| Phytoremediation is the use of plants to remove pollutants from the environment or make them stabilized or less toxic.Dendroremediation is the use of trees for phytoremediation.It has been a promising solution to heavy metal pollution,especially when using fast-growing species of trees,due to its many advantages,such as large biomass,high growth rate,economic value,low nutrient demands,site stability,etc.Poplar is a wellrecognized tree species for heavy metal extraction and is also a typical fast-growing species.Leaning growth happens in reality and that produces tension wood above the inclined trunk of broad-leaved trees.Tension wood has certain special wood structure,such as a gelatinous layer with high content of cellulose and more micro pores among the microfibril bundles,which might be helpful in the accumulation of heavy metals.There has not been such study on comparing the dendroremediation by upright and leaning growing trees.In this study,a pot experiment was designed to study the potential of 69 poplar trees(Populus deltoides Bartr.cv.’Lux’(I-69/55)’)in the extraction of cadmium(Cd).The soil was treated with different concentrations of Cd(5,20,50,and 100 mg/kg dry soil),plus one control treatment,where no extra Cd was applied.Two growth patterns,i.e.upright plantation and artificially inclined by 45° angle plantation,were included.Height and basal diameter were measured every month.In December of each year the one-year old and twoyear old poplars were sampled for the root,leaf,the bark and stem(along three trunk heights),branches and stem wood part in different annual rings(for only two-year old trees)for both upright and leaning poplars,as well as the bark and stem at the tension and the opposite zones,also at the three heights,for the leaning poplars.Cd concentrations were analyzed for each parts.Bio-concentration Factor(BCF)and Translocation Factor(TF)were calculated for various parts to understand the phytoextraction ability.The results demonstrated that:(1)The inhibitory effect of Cd addition on poplar growth was significant and the inhibitory effect was not proportional to the concentration of Cd in soil.In spite of the inhibition effect,both upright and leaning poplars can grow until at the treatment with100 mg/kg Cd addition,indicating strong tolerance of poplar to Cd.Leaning plantation inhibited poplar growth significantly.(2)Cd contents in various tissues increased with the increase of Cd in soil for both upright and leaning poplars.The highest Cd was accumulated in the above ground parts of transpiration,i.e.the bark or the leaf.Cd in the wood part of branch and stem was always the lowest.Along the trunk height,the largest Cd content was in the top for the stem,but in the basal for the bark,for both one-year-old and two-year-old poplars.Between the two radical wood rings of the two-year-old poplars,the second ring(near the bark part)contained significantly more Cd than that in the first ring(near the pith).(3)Different parts of poplar demonstrated average BCFs ranges between 0.08 to 4.27,and average TFs between 0.28 to 4.81,indicating big difference among the parts,between the two plantations and between the ages of tree.Cd levels in the soil influenced both BCF and TF significantly.BCFs declined with the increase of Cd addition in the soil,but the change of TF was not proportional to Cd levels in the soil.Bark or leaf demonstrated the highest BCF and TF,while that of the branch and stem were the lowest,indicating very poor phytoextraction ability in wood parts.(4)More accumulation of Cd was found in the stem wood tissue of leaning poplars than in the stem of upright poplars,not significant for one-year-old poplars but significantly for the two-year old poplars,though it might not be the case for other parts.Average BCFs of leaning poplars were generally not higher than that of upright poplar,while average TFs of leaning poplar was larger than that of upright poplar.(5)The two-year-old poplar accumulated higher average concentrations of Cd in various parts and demonstrated higher BCFs than the one-year-old poplars,indicating possible increasing phytoextraction ability with poplar growth.But the TFs of the two-year-old poplar was lower than that of the one-year-old poplars.Further study would be interesting to understand more about dendroremediation,especially with a focus on the cell and cell wall level to recognize the processes and pathways of phytoextraction,on the promotion efforts to increase phytoextraction ability and to promote the accumulation in the stem,and on tracking the whole growth period.It is also necessary for the practical pilot-scale study before using the dendroremediation in soil restoration. |
PDF Full Text Request