Phytolacca americana is a high-biomass rare earth element (REE) hyperaccumulator, which accumulates more than 0.2% REEs (dry biomass) in the shoots. Previous studies found that P. americana could grow on ion-adsorption REE mine area of Southern China, and therefore could be a potential candidate for phytoremediation of REE mine wastelands and agromining of REEs. However, the ionic REE tailings are characterized by extremely low content of organic matter and nutrients, which markedly limits the development of P. americana. Therefore the soil properties should be improved (e.g. addition of soil amendments) prior to implementing agromining. On the other hand, the addition of amendments risks to decrease REE availability and uptake by P. americana. So there is a need to investigate the type and dosage of amendments to optimize the plant performance and agromining efficiency.

In this study, a pot experiment was carried out with P. americana planted in REE tailings amended with organic fertilizer or biochar at different dosages (1.0% and 5.0%, w/w). For each treatment, a control without plant was also set up.

The preliminary results showed that: 1) The biomass of P. americana was significantly increased by the addition of amendments, the dosage of 5% showing the best effect. Compared to the non-amended treatments, the addition of organic fertilizer and biochar increased the total biomass by 7-13 times and 6-9 times. 2) After adding amendments, the pH in the soil showed a significant increase regardless of the presence or absence of plants, which improved the soil acid environment and significantly increased the germination rate of the P. americana. 3) The activity of urease and acid phosphatase significantly enhanced in REE tailings amended with organic fertilizer; however, the addition of 1% biochar had no effect on soil enzyme activity and 5% biochar inhibited them. Compared to the non-planted treatments, the activity of urease and acid phosphatase increased significantly after growing P. americana with different amendments. 4) After amending with 1% biochar, REE concentrations in the whole plants was 812 mg kg^-1, which was not significantly different from the non-amended treatments (1089 mg kg^-1). For the other treatments, REE concentrations were significantly reduced in P. americana (1% OM, 371 mg kg^-1; 5% OM, 31 mg kg^-1; 5% BC, 374 mg kg^-1). 5) The total amount of REEs extracted by P. americana shoots increased significantly (mg pot^-1) when grown in REE tailings amended with 1%, 5% biochar and 1% organic fertilizer compared to non-amended soils. In particular, the yield obtained with 1% biochar is four times higher than that of non-amended treatment. Currently, the effects of amendments on the REE availability of the tailing soil are still under analysis.

This work was supported by the National Natural Science Foundation of China (NSFC) [Grant No. 41771343]; Science and Technology Planning Project of Guangdong Province [Grant No. 2016A020221013; No. 2014A050503032], and the 111 project [Grant No. B18060].