Phytoremediation of Lead in the presence of individual and combined inoculation of earthworms, arbuscular mycorrhizal fungi and rhizobacteria by maize

Soils may become polluted with high concentrations of heavy metals both naturally, as a result of proximity to mineral outcrops and ore bodies and anthropogenically, as a result of industrial activities. Lead (Pb), commonly caused soil pollution and considered to be responsible for significant decreases in biological activities in soils. Phytoremediation is an emerging and low cost technology that utilizes plants and associated organisms to remove, transform, or stabilize contaminants located in water, sediments, or soils. Phytostabilization focuses on the formation of a vegetation cover where sequestration (binding and sorption) processes immobilize metals within the plant rhizosphere reducing metal bioavailability. Therefore, the success of phytoremediation depends on the interactions between macro- and microorganisms and plant roots in the rhizosphere.
The contaminated soil was collected from Bama mining site located in the southwest of Isfahan. After surface-sterilization and germination, maize seeds were transplanted into each plastic pot containing 4 kg of contaminated soil that already autoclaved at 121 oC for 2 h. A completely randomized design with 2×2×2 factorial treatment combinations was used with the following factors: with or without earthworm treatments (Eisenia foetida), with or without arbuscular mycorrhizal (AM) fungal treatments (co-inoculated with Funneliformis mosseae and Septoglomus constrictum) and with or without rhizobacteria (co-inoculated with Bacillus sp. and Bacillus licheniformis). After three months of growth under greenhouse conditions, maize shoots were harvested. Shoot and root were oven dried, weighed and milled used to determine Pb concentration. Concentration of Pb in roots and shoots were measured by dry ash method and soil Pb concentration was determined with DTPA-TEA method. Bioaccumulation (BF) and translocation (TF) and remediation (RF) factors for each treatment were calculated.
In general, inoculation of these organisms increased plant growth, availability of Pb in soil, plant Pb concentration and bioaccumulation factor. The highest shoot dry weight was observed in earthworms-AM fungi (EM) and AM fungi-bacteria (MB) co-inoculations with 3.2 times increase compared to un-inoculated plants. Available Pb in soil in earthworm-AM fungi-bacteria co-inoculation (EMB) was about 3 times higher than un-inoculated treatments. The higher Pb uptake in maize shoot and root were recorded in EMB. Furthermore, the BF for root maize in all treatments was higher than 1, especially in AM fungal treatment, alone. Although the TF for maize was lower than 1, it was increased above 1 in polluted soil co-inoculated with earthworm and bacteria. However, AM fungi tended to decrease the TF compared to un-inoculated maize. The highest RF (0.14%) with 23 times increase compared to un-inoculated was showed in EMB treatment. Conclusions Despite the substantial enhancement of Pb concentration in the maize, Pb absorption was not high enough to achieve extraction rates which would be necessary for practical use. Furthermore, the amounts of BF, TF and RF in this study demonstrated that maize could useful for Pb phytostabilization. Hence, it appears that the presence of AM fungi (as factor improve phytostabilization) could result in a better plant growth and tolerance against Pb toxicity, when soil is co-inoculated with earthworm and/or bacteria, especially under natural conditions that the presence of these organism's together, could reduce Pb toxicity and improve maize.