Sedum plumbizincicola , a native Chinese cadmium (Cd) and zinc (Zn) hyperaccumulator, is suitable for removing Cd and Zn from contaminated acid soils. However, there is still little information on the influence of excessive aluminium (Al) in acid soils on the metal phytoextraction efficiency of hyperaccumulators. Here, we conducted hydroponics and glasshouse pot experiments to investigate Al toxicity to S. plumbizincicola at different Al levels. In hydroponics culture there were six Al addition levels, namely 0 (CK), 50 (Al50), 100 (Al100), 200 (Al200) and 500 μmol L⁻¹ (Al500) and in the pot experiment there were five levels (0, 50, 200, 500 and 1000 mg kg⁻¹). We used two soils, a Hydragric Anthrosol from Guangdong province with a pH of 5.52 (GD) and a Haplic Alisol from Guizhou province with pH of 4.86 (GZ). Hydroponics results indicate that the shoot biomass of S. plumbizincicola decreased with increasing Al addition level but no significant differences were observed in either shoot or root biomass among the different Al treatments. Aluminium addition significantly decreased plant root Cd and Zn uptake. Compared with the control, root Cd and Zn uptake across the different Al treatments declined by 34-69 % and 31-55 %, respectively. However, Al addition did not decrease shoot Cd or Zn uptake significantly. Further study shows that Al stress led to a decline in root diameter, root volume and Cd and Zn adsorption on the root surfaces. Aluminium toxicity symptoms were more apparent in the glasshouse pot experiment as it involved a longer growing period. In both soils, except for the 50 mg kg⁻¹ Al treatment which slightly stimulated the biomass of S. plumbizincicola , Al addition significantly inhibited plant biomass and the plants subjected to the highest Al concentration (1000 mg kg⁻¹) died.

Compared with the controls with no Al addition, 91 and 97 % declines in shoot Cd uptake and 90 and 98 % declines in shoot Zn uptake in GD soil and 80 and 84 % declines in shoot Cd uptake and 46 and 62 % declines in shoot Zn uptake in GZ soil were found at 500 and 1000 mg Al kg⁻¹. Analysis of metal distribution and metal chemical forms in the plants indicates that Al taken up was mainly sequestered in the roots and the dominant fractions of Al transported to the shoots were the fraction extracted by 0.6 M HCl and the residual fraction, which had low mobility in the plants. Furthermore, Al addition significantly influenced the chemical forms of Cd and Zn in the stems of S. plumbizincicola , reducing the mobility of Cd and Zn the stems and thus restraining Cd/Zn transfer to the leaves. Significant decreases in pH were observed in the soils after repeated phytoextraction by S. plumbizincicola , therefore Al toxicity may be an important limiting factor in the decontamination of acid soils using S. plumbizincicola.