There has been increasing interest in developing a plant-based technology (phytoremediation) to remediate heavy metal-contaminated soils. The primary objective of this technology is to maximize the transfer of heavy metals to plants so that the greatest total mass of contaminant is removed by each cropping. Slow growth rate and low biomass of hyperaccumulating plants may limit the utility of phytoremediation technology. In addition, the low bioavailability of heavy metals in the soil also restrains this technology application.
Earthworm is an important components of plant rhizosphere ecosystem, and it significantly contributes to total soil organic matter, enhance nutrient cycling, improve soil physical conditions, modify soil pH and promote plant growth, and able to increase metal bioavailability in soil through burrowing and casting. The arbuscular mycorrhiza (AM) fungi are important rhizospheric microorganisms. They can increase plant uptake of nutrients and consequently increase root and shoot biomass and improve plant growth. Available evidences suggest that AM fungi can colonize plant roots in metal contaminated soil, while their effects on metal uptake by plant are conflicting in previous studies. In order to understand thecomplex interactions between roots, earthworms and arbuscular mycorrhiza (AM) in the rhizosphere in metal contamination soil, present study focuses on investigating the effects of inoculation of earthworms and/or arbuscular mycorrhiza (AM) on ryegrass growth and bioavailability of Cd in Cd-contaminated soil.
Aquods from Laitiao Village, Hong Kong was used in the laboratory incubation experiment. The soil had a pH (in water) of 6.73, and the concentration of DTPA extractable Cd (at pH 7.3) was undetectable. The soil was steam-sterilized (121°C for 2 h) by autoclaving to eliminate native AM propagules. Soil in pots (1.0 kg per pot) was amended to contain 0, 5, 10 and 20 mg Cd/kg by adding CdCl2. After incubation for 2 months at 20°C and moisture content of 70%, all the pots were divided into four groups, with one following treatment respectively: earthworm [8 individual earthworms (Pheretima sp.) per pot], arbuscular mycorrhiza (AM) [30 g/kg soil], combination of earthworms and AM, and without earthworm and AM. Each pot was received 15 pre-germinated ryegrass (Lolium multiflorum) seeds. The earthworms used for the present experiment were washed free of surface soil with distilled water and kept in sterilized glass vessels for 24 h to minimize the number of naturally-occurring mycorrhizal propagules associatied with their surfaces or gut contents. Eight earthworms with similar fresh weight (0.6 g). The arbuscular-mycorrhizal fungi (AMF) inoculum was a mixture of Glomus mosseae and Glomus intraradices (Inoculum Endorize-Mix2) purchased from Biorize Sari, France. Seeds of ryegrass (L. multiflorum) were surface sterilized in a 10% (v/v) solution of hydrogen peroxide for 10 min.
The results from this study showed that that both earthworms and mycorrhiza were able to survive in all the Cd treatments after five weeks, but the growth of earthworms declined with the increase of Cd added. Compared with non-earthworm, earthworm treatment increased 9% root infection rate, significantly increased shoot biomass of ryegrass, increased Cd concentration in soil extracted by 0.01 mol/L CaCl2 in all Cd treatments, and resulted in the increase of root Cd concentration. Compared with non-inoculation, inoculation mycorrhiza alone did not affect ryegrass biomass, however, significantly increased Cd concentration in shoot and root. Earthworms-mycorrhiza combination decreased shoot biomass of ryegrass compared with earthworms alone, and increased shoot Cd concentration in the treatments of 5 and 10 mg Cd/kg soil, when compared with earthworms or mycorrhiza alone.
In conclusion, earthworms, mycorrhiza and their combination may have potential roles in increasing plant biomass and enhancing metal uptake by plant, and consequently elevating phytoextraction efficiency in low to medium level metal contaminated soil. Further investigation is necessary to study host plant-AM fungi associations in the other metal polluted soils and in particular the interactions between roots, microorganisms and animals in rhizosphere. Copyright © 2005 中國生態學學會.
|Published - Jun 2005
- Soil contamination
- Alt. title: Roles of earthworm-mycorrhiza interactions on phytoremediation of Cd contaminated soil