Soil preparation
One and half kilogram of air-dried soil was placed into each of the 52 plastic pots (17 cm in diameter and 20 cm in height) with a 0.1 mm plastic sieve placed at the bottom to retain the fine soil particles. The soil was then supplemented with a standard nutrient solution containing 150 mg kg−1 N (NH4NO3), 83 mg kg−1 K (KCl), and 44 mg kg−1 P (NaH2PO4) (Blaylock et al., 1997). Two days later, metal solution containing 100 mg l−1 As (NaAsO2), 600 mg l−1 Zn (ZnSO4 · 7H2O), and 150 mg l−1 Cu (CuSO4 · 5H2O) was spiked into the soil in plastic pots and mixed thoroughly. The soil was then allowed to equilibrate for 15 days in the greenhouse with 3 cycles of saturation with deionized water and air-drying.
Control and blank pots were also prepared: the control pots consisted of the same levels of spiked metal concentrations without amendments of chelating agents, while the blank pots did not receive metal solution.