Research On Remediation Effect Of Compound Microbial Inoculum On Cadmium Contaminated Wheat Soil

The situation of soil cadmium（Cd）pollution in China is severe.According to the National Soil Pollution Status Survey Bulletin released in 2014,the exceeding rate of soil Cd content in China reached 7%.Microbial inoculum have small size,large specific surface area,surface charge,and certain metabolic activity.They can effectively adsorb and biotransform heavy metals in soil,and have great potential in heavy metal pollution control.In this study,firstly,the adsorption characteristics of microbial inoculum on Cd²⁺were analyzed by batch processing experiments,and the surface structure characteristics of microbial inoculum before and after adsorption and its interaction mechanism with Cd²⁺were clarified.The remediation effect of compound microbial inoculum on Cd contaminated soil was studied by soil culture experiment.Finally,the application effect of compound microbial inoculum on Cd contaminated soil was verified by field experiment.Through the above research,we hope to provide technical support for Cd contaminated soil remediation.The main results are as follows:（1）The adsorption capacity of compound microbial inoculum reaches the maximum（12.23mg/g）at p H 7.0,while the adsorption capacity of organic material decomposing agent reaches the maximum（13.23 mg/g）at p H 4.Freundlich isotherm model and pseudo second order kinetic model are more consistent with the adsorption process of Cd²⁺by the two microbial inoculum.The analysis results of scanning electron microscope（SEM）and energy dispersive spectroscopy（EDS）showed that the surface structures of the two microbial inoculum were damaged in varying degrees after the adsorption of Cd²⁺,and Cd elements were distributed on the surfaces of the two microbial inoculum,which proved that Cd was successfully adsorbed on the surface of the microbial inoculum.XRD results showed that Cd CO₃and Cd（OH）₂were formed on the surface of the compound microbial inoculum and organic material decomposing agent after adsorbing Cd²⁺,respectively.FTIR analysis showed that the functional groups of compound microbial inoculum involved in the adsorption of Cd²⁺were-OH,C-H,C=O,-NH and-CH,while the organic material decomposing agent were-OH,C-H,C=O and-CH.Therefore,the interaction mechanism between the two microbial inoculum and Cd²⁺is mainly the complexation of bacterial surface functional groups.（2）After 40 days of incubation with compound microbial inoculum,the soil p H and EC increased by 0.84%-2.43%and 31.6%-241.48%,the content of soil organic matter increased by8.11%-24.1%,and the soil cation exchange capacity decreased by 3.8%-40.51%.Compared with the control treatment,the content of soil available Cd decreased significantly by 15.48%-29.68%,and the occurrence form of soil Cd changed to a more stable residual form.The activities of soil urease,sucrase,catalase and dehydrogenase showed a trend of gradual enhance with the increase of compound microbial inoculum.（3）Through the field demonstration experiment,it was found that the soil p H value and organic matter content increased with the increase of the application amount of compound microbial inoculum,while the soil conductivity and cation exchange capacity decreased with the increase of the application amount.The content of soil available Cd in wheat filling stage and mature stage decreased in varying degrees with the application of compound microbial inoculum.When the application amount of compound microbial inoculum in mature stage was 600g,the content of soil available Cd decreased significantly by 8.64%compared with the control treatment（p<0.05）.During the grain filling stage,the proportion of iron and manganese oxides Cd in soil decreased by 1.85%-4.27%,and the proportion of residual state increased by 0.87%-4.83%.At maturity stage,the exchangeable state ratio of soil Cd decreased by 1.67%-2.48%,and the residual state ratio increased by 3.32%-6.6%.Wheat yield increased by 4.31%-15%under different concentrations of compound microbial inoculum.Compared with the control group,Cd in wheat roots decreased by 13.25%-32.6%and 4.3%-28.38%at grain filling stage and maturity stage,respectively.Under B300,B450 and B600 treatments,the Cd content in wheat stems and leaves decreased significantly by 19.64%,20.63%and 28.65%（p<0.05）.The content of Cd in wheat grain decreased significantly by 12.83%-33.15%（p<0.05）,among which B600 treatment decreased the most,up to 33.15%.Compared with the control group,urease and dehydrogenase activities increased significantly under B450 and B600 treatment（p<0.05）.After adding compound microbial inoculum,the activities of soil sucrase and alkaline phosphatase increased in different ranges compared with the control group,but there was no significant difference in each treatment（p>0.05）.The number of OTUs sequences and Shannon index of soil microorganisms increased in varying degrees,and the species diversity of bacterial community increased.Under different treatments,the relative abundance of soil Bacteroidota decreased,while the relative abundance of Proteobacteria,Acidobacteria and Firmicutes increased to a certain extent.