BackgroundPhytoextraction is an environmentally acceptable and inexpensive technique for mine tailing rehabilitation that uses metallophyte plants. These plants reduce the soil trace metal contents to environmentally acceptable levels by accumulating trace metals. Recently, whether more trace metals can be removed by species-rich communities of these plants received great attention, as species richness has been reported having positive effects on ecosystem functions. However, how the species richness affects trace metals removal of plant communities of mine tailing is rarely known.Methodology/Principal FindingsWe examined the effects of species richness on soil trace metal removal in both natural and experimental plant communities. The root lengths and stem heights of each plant species were measured in order to calculate the functional diversity indices. Our results showed that trace metal (Cu, Cd, Pb and Zn) concentrations in mine tailing soil declined as species richness increased in both the natural and experimental plant communities. Species richness, rather than functional diversity, positively affected the mineralomass of the experimental plant communities. The intensity of plant-plant facilitation increased with the species richness of experimental communities. Due to the incremental role of plant-plant facilitation, most of the species had higher biomasses, higher trace metal concentrations in their plant tissues and lower malondialdehyde concentrations in their leaves. Consequently, the positive effects of species richness on mineralomass were mostly attributable to facilitation among plants.Conclusions/SignificanceOur results provide clear evidence that, due to plant-plant facilitation, species richness positively affects the removal of trace metals from mine tailing soil through phytoextraction and provides further information on diversity conservation and environmental remediation in a mine tailing environment.
The criterion for judging the successful revegetation largely focuses on the aboveground indicators, whereas the information for soil ecosystem during the revegetation is often ignored. To better understand the effects of the revegetation on the development of the soil ecosystem near Shaoguan Pb/Zn Smelter, Guangdong Province of Southern China, we compared the difference of the microbial and physico-chemical parameters between the four revegetated sites and two control sites (bare ground and native forest area). The soil organic C, total N, total P, NH 4 -N, NO 3 -N, available P, WHC and porosity significantly increased and bulk density decreased in the four revegetated sites compared with those in bare ground, indicating the processive effects of the revegetation on the reestablishment of the soil nutrient pools. The heavy metal contents were higher in the four revegetated sites than in the bare ground, thus the revegetation resulted in the accumulation of heavy metals released from smelter in surface soil. The soil microbial composition and activities, except that the oligotrophic bacterial number decreased over revegetation time, significantly increased in the revegetated sites compared with those in the bare ground, and predominantly correlated with soil organic C, total N, NH 4 -N, NO 3 -N and WHC. The soil oligotrophic bacteria was negatively related to all individual heavy metal contents, thus was the most sensitive indicator in reflecting heavy metal stress, while other microbial parameters, despite not showing negative relationships to the individual heavy metal contents, were sensitive to the potential availability of Pb and Cu (ratio of available to total heavy metal contents), but less sensitive to those of Zn and Cd. Both the principal component analysis (PCA) and the discriminant analysis (DA) resulted from microbial and physico-chemical datasets not only revealed the shifts of the soil physico-chemical and microbial patterns from the unrevegetated to non-polluted conditions, but also implied the possible loss of effects of revegetation on soil remediation in the sites revegetated for four (RIV) and five (RV) years, respectively.
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