Rhizosphere Mechanism Involved In Chemical-Induced Phytoextraction Of Cd-Contaminated Soil By Brassica Napus L.

Soil heavy metal contamination impacts have long incubation period and hardly were discerned at the early stage due to slow toxic accumulation process.It is however difficult to be removed once environment receives contamination.How to remove pollutants from the habitat or render them harmless hence has been the priority in environment contamination clean-up research field. Phytoremediation,which acts as a potential strategy to remediate contaminated soils in the use of plants to remove pollutants,is now becoming an established technology for environmental clean up and protection,and increasing acquires concern due to its cheap,green and clean.In general,there are currently two means to employ Phytoremediation.One is to screen wild hyperaccumulation plants from available natural resources,which rather often gets some problems like the hyperaccumulation plants slow growth,small harvests and weak competition to native plants. Another is to utilize physical,chemical,biological,agronomical technologies to induce hyperaccumulation and native dominant plants.Among of these technologies,chemical induction as its distinctive advantages has got wide application.Currently,most researches focus on the fate of heavy metal under soil-plant ecology system, and heavy metal chemical behavior of Rhizosphere eco-system as well as Phytoremediation process are nonetheless rarely covered.Meanwhile many researches have showed that cruciferous vegetables are of strong heavy metal accumulation sorption and also liable for good annual harvest.Brassica napus L.as one of chinese main crops and cruciferous vegetables,mostly distributed in plants cutting parts can provide strong pollutants remediation.Chongqing agricultural land purple soils for many years have got heavy metal contamination and the cadmium（Cd） contamination is typically serious.In this study,Cd was chosen as heavy metal representative and Brassica napus L.was adopted as Phytoremediation material.Rhizobox cultivation,sorption-desorption behavior examination,soil plot planting and nutrient solution experiments were employed to investigate the sorption-desorption process of Cd in the Rhizosphere soil of rape and its distribution as well as forms transformation.The inductions of elevated nitrogen, chelating agents,organic acid and other chemical constitutes on rape sorption of Cd were also given detailed discussion.The main results were described as follows:Sorption-desorption behavior of Cd²⁺ in the Rhizosphere and bulk soil of rape was studied by the batch method.The results indicated that there were obvious difference between sorption capacity of Rhizosphere soil and that of bulk soil although both sorption capacities increased with the Cd²⁺ concentration growth.The sorption capacity of Rhizosphere soil to Cd²⁺ was higher than that of bulk soil.Isothermal curves of Cd²⁺ sorption by Rhizosphere and bulk soil fitted Langmuir,Freundlich equations well.The biggest sorption capacity of Rhizosphere soil and bulk soil to Cd²⁺ was 112.3 mg kg^-1,105.4 mg kg^-1 respectively.Compared with bulk soil,the desorption of Rhizosphere soil to Cd²⁺ was more difficult.The results also showed that pH and organic acid prohibited sorption-desorption of soil Cd²⁺.With the growth of pH,the desorption of Cd²⁺ exhibited S curve,which consisted of biggest desorption phase（pH≤2.0）,desorption rapid decrease phase（pH2.0～5.0） and precipitation & least desorption phase（pH5.0～9.0）.Desorption of Cd²⁺ reached peak at pH≤2.0（Desorption capacity surpassed 80%） and it decreased sharply from nearly 90.0%to 20.0%within pH 2.0～5.0 in acidic purple soils,and then decrease diminished at pH＞5.0.Meanwhile the results found that the types and concentration of root exudates organic acids also affected the sorption-desorption behavior of soil Cd²⁺.With the concentrations of citric acid,salicylic acid,acetic acid increased from 10^-4 to 0.1 mol L^-1,desorption of soil Cd²⁺ showed U curve trend.Organic acid at relatively low concentrations (≤10^-3 mol L^-1) slightly inhibited Cd²⁺ desorption,but enhanced Cd²⁺ desorption at higher concentrations with citric acid,salicylic acid,acetic acid(≥10^-3 mol L^-1) and tartaric acid(≥10^-2 mol L^-1).Citric acid,salicylic at higher concentrations(≥10^-1 mol L^-1 had the greatest improvement of Cd²⁺ desorption,followed by malic acid,acetic acid and the smallest was tartaric acid.Chemical forms of cadmium in the Rhizosphere and bulk soil of Brassica napus L.were investigated using rhizobox cultivation.The results showed that the continuous changes in cadmium fraction within the Brassica napus L.Rhizosphere might derive from DOC,Eh and pH variance.The amount of exchangeable cadmium increased before dropping below the initial level after 40 days or so.Carbonate associated cadmium followed a similar trend of change,but with a slower pace than the exchangeable cadmium.The increase in carbonate-associated cadmium reached the top,but with the net loss occurring after 55 days.There were also initial increase in oxide bound cadmium as well as decrease in the organic matter associated cadmium.The accumulation of cadmium in the Brassica napus L.was found to be positive correlation with biomass.The amount of accumulated cadmium absorbed in the plant material exceeded the initial quantity of the exchangeable cadmium in the soil, revealing a transformation from less bioavailable to more bioavailable fractions.During cultivation, decreases in redox potential and increase in pH,dissolved organic carbon（DOC）,in Brassica napus L.Rhizosphere were observed.The change in cadmium speciation may result from root-induced change in DOC,redox potential in the Rhizosphere.Soil plot planting results showed that the N form has a deep impact on the Cd in shoots of Brassica napus L..The ammonium-fed oilseed Brassica napus L.could significantly increase the content of Cd in shoots.However,the nitrate-fed oilseed Brassica napus L.could not increase the content of Cd in shoots of oilseed Brassica napus L.The rhizophere pH was significantly decreased with ammonium-based fertilizers.This changed the form of Cadmium bound to iron and manganese, cadmium bound to carbonates and Cd residual to exchange Cd.It enhanced the bioavaility of Cadmium.Nutrient solution experiments investigated the influence of the EDDS and EDTA on uptake and transport of Cd by Brassica napus L..The results showed that Cd were decreased in roots and increased significantly in shoots in the presence of EDDS and EDTA.The transportation of Cd from roots to shoots was enhanced in the presence of EDDS and EDTA and the results of Cd transportation indicated that EDTA was more effective than EDDS.It might be related to the fact that the concentration of Cd can be enhanced by EDTA via the xylem.Nutrient solution experiments also found that organic acids could mobilize soil Cd,the sequences of mobilizing ability of organic acids were followed,citric acid＞humic acid＞oxalic acid＞malic acid＞fulvic acid＞tartaric acid＞salicylic acid＞water.Compared with control,citric acid and humic acid both contributed to Cd content increase in shoot.In this dissertation,the author investigated Rhizosphere processes and mechanism involved in hytoremedition of cadmium contaminated purple soil.In addition,the results can provide scientific support for better understanding the chemical behavior of heavy metals in the hytoremedition. However,Soil contamination remediations involve many other complex factors,like organic and inorganic compound contamination,Rhizosphere organic pollutants abiotic oxidation,Rhizosphere microbial communities change,chemical induced soil and underwater risk,still further work should be employed in future research.