蚯蚓-菌根在植物修復鎘污染土壤中的作用

以灰化土(Aquods)為供試土壤，分別加入4個濃度的Cd²⁺ (0,5,10,20mg/kg)模擬土壤污染，設置每缽接種8條蚯蚓(Pheretimasp.)、接種菌根(InoculumEndorize-Mix2)和同時接種蚯蚓和菌根的處理，以不加蚯蚓和菌根為對照，並種植黑麥草(Loliummultiflorum)，研究蚯蚓菌根相互作用對Cd污染土壤中黑麥草生長及土壤中Cd生物有效性的影響。結果表明菌根浸染率不受添加Cd濃度的影響，平均浸染率為22%，加入蚯蚓能使菌根的侵染率提高9%。在Cd污染土壤上，引進蚯蚓顯著增加了黑麥草地上部的生物量，接種菌根對黑麥草地上部分產量沒有明顯影響，同時接種蚯蚓和菌根與只接種蚯蚓相比沒有顯著差異。蚯蚓活動顯著提高了土壤中CaCl₂-Cd的含量，而菌根只在低濃度Cd處理上增加了土壤中CaCl₂-Cd含量，二者對H₂O-Cd、DTPA-Cd均無顯著影響，蚯蚓和菌根對增加土壤有效態Cd含量不存在協同作用。蚯蚓活動促進了黑麥草對Cd的吸收，但吸收的Cd積累於黑麥草根部。接種菌根不僅能促進黑麥草對Cd的吸收，而且還能促進Cd從植物的根部向地上部分轉移，由於接種蚯蚓可以提高菌根的浸染率，所以二者具有促進Cd向地上部轉移的協同作用。這對於重金屬污染土壤的植物修復具有十分重要的意義。

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.