Effects Of Phosphate-solubilizing Bacteria On Immobilization Of Soil Cd By Weathered Coal

Large areas of agricultural soil in China are contaminated by cadmium（Cd）,which is easily accumulated in crops and harmful to human health through the food chain.Weathered coal is a potential effective material for soil Cd immobilization.However,whether the stability of Cd after immobilization is affected by the presence of a large number of rhizosphere-promoting phosphate-solubilizing bacteria（PSB）in the soil is unclear.In this study,a Bacillus TH-35 with Cd mobilization function was screened out from the rhizosphere-promoting PSB,which was from our research group in the early stage.The effects of rhizosphere-promoting PSB on stability of weathered coal immobilized Cd was studied by inoculating this strain in amaranth rhizosphere.This study can provide a reference information for the research and development of technology of weathered coal to immobilization soil Cd.The main results and conclusions were listed as below:（1）Cd tolerance of ten strains of PSB was tested,and the Cd-tolerant concentration was set up from 300 to 400 mg·L^-1.Due to strains of TH-32,TH-35 and TB-44 had relatively good Cd-tolerance,these strains were selected to mobilize Ca₃（PO₄）₂ and Cd CO₃.Meanwhile,the results showed that the contents of soluble phosphorus in the medium,inoculating with TH-32,TH-35 and TB-44,respectively increased by 111%,293%and 281%（p<0.05）,and the concentration of soluble Cd increased by 50%,115%and 108%,respectively（p<0.05）.In addition,a number of extracellular metabolites,such as organic acids,amino acids and carbohydrates,have been detected from the above culture medium.Then,TH-35 was selected for Gram staining and physiological and biochemical reaction determination,due to it had a relatively strong ability of dissolving phosphorus and cadmium.Finally,TH-35 was identified as Bacillus cereus.（2）A pot experiment of Cd contaminated soil was designed to evaluate the immobilization effect of soil Cd by weathered coal.The results indicated that the weathered coal was added on soil significantly reduced the Cd concentrations in amaranth roots,shoots,and rhizosphere soil solution（26.1%-27.5%in roots,21.2%-32.6%in shoots,13.5%-39.9%in rhizosphere soil solution）（p<0.05）,and the Cd concentration in rhizosphere soil solution was highly positively correlated with the Cd concentrations in roots and shoots（p<0.01）.Therefore,it could be concluded that weathered coal can effectively decrease the concentration of Cd in rhizosphere soil solution,thereby reducing the absorption of soil Cd by plants.（3）A pot experiment was conducted in a Cd-contaminated soil to study the mobilization effects of soil Cd by TH-35.The results indicated that Cd concentrations of amaranth roots and shoots increased by 25.8%and 18.8%（p<0.05）,when the strain TH-35 was inoculated alone.Meanwhile,the Cd concentrations increased by 20.7%and 34.8%in roots and 36.3%and 26.1%in shoots,when TH-35 was inoculated after 2‰and 3‰weathered coal was added（T4,T5）,compared with adding weathered coal alone（T1,T2）（p<0.05）.That is whether or not weathered coal was added to the soil,the inoculation of TH-35 in the rhizosphere significantly promoted the absorption of soil Cd by amaranth（p<0.05）.The analysis of soil Cd speciation showed that the addition of weathered coal transformed soil exchangeable Cd into organic fraction,but inoculation of TH-35 caused the exchangeable Cd concentrations to increase by 19.3%（p<0.05）,and Cd in the carbonate and Fe-Mn oxide fraction decreased by 29.1%and 8.73%（p<0.05）,respectively.This finding showed that the PSB enhanced the effectiveness of soil Cd in plants.（4）In order to investigate the effect of strain TH-35 on the plants effectiveness of weathered coal bond state Cd,sand culture was used to design a pot experiment.The results showed that the inoculation of TH-35 in the rhizosphere after adding weathered coal in sand culture had no significant effect on Cd absorption by amaranth.Scanning electron microscopy（SEM）,Zeta potential and Fourier transform infrared（FT–IR）spectroscopy analysis showed that the inoculation of TH-35 after weathered coal immobilization Cd didn’t change the microstructure,amount of negative charge,type and content of functional group of weathered coal（p>0.05）,indicating that the stability of organic fraction Cd of weathered coal is not affected by PSB.That is PSB will not significantly remobilize soil Cd remediated by weathered coal.