Study On Remediating Mechanisms Of Poplar In Highly Metal-Contaminated Calcareous Soil

The use of bioenergy plants such as poplars to manage heavy metal-contaminated soil is a green,economical and sustainable remediation technology that has emerged in recent years.However,the long-term phytomanagement process and the plant repair mechanism is still not clear.In this study,to investigate soil physical and chemical properties were driven by stand age,heavy metals concentration in the soil,bioavailablity,species and the changes of heavy metals concentration in poplar organs and soil biological characteristics,field positioning sampling experiments were carried out.The main findings are as follows:?1?Compared with the bulk soil,pH did not change significantly in the rhizospheric soil.But EC was reduced in rhizospheric soil.At the same time,the concentration of CaCO₃ in rhizospheric soil gradually decreased with stand age.Compared with bulk soil,the concentration of different species of C in the rhizospheric soil did not change significantly.However,compared with the accumulation of SOC?Soil total carbon?at young stand age,the significantly decreased SOC concentration in old stand age was observed.The early?young stand age?and late establishment?old stand age?of poplar phytomanagement systems significantly reduced the N concentration in the rhizospheric soil,but had no significant effect on the concentration of P in the soil and the stoichiometric ratio of nutrients.The level of Olsen P in the soil could not be significantly reduced throughout the phytomanagement process.?2?Total concentrations of Cd and Zn were decreased with an increasing stand age in the rhizospheric soils.The total removal rates ranged from 7.72%to 31.77%and 0%to 45.33%,respectively,and the average annual removal rates ranged from.81%to 1.44%and 0%to 3.35%,respectively.The bioavailable Cd concentration in the rhizospheric soils was generally higher than that in the bulk soil.Bioavailable Cd concentration was positively correlated with Bioavailable Cd concentration was positively correlated with C/N ratios,DOC,but negatively correlated with C/P ratios,N/P ratios,total N content,total P content and SOC.Bioavailable Zn concentration was significantly higher in the early stage of establishment of the phytomanagement system,and it was negatively correlated with pH.OM-bonded and Carbonate-bonded Cd concentration were significantly decreased in old stand age,while the OM-bonded and Carbonate-bonded Zn concentration were significantly lower than that in the bulk soil.?3?Results from the univariate multi-factor ANOVA showed that the single effects of plant organ and stand age on Cd and Zn concentrations were significantly?P<0.05?.The interaction effects was also significantly?P<0.05?.T Mean concentrations of Cd and Zn in the organs of P.simonii were in the descending order of bark,leaves>branches>wood.Cd and Zn are mainly distributed to the bark in the young stand stages of the establishment of the phytomanagement system,while the highest accumulation of Cd and Zn in leaves was observed in the old stand ages of the establishment of the phytomanagement system.The concentration of Cd and Zn in the wood of the trunk was always the lowest during the growth period,which were not higher than 10 and25 mg kg-1,respectively.Compared to leaves in the growth period,leaf litter has a relative higher enrichment ratio for heavy metals.?4?DHA and GLU activity was significantly inhibited in the stages of establishment of the poplar phytomanagement system.However,the activity of URE associated with N cycle in rhizospheric soil was always significantly higher than that in bulk soil?P<0.05?.ALP activity was gradually declined with the increasing stand age.Redundancy analysis showed that the activity of ALP was significantly negatively correlated with the concentration of bioavailable Cd,while it was significantly positively correlated with the content of C,N and P and the stoichiometric ratio of nutrients in the soil.In summary,during the long-term phytomanagement period,the conversion of nutrients in the soil induced by the activities of poplar trees will enhance the bioavailability of heavy metals,and poplars will mainly detoxify the negative influence of heavy metals through accumulate metals in bark and remove metals by leaf litter.Therefore,poplars have the ability of plant extraction and recovery in heavy metal-contaminated calcareous soil.ALP can be used as a sensitive biological indicator to indicate the changes of nutrients and heavy metals in the soil.