The Synergistic Remediation Of Cadmium Pollution In Purple Soil Farmland By Alfalfa And Plant Growth-promoting Bacteria

Cadium,（Cd）is a non-essential but highly toxic heavy metal element for the human body.When the environment is polluted by Cd,it will be accumulated in organisms and then enter the human body through the food chain,posing a threat to human life.In recent years,with the aggravation of heavy metal pollution in the environment,people pay more and more attention to and study the use of microorganisms to enhance phytoremediation technology.Plant growth-promoting bacteria（PGPB）increase the efficiency of phytoremediation by promoting plant development and resistance to heavy metals.Therefore,screening the PGPB with a friendly environment,strong resistance,and growth-promoting performance is the key to repair.For this purpose,the PGPB with metal resistance was screened and isolated from the metal-contaminated farmland,and the promotion effects on plant growth and metal remediation by two inoculation methods,i.e.,direct inoculation and preparation as sodium alginate gel beads,were studied through pot experiment.This study will provide a basis for the use of microbial-plant synergy to repair metal pollution in farmland soil.The following are the main results of this study:（1）After co-isolation and screening,30 strains of PGPB were obtained from metal-contaminated farmland soils and plants,which showed resistance to Cd(591.6mg·L^-1).Six strains were selected,which showed high plant growth-promoting properties,including indoleacetic acid production,iron carrier production,phosphorus solubilization,nitrogen fixation,1-aminocyclopropane-1-carboxylic acid（ACC）deaminase activity,and plate germination test.Through phylogenetic identification,five strains were identified as Klebsiella sp.and one strain was identified as Serratia sp.All the isolated strains showed good metal resistance and growth-promoting performance.Finally,strains Serratia surfactantfaciens Y15 and Klebsiella variicola Y38 were selected,which showed high IAA production capacity,siderophore-producing and phosphate solubilitation capacity,and used them as biological materials for pot experiments.（2）A pot experiment was conducted to study the colonization of PGPR in soil using the green fluorescent protein（GFP）labeling technology.Fluorescence signals were detected by a fluorescence microscope.The fluorescence signal was detected by a fluorescence microscope,and the effects of PGPB on alfalfa biomass and Cd absorption and accumulation under different inoculation methods were explored.The pot experiments showed that the endophytic growth-promoting bacteria S.surfactantfaciens Y15 and rhizosphere growth-promoting bacteria K.variicola Y38improved the photosynthesis and antioxidant effects,promoted root development and the absorption of Cd by M.sativa.These strains also promoted the transport of Cd from the root to the shoot.At the same time,the mixed-use of these PGPB was more conducive to promoting the growth of M.sativa and the absorption and accumulation of Cd by the plants than the single use.Therefore,the combined use of PGPB was more effective than the single use.In addition,it was found that the PGPB-coated sodium alginate gel beads were more conducive to the growth of plants and the repair effect of metals than direct inoculation.This also proved that the sodium alginate gel beads could effectively alleviate the influence of the external environment on the strain,make it play a better role and significantly improve the survival rate of probiotics.（3）Through the analysis of high-throughput sequencing technology,this study explored the influence of different strains on the rhizosphere bacterial community structure of M.sativa under Cd stress.The results showed that different inoculation treatments had different effects on the richness and diversity of rhizosphere soil bacterial communities of M.sativa.In particular,the sodium alginate gel bead treatment group,which co-wrapped the endophytic growth-promoting bacteria S.surfactantfaciens Y15 and rhizosphere growth-promoting bacteria K.variicola Y38,showed significant advantages in the diversity of bacterial communities,surpassing the other inoculation treatment groups.In summary,the two strains S.surfactantfaciens Y15 and K.variicola Y38showed the potential to promote plant growth and improve metal remediation efficiency,with practical application value.In this study,PGPB with heavy metal and growth-promoting properties was screened from metal-contaminated farmland soil,and the effects of PGPB application on plant growth and metal remediation were comprehensively explored through various studies,including plant biomass,antioxidant enzyme activity,changes in heavy metal content,and microbial community structure.Therefore,this study provided a valuable resource and scientific basis for the use of microbial-plant synergy in the remediation of heavy metal-contaminated soil.