Abstract: Back ground, aim and and scope: The compound pollution of heavy metals in farmland caused by mining and transportation is becoming more and more concerned. Bioremediation of contaminated farmland is more friend than physical and chemical remediation. Ramie, as a unique crop in China, has been considered to adsorb the heavy metals in polluted farmland effectively, such as As, Cd, Pb and Sb. Meanwhile, the mycorrhiza formed by AM fungi and ramiecan improve resistance of ramie to heavy metals. The questions discussed in this study include: 1) whether AM fungi and ramie can form mycorrhiza; 2) whether the mycorrhizal fungi play a role in promoting the remediation of heavy contaminated soil. Materials and methods: In this study, the farmland soil around the tailings of Hunan Tin Mine (1 km) was used as the experiment medium. AM fungi and ramie 3 were used as the experiment material, and the aim in this study is to investigate the effects of AM fungi on ramie colonization rate, biomass, contents of phosphorus in shoots, contents of heavy metals, transport coefficient and antioxidant enzyme system. In addition, the established method was used to remediate the contaminated soil induced by heavy metal. Results：The results showed that (1) AM fungi could form mycorrhizal fungi with ramie, and the colonization rate was 33.7%. The AM fungi inoculated with AM fungi could significantly increase the aboveground biomass (22.2%) and underground biomass (24.0%). Compared to the non-inoculated groups, the aboveground contents of phosphorus (50.4%) were significantly increased for the inoculated groups. (3) Contents of Zn and Cd in the shoots of AM fungi were significantly increased by 50.3% and 100.0%, respectively. Additionally, the contents of Cu and Sb in the roots of AM fungi were significantly increased by 30.4% and 114.3%, respectively. However, the inoculation for AM fungi inhibited the absorption of As in ramie, and the contents of As in the aboveground and underground parts of ramie were reduced by 121.6% and 416.4%, respectively. (4) the inoculation for AM fungi significantly increased the transport coefficients of Zn, As and Cd in ramie by 58.6%, 148.1% and 49.8%, respectively. The transport rate of Sb was significantly decreased by 64.1% in ramie. (5) the inoculation for AM fungi significantly increased the activity of SOD (17.47%) and CAT (31.75%) in ramie, while the inoculation for AM fungi had no significant effects on POD activity. Discussion: (1) Inoculation of AM fungi increased ramie biomass and metal tolerance, which may be due to: 1) AM fungi could expand the range of ramie roots and absorption area through their external hyphae, and further improve ramie absorption of P; 2) the hyphae secrete acetic acid, citric acid and other organic acids could activate the soil in the insoluble phosphate to promote the absorption of ramie P and improve the nutritional status of ramie to P and the ramie biomass. Thereby it enhanced the resistance of ramie to heavy metals (2) AM fungi could promote the absorption and transport of Zn and Cd for ramie, so it can remediate compound pollution of heavy metal contaminated soil by plant extraction, and Cu, Sb immobilized in the ramie root may be mycelial "filtration" effect. AM fungi could reduce the absorption of As in the ramie, which may be due to the absorption of P and As through the phosphate transport system into the plant. Moreover there is a competitive relationship for promoting the P absorption and reducing the absorption of As. (3) AM fungi could improve the resistance of ramie to heavy metals by increasing the activity of antioxidant enzymes, the activation of O2- •, H2O2 and other reactive oxygen species on ramie cells under the stress of heavy metal. Conclusions: Under the stress of compound heavy metal, the experimental conclusions showed that: (1) AM fungi could form mycorrhiza with ramie. AM fungi could increase the uptake of P in the upper part of ramie, promote the increase of biomass and improve the tolerance of heavy metals. (2) Inoculation of AM fungi increased the absorption of a variety of heavy metals for ramie, so the combination of AM fungi and ramie could be a new method for the remediation of heavy metal pollution by the combination of plant and microorganism. (3) Inoculation of AM fungi significantly increased the activity of SOD and CAT in the aerial part of ramie and promoted the activity of POD to a certain extent. Recommendations and perspectives: It has been found that the combination of AM fungi and ramie can be used as a bioremediation method to repair the contaminated soil by heavy metals. In addition, the specific mechanism of the combination of AM fungi with ramie on the remediation of contaminated soil remains to be further studied.
Keywords: arbuscular myhorrhizal fungi; compound heavy metal; ramie; phosphorus; biomass; antioxidant enzyme