Applied remediation measures
Screening of metal-tolerant species known as hyper-accumulators applied for remediation of a metal contaminated site has been a worldwide hotspot of research (García et al., 2009; Tie et al., 2005; Zhang et al., 2015). A hyper-accumulator does not only tolerate high concentrations of a metal element in its tissue but also possesses high under-ground to above-ground metal transfer rates (Abreu et al., 2102; Adamo et al.. 2014). The demerits for use of most of the reported hyper-accumulators, however, are their low levels of biomass growth associated thus with low levels of total metal uptake. In addition, the nature of their tolerance to a specific metal element also limits their application on multiple metal contaminated sites. Attentions in related fields have been paid to use of tree species ( Bidwell et al., 2002; Yang et al., 2008; Wang et al., 2010) . As most of the fast-growing tree species cannot survive under heavily metal contaminated conditions, the commonly applied methods for forestation at a metal tailing site are either to seal its upper layer with an uncontaminated soil (Gomez-Ros et al., 2013) or to use an immobilization reagent to reduce its upper layer’s metal toxicity (Vargas-García et al., 2012, Yang et al., 2017). The disadvantages of soil sealing and metal immobilization are mainly their high costs.
The experimental site under study was a manganese tailing wasteland located at Xiangtan, Hunan, China. The site was poor in nitrogen and phosphorus but very high in contents of manganese, lead, zinc, copper, cadmium and chromium. The applied remediation strategy was a combination of the above mentioned methods using tree species, metal-tolerant species and remediation reagent. Two fast-growing tree species (Paulownia fortunei and Koelreuteria bipinnata ) screened from the native species naturally distributed in the uncontaminated region nearby the tailing site were used as dominant phyto-remediation species. Before transplanting their seedlings, a bacteri-rich organic manure specifically prepared for the tailing site was amended to their rhizospheric areas. The enrichment of the site’s biodiversity was counted on natural germination of the native metal tolerant plant species based on the soil seed bank theory (Tang et al., 2006). To prevent the spread of metal contamination to surrounding areas, an ecological interception belt along the border of the tailing site was established and its outlet was connected to a constructed wetland system for treatment of runoff.
Detailed experimental information has been presented in previous work (Ouyang et al., 2016; Wu et al., 2017). The experiment plots, sample analysis and equations for calculation are briefly described below.