| In recent years, the increased demand for rare earth and the extensive exploration of rare earth resources have resulted in the migration of more and more rare earth elements (REE) into the environment, causing serious ecological damage and environmental pollution problems, of which the soil environment problems are particularly serious and are gradually attracted widespread attention. Therefore, it is urgent that the soils contaminated by REE be controlled and it has important practical significance for environmental protection. Soils in rare earth mine and surrounding area are generally characterized by the co-contamination of REE with heavy metals. Studies have shown that there are complex interactions between REE and heavy metals in soils. At present, arbuscular mycorrhizal (AM) fungi has been widely applied to the phytoremediation of soils contaminated by heavy metals, but few studies have focused on the effects of AM fungi on alleviating the toxicity of REE in plants. Moreover, it is unclear what roles of the interaction of REE and heavy metals played in the effects and mechanisms of AM fungi colonization when REE and heavy metals exist in the soil simultaneously. In this study, light rare earth element lanthanum (La) and heavy metal lead (Pb), the main pollution elements in Baotou rare earth mine tailings, were selected to simulate La-Pb co-contaminated soils at different levels. The aims were to investigate the effects of AM fungi on the growth and La, Pb uptake of maize (Zea mays L.) in co-contaminated soils under the influence of La-Pb interactions, and to explore the possible mechanisms of AM fungi in the phytoremediation of soils co-contaminated with REE and heavy metals.Simulating La-Pb co-contaminated soils at different levels (La:0,100, and 500 mg-kg’1; Pb:0,50 and 400 mgkg-1), a greenhouse pot experiment was conducted to investigate the effects of AM fungi G. versiforme on AM colonization rate, biomass, plant height, nutrient uptake, C:N:P stoichiometric, and La and Pb uptake of maize. The results indicated that symbiotic associations were successfully established between G. versiforme and maize, and the AM colonization rates ranged from 20.83% to 72.22%. The increasing La and Pb concentrations of soils significantly decreased mycorrhizal colonization rates, biomass, plant heights, nutrition contents, and increased C:P and N:P ratios of maize. AM fungi inoculation can alleviate toxic effects of interactions of La with Pb in plants, promoted the plant growth, improved nutritional status of maize, and decreased C:P and N:P ratios of maize to a certain degree. With increasing La and Pb concentrations of soils, La and Pb concentrations in shoots and roots of maize showed a significant upward trend. With increasing exogenous La concentrations, Pb concentrations in shoots and roots of maize also increased in Pb-contaminated soils. The effects of exogenous Pb on La uptake by plants are complex. Pb application at 50 mg·kg-1 significantly reduced root La concentration in uncontaminated soils. Pb application at 400 mg·kg-1 significantly increased La concentrations in shoots and roots of maize in slightly La contaminated soils. Pb application at 400 mg·kg-1 significantly decreased shoot La concentrations, but significantly increased root La concentrations of maize in moderately La contaminated soils. Inoculating with AM fungi decreased La and Pb concentrations in shoots and roots of maize under the interactions of La with Pb.The results indicated that AM fungi inoculation can alleviate toxic effects of interactions of La with Pb in plants. Based on the above results, another greenhouse pot experiment was conducted to further investigate the effects of G. versiforme under interactions of La with Pb on the activity of SOD, CAT and POD, the contents of MDA, PRO, soluble protein and soluble sugar, rhizosphere pH and the contents of easily extractable glomalin protein and total glomalin protein. This study also investigated physiological and biochemical mechanisms of AM fungi in alleviating co-contamination of REE with heavy metals. The results indicated that inoculation with G. versiforme increased the antioxidant enzyme activity of SOD, CAT, POD in maize leaves, and they were increased by 0.5%～19.6%,0.9%～5%, and 0.7%～22.5%, respectively. Inoculation promoted the synthesis of osmotic adjustment substance synthesis in maize to some extent, especially significantly increased the contents of PRO and soluble protein, which were increased by 21.3%～62.1%and 10.6%-66.0%, respectively. Rhizosphere soil pHs were significantly increased by 0.3%-3.4%. AM fungi inoculation also increased the contents of easily extractable glomalin protein and total glomalin protein, which were increased by 0.3%～10.5% and 0.6%～8.2%, respectively.The experiment demonstrates that AM fungi can alleviate toxic effects of interactions of La with Pb in plants and play a potential role in the phytoremediation of soils contaminated by REE. Further systematic research should focus on the mechanisms of AM fungi on plant REE uptake to provide a theoretical basis for the application of AM fungi on the ecological restoration of mining soils. |
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