Study On Phytoremediation Of Cd Contaminated Soil By Medicago Sativa Treated In Different Levels Of Inorganic Nutrient And EDDS

Due to the great progresses of the social development,human activities have a stronger impact on the natural environment than before.A heavier soil pollution has been formed consist in the unreasonable emissions of the contaminants which are discharged from various businesses,such as mining of minerals,smelting of metals and utilizing of energy.The potential of phytoremediation to restore heavy metals polluted soil has been recognized widely,the positive role of nutrient elements and chelating agents played in enhancing phytoremediation also has been proved by numerous studies.Because there are great differences in degree of pollution in different regions,and the related factors（soil,climate,plants…）have obvious regional features,it is still worth exploring how nutrient elements and chelating agents work in phytoremediation.We take potted Medicago sativa as the research objects and add a Cd Cl₂·2.5H₂O solution to simulate Cd polluted soil（15 mg/kg soil）.This study consists of three parts:treatments of inorganic nutrient elements（nutrient treatments）,treatments of EDDS and combined treatments of nutrient elements&EDDS（combined treatments）.Different nutrient levels are set in nutrient treatments:Hoagland Nutrient Solution（H）,double the concentration of nitrogen（2N）,double the concentration of phosphorus（2P）,double the concentration of potassium（2K）.Different concentration gradients are set in EDDS treatments:0.5mmol/L,1.0 mmol/L,0.5 mmol/L+0.5 mmol/L,2.0 mmol/L,1.0 mmol/L+1.0 mmol/L,3.0 mmol/L,1.5mmol/L+1.5 mmol/L.The growth of plants,accumulation of Cd,forms of Cd,physical/chemical properties of soil and changes of evaluation indexes are the main research contents.We can understand the working effects of nutrient elements and chelating agents on phytoremediation through the above contents.Meanwhile,we hope to provide some scientific references for the large-scale application of phytoremediation as well.The main findings are as follows:（1）Nutrient treatments are able to alleviate the Cd stress,and all Medicago sativa growth indexes reach the peak on day 100.The relative over-application of N,P,K causes slight inhibition.On the whole,Medicago sativa growth condition shows a trend of“increasing first and decreasing later”as the EDDS concentration increases.Low concentration of EDDS significantly promotes plants growth,and 0.5mmol/L+0.5 mmol/L EDDS on the 100_th day（0.5+0.5E-3）works the best,which shows intermittent adding mode（interval of 5 days）is helpful to reduce the Cd stress and promote growth.Such improvement gradually diminishes as the EDDS concentration increases.Combined treatments are good to the accumulation of plants biomass,and available to alleviate Cd stress,Hoagland Nutrient Solution+0.5mmol/L EDDS（H+0.5E）works the best.（2）Nutrient treatments is helpful to accumulate Cd by Medicago sativa.In all growth cycles,plants show similar abilities to accumulate Cd in aboveground part in each nutrient treatment.2P treatments lead to a better enrichment in underground part.Cd accumulation goes weaker under 2K treatments when growth time increases.The addition of EDDS at the later growth stage（100 days）is beneficial to the enrichment of Cd,and 0.5+0.5E works the best.2P+EDDS treatments have a better effect of Cd accumulation than other combined treatments,and 2P+0.5E works the best.（3）All nutrient treatments is helpful to reduce the soil Cd,and it works better as the growth cycles increases,Hoagland Nutrient Solution on the day 100（H-3）works the best among the nutrient treatments,2K works the worst.Adding EDDS in the middle/late stages of plants growth has an effective role of reducing soil Cd,and 0.5+0.5E works the best on day 80 and day 100.H treatments added with low concentration works the best on reducing soil Cd.However,relative over-application of N,P,K will weaken the Cd reduction effect.（4）The content of residue state Cd（F5）and organic bond state Cd（F4）are higher in the soil used in this study.All nutrient treatments can activate the soil Cd,and 2P treatments have a lower effect.In the early and late stages,relative over-application of K contributes to activating soil Cd,but it shows opposite effect in middle stage（80 days）.H treatments show great activation.EDDS is helpful to soil Cd activation in early stage,and a single adding works better.On the whole,there is a better activation effect when the total EDDS concentration reaches 2.0 mmol/L.2N+EDDS treatments reduce the content of the exchangeable Cd（F1）,increase the content of F5 and are bad for plants Cd accumulation.2P+EDDS and2K+EDDS treatments enhance the percentage of F1 and increase the Cd bioavailability.（5）In this study,p H stays stable in all treatments.The relative over-application of N increases the activity of soil dehydrogenase（DED）,soil catalase（CAT）,soil urease（URE）and soil sucrase（SUC）in early growth stage in nutrient treatments,but the relative over-application of P doesn’t.EDDS treatments show an inhibitory effect on soil DED activity in early stage which reaches peak under 2E-1（2.0 mmol/L EDDS on day 60）;then show promoting effect in middle and late phases.During the early and middle phases,EDDS enhances the activity of soil CAT,and 0.5E performs the best;however,it has no significant effect in late growth stage.Soil URE activity is at a high level under EDDS treatments in early stage,and goes up higher as the EDDS concentrations increases in late phase.After 60 days of growth,both lower and higher concentrations promote the soil SUC activity,while middle concentrations show the opposite effects;in late stage,the soil SUC activity reaches the peak at 2E-3（2.0 mmol/L EDDS on day 100）,and goes up higher as the EDDS concentrations increases.In combined treatments,the activity of soil CAT is promoted;the relative over-application of nutrient elements reduces the soil SUC activity;H treatments enhance the activity of soil DED.The soil CAT activity is at a higher level under 2P+EDDS treatments,and2P+0.5+0.5E performs the best.All combined treatments have a similar ability to enhance the soil SUC activity;the soil SUC activity increases when 2N or 2P treatments are applied with EDDS,and 2N+0.5E works the best.H treatments added with low EDDS concentration treatments increase the activity of soil SUC,but they don’t work as well as 2N+0.5E.（6）The aboveground bioaccumulation factor（ABF）,underground bioaccumulation factor（UBF）and remediation efficiency（RE）are at a high level in late growth stage.Having added EDDS in late phase increases the ABF,UBF,transfer factor（TF）and RE,and 1E-3 performs the best.ABF and UBF are enhanced under 2P+EDDS treatments;2K+EDDS treatments lead to a higher TF.