Effects of Metal Phytoextraction Practices on the Indigenous Community of Arbuscular Mycorrhizal Fungi at a Metal-Contaminated Landfill

Abstract

Phytoextraction involves use of plants to remove toxic metals from soil. We examined the effects of phytoextraction practices with three plant species (Silene vulgaris, Thlaspi caerulescens, and Zea mays) and a factorial variation of soil amendments (either an ammonium or nitrate source of nitrogen and the presence or absence of an elemental sulfur supplement) on arbuscular mycorrhizal (AM) fungi (Glomales, Zygomycetes) at a moderately metal-contaminated landfill located in St. Paul, Minn. Specifically, we tested whether the applied treatments affected the density of glomalean spores and AM root colonization in maize. Glomalean fungi from the landfill were grouped into two morphotypes characterized by either light-colored spores (LCS) or darkcolored spores (DCS). Dominant species of the LCS morphotype were Glomus mosseae and an unidentified Glomus sp., whereas the DCS morphotype was dominated by Glomus constrictum. The density of spores of the LCS morphotype from the phytoremediated area was lower than the density of these spores in the untreated landfill soil. Within the experimental area, spore density of the LCS morphotype in the rhizosphere of mycorrhizal maize was significantly higher than in rhizospheres of nonmycorrhizal S. vulgaris or T. caerulescens. Sulfur supplement increased vesicular root colonization in maize and exerted a negative effect on spore density in maize rhizosphere. We conclude that phytoextraction practices, e.g., the choice of plant species and soil amendments, may have a great impact on the quantity and species composition of glomalean propagules as well as on mycorrhiza functioning during long-term metal-remediation treatments.Health hazards posed by the accumulation of toxic metals in the environment accompanied by the high cost of removal and replacement of metal-polluted soil have prompted efforts to develop phytoremediation strategies that would utilize plants to extract excessive soil metals. Several plant species or ecotypes associated with heavy-metal-enriched soils accumulate metals in the shoots (15). These plants can be used to clean up metal-contaminated sites by extracting metals from soil and concentrating them in aboveground biomass (9, 34). The metal-enriched biomass can be harvested using standard agricultural methods and smelted to recover the metal.Metal phytoextraction is not as extreme as conventional metal removal methods but still involves considerable alterations to the environment, including elimination of the existing vegetation cover and the application of fertilizers and soil amendments, such as synthetic chelates or sulfur, to increase metal availability to plants (14,19,38). Achieving a desired reduction in soil metal concentration may require cultivation of metal-accumulating plants at the remediated site for several cycles that include harvest and removal of metal-enriched biomass (16). The decontaminated area may be subsequently revegetated with a more appropriate and/or desirable plant cover.Despite the important role that rhizosphere microorganisms p...

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