| Soil pollution is one of the most serious environmental concerns on a global scale.In China almost 20% of arable lands are contaminated by various pollutants particularly cadmium(Cd)with its highest 5.28 pollution index(PI)in farmland soils.Therefore,eco-friendly strategies should be established to ameliorate the Cd polluted soils.However,amelioration of contaminated soils with toxic metals using chemical and mechanical methods is expensive.Thus,cheap and sustainable approaches,for instance growing of plants that accumulate large amounts of metals in their tissues,are highly needed for soil health improvement.The castor(Ricinus Communis L.)plant has good potential to remediate metals and tolerate Cd stress and is known as an excellent Cd-hyper-accumulator.The chelates play an important role in amelioration through breaking the equilibrium between the soil liquid and solid phases and thereby degrade phytotoxic compounds.Biochar is also a good organic material that can ameliorate heavy metals.Biochar increases soil pH and adsorb heavy metals and thereby alleviate the toxic effects of metals for plant roots.The arable soils were collected from Wuhan,Hubei Province,China.Air dried soils were treated with different Cd levels 0-400 mg kg-1 soil,using Cd(NO3)2.4H2 O salt respectively and incubated at 25 0C.Several treatments were applied to Cd contaminated soil to examine the efficiency for amelioration chelates were used as following: ethylenediaminetetraacetatic acid(EDTA)3--15 mmol kg-1,nitriloacetic acid(NTA)3-10 mmol kg-1 and ammonium citrate(NH4 Citrate)10-25 mmol kg-1 and rice straw biochar(RSB)3% and 6% respectively.Castor plant was grown in the treated soils for 30-40 days in various experiments.The main results showed:(1)The growth of castor plants and metal-remediating efficiency exposed to Cd concentrations can be affected by chelators.The application of chelators significantly reduced the plant height with increasing concentration of chelators,such as EDTA 3 to15 mmol(29-41 %),NTA 3 to 10 mmol(55-22%)and NH4 citrate 10 to 25 mmol kg-1(18-21%)respectively,as compared to control.In addition of chelators,dry biomass of castor plant significantly decreased by 64-114%,125-51% and 36-42% under EDTA 15,NTA 10,NH4 citrate 25 mmol kg-1,respectively,to Cd-contaminated soil as compared to control.The higher Cd concentration in the soil was up to 54 %,53% and 26% under NH4 citrate 25 mmol,NTA 10 mmol and EDTA treatments as compared to control.The higher uptake of Cd concentration in the castor plant tissue was recorded in leaf,shoot and root was up to 2%,5% and 3% under NH4 citrate 25 mmol as against to control.While,the significantly increased biocencentration(BCF)value in leaf,shoot and root were noted in 0.4,0.8 and 1.2%,under EDTA 15 and NH4 citrate 25 mmol as against control treatments.Furthermore,the higher Cd concentration in the remediation factor(RF)leaf-to-soil and leaf-to-root ratios was up to 0.6% under EDTA 15 mmol treatment as compared to control.The FTIR analysis suggest that chelators with Cd treated soil had strong adsorption of metals,due to complexity with chelators and metals by functional groups and may create stable complexes with the metal ions.(2)The synthetic chelating agents have played prospective role to inflate the metal solubility and to intensify the microbial biomass carbon(MBC)and dissolved organic carbon(DOC)in the polluted soil for sustainable environment.The treatments are comprised of two levels of Cd(-Cd and +Cd)with three chelating agents EDTA,NTA and ammonium citrate were applied at the rate of 5,10,and 50 mmol with five kg of soil per pot and replicated with three times.Our results are satisfied that the application of chelators such as NH4 citrate and EDTA treatments significantly increased Cd concentrations in soil by 0.82 and 0.64 mg kg-1 respectively,under Cd polluted soil as compared to non-polluted soil.Furthermore,the Cd absorptions superior in castor plant tissue was significantly(p< 0.05)inflate under NH4 citrate and EDTA treatments(1.03 and 0.61 mg kg-1)in Cd polluted soil as against non-polluted soil.The structural characterization of FTIR analysis using spectral 1032-1053 cm-1 NH4 citrate chelators revealed a clear difference between the non-polluted and Cd polluted soil.While,the NTA and NH4 citrate treatments were significantly inflate MBC and DOC content under polluted soil by 81.98 and 80.37 and 1.96 and 1.90 mg kg-1 was increased under NH4 citrate and NTA in Cd polluted soil in comparison to non-polluted soil.However,the phytoextraction potential were significantly(p<0.05)increased under EDTA treatments in Cd polluted soil.Furthermore,the significantly relationship was found with DOC,MBC and available metal in the polluted soil as against non-polluted soil.While,there was no significant relationship found with MBC in the non-polluted soil as against polluted soil.More attention should be paid to explore the appliance of chelators might be appropriate candidate in Cd polluted soil.(3)The growth of castor predictable in term of plant height and biomass was significantly reduced at the 25 and 50 mg kg-1 Cd stress treatments as against control.The Ricinus communis L.was extremely affected by the plant growth and biomass decline under 25-50 Cd mg kg-1,which caused 71%-116%,166%->200% and 48%-84% in stem,root and plant height,significantly declined under 25-50 mg kg-1 Cd stress as compared to control.However,the satisfy of Cd in the Ricinus Communis L.in root,stems and leaves were increased with increasing Cd concentrations in the soil 25 and 50 mg kg-1 Cd stress treatments as against control.Our results specify that 50 mg kg-1 Cd significantly increased Cd concentration in plant tissue exhibited 2 fold,1.8 fold and 1.5 fold in leaves,stem and roots respectively as compared to control treatments.While,the peroxidase(POD)and super oxide dismutase(SOD)enzyme activities were significantly increased(215.30 ?mol/g-1 and 53.20 U/g)under 50 mg kg-1 Cd stress as against control.Therefore,the malondialdehyde(MAD)and chlorophyll content were dramatically decreased(3.11% and 0.48%),proline content values 2.23 to 1.75 ?g-1 under 25 and 50 mg kg-1 Cd stress as against control.(4)The concentration of soil Cd extracted with CaCl2 significantly decreased 79 % by rice straw treated with 6% biochar in castor as against control.The concentration of Cd in shoot and root of castor plant decreased by 60% and 74% with 6% with addition of 6% rice straw biochar as against to control.The application of rice straw biochar(RSB)significantly increased soil pH under polluted soil.The enhanced soil pH and organic carbon may contribute to a decrease in the available concentration of metals by reducing metal mobility and bonding metals to more stable fractions.Biochar treated soil has higher zeta potential and adsorbed more Cd than no biochar.The results demonstrated that the Ricinus Communis L.as a hyper-accumulator plant,to clean up contaminated soils with the assistance of chelators and biochar,castor might be appropriate candidate for the phytoextraction of heavy metals from contaminated soils. |
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