Submitted to: Canadian Journal of Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/5/2001
Publication Date: 6/1/2001
Citation: Interpretive Summary: Phytoremediation is a new technology which uses metal hyperaccumulating plants to phytoextract soil metals to their shoots which can be harvested to remove metals from contaminated fields. Little information was available about the effects of such plants on soil microbes which are required for normal biochemical processes in soils. An experiment was conducted to examine the effect of growing the hyperaccumulator Thlaspi caerulescens and the non-accumulator species Trifolium pratense (red clover, a legume), and of raising the pH of the soil, on the density of soil bacteria and fungi, and the fraction of total microbes which were resistant to soluble Zn or Cd. The soils were adjusted to pH 5.8 and 6.8 (liming reduces metal solubility in soils), and fertilized, and then the two plant species were grown for 4 weeks. At harvest, soil adhering to the roots (rhizosphere soil) was examined for density of microbes. Both plant species increased microbial density in the rhizosphere soil compared to non-rhizosphere soil and non-planted soil incubated under similar conditions. When the metal tolerance of the soil microbes in the different plant rhizospheres were tested, it was evident that microbes from the rhizosphere of T. caerulescens were more resistant to Zn and Cd than were the random microbes of the unplanted controls or the rhizosphere soil from T. pratense. Although the mechanism of this effect remains uncertain, we hypothesize that the microbes in the rhizosphere of T. caerulescens were more metal resistant because this species causes higher availability of soil metals by local acidification of the rhizosphere soil to increase its ability to absorb the soil metals, thereby increasing Zn and Cd stress to microbes.
Technical Abstract: Metal hyperaccumulator plants like Thlaspi caerulescens are used for phytoremediation of contaminated soils. Because little is known about the rhizosphere of hyperaccumulator plants, the influence of T. caerulescens was compared to the effects of Trifolium pratense L. (red clover, a legume) on soil microbes. High and low metal soils were collected near a zinc smelter in Palmerton, PA. Soil pH was adjusted to pH 5.8 and pH 6.8 by addition of Ca(OH)2. Liming increased bacterial populations and decreased metal toxicity to levels allowing growth of both plants. The effects of plants on total (culturable) bacteria, total fungi, and on Cd- and Zn-resistant populations were compared to non-rhizosphere soil. Microbial populations were higher in soils planted with T. pratense, but higher ratios of metal resistant bacteria were found in the presence of T. caerulescens. We hypothesize that T. caerulescens acidifies its rhizosphere as it hyperaccumulates soil cations. Soil acidification in the rhizosphere of T. caerulescens would affect metal uptake by increasing available metals around the roots and consequently increasing the selection for metal resistant bacteria. Soil acidification may be part of the hyperaccumulation process enhancing metal uptake from soil. Phytoextraction offers an alternative for remediation of large area mining-related Superfund Sites.