Microbially-Enhanced Phytoremediation Of Acidic Cadmium-Contaminated Soil By Turfgrass And Its Mechanism

Heavy metal pollution has attracted wide attention due to its great threat to the environment and human health,especially cadmium pollution occurs frequently all over the world.A large amount of cadmium is released into water and soil environment,exposing crops to high risk of cadmium pollution,seriously affecting ecosystem health and drinking water safety.Therefore,it is urgent to develop feasible remediation technologies.At present,microbial enhanced phytoremediation is considered as a sustainable remediation strategy for contaminated soil.However,there are few studies on the changes of soil property and soil restoration in the process of intensive phytoremediation by tolerant microorganisms selected from contaminated soil.Some turfgrasses,as high biomass plants with heavy metal tolerance,can be used as ideal materials for phytoremediation.Therefore,the purpose is to screen cadmium-resistant strains from acidic cadmium-contaminated soil in mining areas,and set five application concentrations（0,10³,10⁵,10⁷ and 10⁹ CFU/mL）.To evaluate the practical application effect of different concentrations of bacterial solution on the remediation of cadmium-contaminated soil by tall fescue（Festuca arundinacea ’Arid Ⅲ’）and Kentucky bluegrass（Poa pralensis ’Midnight Ⅱ’）,and study the relationship between soil physical and chemical properties,plant enzyme activities,soil and plant Cd concentrations,Cd availability and microbial community.It shows the potential of the tested strains in the bioremediation of Cd contaminated soil,in order to provide a new technical idea for the treatment of acid Cd contaminated soil in mining areas.The main conclusions are as follows:（1）A newly isolated strain SP1 was screened from acidic heavy metal contaminated soil in alum mining area of Lujiang County,Anhui Province by plate streaking method.Strain SPl could still grow when Cd²⁺ concentration was 200 mg/L.The colony of SP1 on solid medium was round or nearly round,milky white opaque,with protrusions in the center of the colony,and the length was 0.75-1.30μm.The morphological characteristics were similar to those of Bacillus cereus,and the GenBank accession number was KR063181.1.Combined with 16S rRNA sequence homology analysis,SP1 was identified as Bacillus cereus.The optimum growth conditions of strain SP1 were determined as temperature=30℃,pH=7 and NaCl concentration=1%.（2）The adsorption characteristics of SP1 were studied,and the optimum adsorption conditions for Cd²⁺were pH=7,temperature=30℃ and inoculation amount=4%.The fitting results of isothermal adsorption model and adsorption kinetic model showed that,the correlation coefficients R² of Langmuir and Freundlich isothermal models were 0.9291 and 0.9631,respectively.The Freundlich model had a better fitting effect on the adsorption process of strain SP1,which was the multi-molecular layer adsorption process.At 25,50 and 75 mg/L Cd²⁺concentrations,the first-order kinetic fitting correlation coefficients were greater than 0.90,and the second-order kinetic fitting correlation coefficients were greater than 0.99,indicating that the entire adsorption process of Cd²⁺by strain SP1 was more consistent with the secondorder kinetic model.（3）The effects of different concentrations of SP1 on the remediation of cadmium contaminated soil by lawn plants were studied by potting experiments.The results showed that the application of strain SP1 increased soil pH value and organic matter content,and increased soil nutrient content.In addition,the application of SP1 improved the bioavailability of Cd in the rhizosphere soil.By promoting the absorption and the removal rate of Cd in soil by plants,the total Cd content in soil was reduced.Among them,G2 and Z2 treatment（105 CFU/mL）had the best effect.Compared with the control treatment G0 and Z0,the bioavailable Cd content increased by 13.43%and 26.54%,the total Cd content in soil decreased by 10.28%and 13.49%,the total Cd accumulation in plants increased,and the performance was tall fescue higher than kentucky bluegrass.The application of bacteria was beneficial to reduce the malondialdehyde（MDA）content in plants.The treatment of tall fescue decreased by 0.07%-32.64%,and the treatment of kentucky bluegrass decreased by 4.9%-23.29%,which significantly reduced the oxidative stress of plants under Cd stress,and the activity of antioxidant enzymes（peroxidase,catalase,superoxide dismutase）was enhanced,thereby increasing plant biomass.（4）The relationship between soil ecological factors and the regulation mechanism between bioremediation and soil microecology were clarified.The application of SP1 bacterial solution increased the diversity and richness of bacterial communities in the rhizosphere soil.Compared with the non-bacterial treatment,the total proportion of Proteobacteria and Actinobacteriota in the soil decreased,and the total proportion of Firmicutes and Bacteroidota increased.The results of LESfe analysis of community markers showed that different treatments caused differences in indigenous microbial groups and active expression in rhizosphere and nonrhizosphere soils.RDA analysis and Spearman correlation analysis showed that soil physical and chemical properties（pH,organic matter,total nitrogen,available phosphorus）,soil Cd content and various forms of cadmium content were the main factors affecting bacterial community structure,indicating that soil microbial community structure was driven by soil nutrients.