Microbial associated plant growth and heavy metal accumulation to improve phytoextraction of contaminated soils

Utilizing plants to remediate heavy metal contaminated soils, a process known as phytoextraction, offers many advantages but has yet to reach levels of efficiency that would make the strategy economically viable. Inoculation of the plant rhizosphere with microorganisms is an established route to improving phytoextraction efficiency. In general, microorganisms can improve phytoextraction by increasing the availability of heavy metals to the plant and by increasing plant biomass. This review uses a meta-analysis of the results from 103 microbial-augmented phytoextraction studies to examine if one of these microbial mechanisms has a greater potential to positively impact phytoextraction. Trends surrounding the use of heavy metal-accumulating versus non-heavy-metal-accumulating plants in phytoextraction are discussed. Microbially induced improvements in the accumulation of heavy metals in plant biomass, a focus of several studies, are always coincident with enhanced net phytoextraction. However, microbial treatments that improved plant biomass are more prevalent in the literature and account for a larger number of studies that reported improved phytoextraction, particularly in non-heavy-metal-accumulating plants. The experimental findings emerging from the literature that implicate specific microbial processes in improving phytoextraction are briefly reviewed and used to underline trends observed from the meta-analysis that indicate future directions regarding the use of microorganisms to improve phytoextraction efficiency.