Phytoremediation of metal-contaminated rare-earth mining sites using Paspalumconjugatum

Over-exploitation of rare-earth elements causes cadmium (Cd)- and lead (Pb)-contamination of rare-earth mine tailings. Here, Paspalum conjugatum was used as a hyperaccumulating perennial herb to evaluate its phytoextraction potential for removing metals in a hydroponic experiment. Further, an in-situ experiment was conducted for two years (2016-2018) to investigate the potential of P. conjugatum for reducing soil metal concentrations and to assess microbiome recovery on abandoned rare-earth mining land. Pinus massoniana was used for control treatments. We found that P. conjugatum produced metal transfer coefficients of 0.85 and 0.89 for Cd and Pb, respectively. The concentrations of Cd and Pb accumulated in P. conjugatum were 98.33 mg kg^-1 and 137 mg kg^-1, respectively. Using P. conjugatum, soil Pb and Cd concentrations were significantly decreased, and Cd concentrations approached acceptable levels (0.209 mg kg^-1). The bacterial diversity in P. conjugatum-restored soil was higher than that in soil of P. massoniana. The bacterial genera Chloroflexi, Acidobacteria, and Actinobacteria were predominant in the restored soils. P. conjugatum was tolerant to drought and exhibited enhanced enzymatic activity. These results suggest that P. conjugatum can be used for efficient phytoremediation of Pb- and Cd-contaminated soils.