing metal mobility. However, both DTPA-extractable Zn and Pb increased significantly as the remediation time progressed, which might be due to soil disturbance during the remedial works. The greenhouse experiment showed that the application of amendments and plant growth effectively reduced DTPA-extractable heavy metal concentrations. The strongest decrease was achieved in the treatments containing both lime and river sediment (L1/L2+RS and L1/L2+NPK+RS) which indicating that the river sediment maight played a specific role in reducing the availability of heavy metals in the mine soils.3. The vegetation development enhanced the nutrient accumulation in the mine soil. The organic matter (OM), total nitrogen (TN), ammoniacal nitrogen (NH4-N), available phosphorus (AP) and available potassium (AK) showed rapid increasing trends with the remediation time. Similar results were found in the greenhouse experiment. Re-vegetation plays an essential role in nutrient accumulation, organic matter turnover, thereby facilitating soil forming processes and accelerating ecological succession of the man-made habitats.4. Another particular concern associated with re-vegetation of mine soils is the accumulation of heavy metals in the shoots of plants. From the viewpoint of stabilizing metal-contaminated sites, a lower metal concentration in the above-ground parts of plants is desirable to prevent metal potentially entering the ecosystem through the food chain. In the present study, most plants accumulated lower levels of Zn, Pb and Cu in the shoots rather than the roots. However, there were great variations of metal accumulation among different metals and within the same metal at different species. For example, a significant increasing trend of Zn was observed in all the species; Pb exhibited an increasing trend in N. reynaudiana and P. repens, while remained constant in E. robusta, C. dactylon and P. purpureum; Cu showed a slight decreasing trend in most species except P. repens. Taken as a whole, among the tested plants in the present study, E. robusta and P. purpureum accumulated the lowest concentrations of Cu, Pb and Zn, and the both species could be a good candidate for re-vegetation of metalliferous mine wastelands.5. All the plant species used in the present study could well establish on the acid metal-contaminated soils and developed a good cover within a relatively short time. Vegetation cover varied from 30% to 80%, the prevention rates of wind and water erosion were between 60-90% and 30-60%, respectively. Among five vegetation plots, Plot V was the best for its plant cover (100% within two years), and its prevention rate of wind and water erosion were above 90% and 60%, respectively.6. The microbial diversity, biomass, activity and soil enzyme activities were determined in mine soils applied the different amendments. The results showed that pH, NAG-pH and NAG were the key factors influencing structure and function of soil microbial community. The total and available heavy metal had significant negative effects to soil microrganism while soil nutrients accelerated microbial diversity and activity.In conclusion, a cost effective revegetation model is established here for remediation of extremely acid metalliferous wastelands. Results presented in present study demonstrate that revegetation is an effective remediation option for soil stabilization, preventing acidification, enhancing nutrient accumulation and the subsequent soil forming processes. Acidification of the mine soils can be controlled effectively in the first two years after plant growth. Vegetation cover and nutrient accumulation are gradually enhanced with the remediation time. However, metal mobility increasing with time is also observed in present experiment, which may result in metal accumulation in the above-ground parts of plants. Therefore, increased metal uptake in the above-ground parts of plants. Therefore, metal accumulation in the soil-plant system should be of a great concern, and long-term monitoring of ecological risk should be further carried out.