Effects of individual and combined pollution of Cd and Zn on root exudates and rhizosphere Zn and Cd fractions in ryegrass (Loliurn perenne L.)

Pot experiments (root rag method) were carried out to investigate the influence of individual and combined pollution of cadmium (Cd) and zinc (Zn) on plant growth, Zn and Cd accumulation in ryegrass (Loliurn perenne L.), rhizosphere Zn and Cd fractions. The results showed that the synergistic effect on uptake between Zn and Cd was observed in the Zn and Cd existing concurrently treatment (8 mmol/kg Zn and 2 mmol/kg Cd) while Zn uptake by ryegrass deceased in the Cd alone treatment (2 mmol/kg Cd). Zn and Cd were distributed mainly in shoot. The highest Zn and Cd concentrations (3108.72, 73.97 mg/kg, respectively) were found with the combined Zn and Cd treatment and this trait could be beneficial in the selection of high Zn accumulator for soil Zn phytoremediation. The results show that ryegrass has a high potential for soil Zn phytoremediation. The concentrations of Zn fractions (EXC, CAB and FeMn) in rhizosphere were higher than that of bulk soil. Rhizosphere and bulk soil Cd fractions were in the order of EXC > CAB > FeMn > RES > OM in Cd alone and Cd and Zn combined treatments. Rhizosphere and bulk soil Zn fractions were in the order of FeMn > CAB > RES > OM > EXC in Zn alone and Cd and Zn combined treatments, while the order was RES > FeMn > OM > CAB > EXC in Cd only treatment. The concentrations of amino acids increased and soil rhizosphere pH decreased with alone Cd, Zn and combined Cd and Zn treatments. The maximum amino acids concentration was found in the combined Cd-Zn treatment which was approximately 1.95, 1.54 and 1.40 times compared to concentration in the control, Zn and Cd individual treatments, respectively. The minimum of soil pH (5.18) in rhizosphere was found in the combined Cd and Zn treatment.