Resistance to and uptake of heavy metals by Vetiveria zizanioides and Paspalum notatum from lead/zinc mine tailings

Abstract

Mine tailings are usually hostile to plant colonization,and also toxic to animals and man through the food chain.It has long been a highlight in the field of restoration ecology how to substantially mitigate the toxicity of adverse factors(e.g.heavy metals,infertility,acidity,etc.)and then to make plant colonization successfully in mine tailings.A greenhouse experiment was implemented for the purposes of evaluating:1)the phyto toxicity of lead/zinc mine tailings located in northern Guangdong Province,China,2)the growing performance of vetiver grass ( Vetiveria zizanioides Nash)and bahia grass( Paspalum notatum Flugge)in the mine tailings,and 3)the heavy metals uptake of both grasses from substrates of the mine tailings with different concentrations of heavy metals.The experiment was conducted with a method of pot cultivation undertaken in a glasshouse,which was ventilated and pervious to light.There were five treatments with foure duplicates for each treatment and they were arranged as follows:100%(w/w)mine tailings,75% tailings+25% domestic refuse,50% tailings+50% refuse,25% tailings +75% refuse,and 100% refuse.The most distinct characteristics of the mine tailings and the domestic refuse were high concentrations of heavy metals(including Pb,Zn and Cu)and organic matter,respectively.Seedlings of vetiver or bahia were planted into the pots treated with the above treatmentsthree tillers of same species for each pot and 20 pots(5 treatments×4 duplicates)for each species.Before plants were put into pots,the shoots and roots of vetiver were pruned to 30 cm and 3 cm,respectively,and one half of the leaves of bahia were removed and its roots also trimmed to 3 cm.After cultivation for 130 days,the height and tiller numbers of vetiver were measured,and then all plants were harvested and prepared for chemical analysis,including the respective contents of Pb,Zn,and Cu in roots and shoots of the two species. The following results were obtained:1) Both vetiver and bahia could survive in lead/zinc mine tailings,refuse or their mixed substrates,but grew worst in pure tailings and best in pure refuse;therefore,refuse could be used as an ameliorant to improve the growth of plants on tailings.2)The biomass of vetiver was influenced by heavy metals more severely than that of bahia,which meant that bahia was more resistant to heavy metals than vetiver under the condition of confined space,e.g.pot cultivation;however,vetiver produced far more biomass than bahia due to its bulk morphology.3)The contents of Pb,Zn,and Cu in both grasses all presented in the order ZnPbCu,but their ability to accumulate different metals into roots from substrates were different;vetiver's accumulative ability to the three metals ranked in the sequence of ZnCuPb whereas bahia's was CuZnPb.4)There was a substantial retention function for roots of the two species to heavy metals,and their retention rates in different treatments were almost all greater than 50%,namely the two plants kept much higher concentrations of heavy metals in roots than in their respective shoots;moreover the retention rates in same species to different elements were not identical,and the biggest value in vetiver was Pb,averagely up to 80%,and that in bahia was Cu,averagely up to 77%.5)Vetiver absorbed the greatest amounts of heavy metals from the 50/50 tailings/refuse treatment,whereas bahia did so at the 75/25 tailings/refuse treatment.Thereby,an addition of 25%～50% refuse into the substrate made the removal of heavy metals from the tailings more effectively and accordingly made plant colonization in the tailings more easily,and the refuse resource could also be utilized more fully and rationally.6)On the whole,vetiver could absorb more amounts of heavy metals in comparison with bahia,but it was not so easy to result in the second pollution or bioaccumulation as bahia,due to its higher retention capacity to heavy metals and its poorer palatability to animals.It may be concluded from this study,therefore,that vetiver grass is more suitable for phytopurification and phytomed