Phytoextraction of lead from contaminated soil by Panicum virgatum L. (switchgrass) and associated growth responses

Abstract

Growing interest in biomass crops for energy production has focused attention on Panicum virgatum L. (switchgrass) as a promising perennial feedstock native to much of North America. Switchgrass may be processed into products such as pulp and paper, ethanol, and fuel pellets. A robust C4 grass, switchgrass typically produces 10-12 dry tonnes/ha on agricultural soils in average growing conditions; this study shows that reduced but substantive yields are also possible on soils rendered unsuitable for food crop production by lead contamination over the acceptable level of 70 ppm. Switchgrass offers not only tolerance to lead in the soil environment, but also the potential for extraction of lead contaminants. Integrating the growth of switchgrass for biomass with phytoremediation could provide greater opportunity for biomass production, while eventually increasing the suitability of such lands for agricultural production. This study examines the potential for using switchgrass to remove lead from the soil and to translocate the contaminant to the leafy portions of the plant, where it could be removed from the site through biomass harvest. Roots of plants treated with 6000 and 10000 ppm lead solution from the time of seeding displayed some morphological changes and growth inhibition, yet, produced biomass comparable to expected yields and removed up to 0.1% of the applied lead solution. Established switchgrass displayed few morphological changes and no significant loss of biomass when treated with lead acetate solution while extracting lead into harvestable tissues. As the primary root is the organ through which water, solutes and heavy metals pass into the plant, the dense, fibrous root system of switchgrass may aid in the ability of this plant to extract contaminants from soil without significantly hindering above ground biomass production. Switchgrass grown on brownfield soil, with contamination levels of up to 23600 ppm translocated lead into harvestable tissues without significant changes in biomass production or growth characteristics. However, symptoms of phytotoxicity were observed in switchgrass grown on soil containing 36100 ppm lead. Atomic absorption spectrometry and inductively coupled plasma analyses indicate that lead accumulation in harvestable tissues of switchgrass occurs at rates comparable to those in previous studies. The presence of dark staining deposits, detected by light microscopy in the root and shoot tissue of switchgrass treated with high concentrations of applied lead (13800, 10000), suggests an adaptive response within the plant. Successful integration of phytoremediation with growth of biomass crops such as switchgrass may provide effective reclamation of contaminated soils, while contributing to a sustainable energy economy.