The Ecological And Environmental Effects Of Cd And Its Mobilization And Removal Driven By Microbes In Polluted Paddy Soil

Cadmium（Cd）pollution in rice does seriously harm to human health,which has become an urgent problem to be solved.In recent years,microbial remediation has become an advanced technology because of its high efficiency,simple operation and green economy.Taking the Cd-polluted paddy fields in the"Chang Zhu Tan"area of Hunan Province as the research object,the environmental risk of heavy metals and its impacts on soil microbial community structure were clarified by comprehensively using the technologies of microbiomics,environmental risk assessment,soil physical-chemical characterization,soil interface analysis.We explored the effects of artificially constructed functional microbial agents on soil microbial species and functions through indoor and field experiments.The effects of microbial agents on Cd removal from paddy soil and Cd reduction from rice,on soil physical and chemical properties and microbial community structure were clarified.Based on the above research,the combined remediation of microbial agent and organic fertilizers was explored,which further reduced the content of available Cd in soil and Cd in rice.The microbes were isolated,and the mobilization and removal mechanism of Cd in soil under alkaline conditions and its potential in improving soil acidification were explored.This paper established the key technology of activated removal and remediation of functional microorganisms in Cd-polluted paddy soil,achieved green,efficient and rapid remediation of Cd pollution in paddy soil.The main conclusions of this study are as follows:（1）The heavy metal pollution in the soil of six typical polluted farmland in the"Chang Zhu Tan"area of Hunan Province were evaluated.The results of pollution index showed that the degree of Cd pollution is extremely high,and more than 90%sites exceeded the standard.Farmland in XT and LL areas were extremely polluted by heavy metals,while the soil in BS and PT area was moderately and slightly polluted,respectively.The results of ecological risk index indicated that the risk of Cd pollution was extremely high,and Cd was the main pollution causing environmental and ecological risks in all study sites.Farmland in XT area were facing the extremely high potential ecological risk,while the total carcinogenic risk caused by Cd in XT has exceeded human acceptable range.（2）The microbial community structure and phenotypic prediction of Cd-contaminated soil was analyzed by 16S r DNA sequencing.The results showed that the dominant phyla were Proteobacteria,Chloroflexi,Acidobacteria and Rokubacteria.At the genus level,the relative abundance of Sulfurifustis,Streptomyces,Arthrobacter,Burkholderia,Bradyrhizobium and some other microorganisms were enriched in higher Cd-polluted soils,indicating that these microorganisms were tolerant to Cd.Severe Cd pollution significantly reduced alpha-diversity of soil microbial community,while the synergistic effects of p H,Cd,and phosphorus shaped community structure,indicating that the change of p H and exogenous nutrients caused by human activities may help to reduce for the adverse impacts of Cd on farmland ecosystem.The potential stress-tolerant phenotypic prediction by Bug Base showed that soil microorganisms may adapt to the harsh environment by recruiting some microbes that directly or indirectly participate in diminishing oxidative damage of Cd.（3）The indoor simulation experiments were conducted to explore the removal efficiency of functional microbial agents on Cd.The results showed that the removal efficiency of Cd by microbial agent treatment（MA）was 32.56%,which was significantly higher than 13.58%of acid control group（AC）.Metagenomic results showed that the species and functional diversity of soil microorganisms in AC group were significantly lower than those in original soil and MA group after acid and microbial agent treatment.KEGG annotation results showed that the four amino acid metabolic pathways were significantly enriched in the process of AC group,but did not change significantly in MA group.Compared with the AC group,MA enhanced the nitrification pathway and denitrification pathway.Compared with MA,AC group improved the pathways of assimilation sulfate reduction to produce sulfide and oxidation thiosulfate to produce sulfate,while the MA group enhanced the pathways of sulfide oxidation to produce sulfite and synthesis of cysteine.Compared with the MA group,the AC group significantly reduced the methane metabolic activity of the soil system.ARDB annotation results showed that bacitracin resistance genes accounted for 91～97%of the total ARGs abundance,and the contribution of Sacharopolyspora to bacitracin resistance genes with the highest abundance in AC and MA groups reached 97%and 83%,respectively.（4）After microbial treatment in field experiments,the total Cd and available Cd in soil and Cd content in rice decreased by 13.8～48.25%,23.4～47.6%and 30.9～72.4%,respectively.The contents of total Cd,available Cd in soil and total Cd in rice reached the standards after one microbial treatment in slightly polluted soil.In moderate and severe pollution areas,the total Cd and available Cd in soil decreased significantly,and the Cd content in rice partly met the national standards.During the process of microbial treatment,soil p H significantly reduced,while the content of nitrogen,phosphorus and potassium significantly increased.After a recovery period of rice growth,soil p H and various nutrient elements nearly restored to the same level as the control group.During microbial treatment,alpha-diversity of microbial community had no obvious change,but the composition and structure of microbial community varied significantly,which was comprehensively affected by the nutrient elements,total Cd and available Cd content.（5）In view of the problem that the Cd content in soils and rice after one-time microbial treatment in moderately polluted rice fields does not meet the standard,after rice planting in two seasons a year,cow dung and milk vetch as organic fertilizers were applied respectively,in order to further reduce the content of available Cd in soil and Cd content in rice.The field experiment showed that the application of cow dung and milk vetch reduced the soil available Cd by 8.9～22.1%,16.9～31.7%and9.4～23.7%before rice planting,tillering and maturation stage,respectively.Applying cow dung and milk vetch significantly reduced the Cd content of rice by 37.04%and 36.74%,respectively.The application of organic fertilizers mainly caused the increase of Firmicutes and TPD-58 abundance in bulk soil and rhizosphere soil at maturation stage,increased by15.94～17.07%and 0.58～1.3%,respectively.（6）A total of 36 strains were isolated from heavily Cd-polluted soil,and the removal efficiency of Cd was 11.5～68.1%.Strain DY-8 had the highest removal efficiency of total Cd in soil（66.39%）,and the content of four fractions of Cd reduced by 38.8～67.1%.The content of Fe³⁺,Fe-O,C-O and O-C=O of soil decreased before and after treatment with strain DY-8,while the C=O and S_n^2-of microbial cell surface decreased after treatment,indicating that iron and sulfur may play important roles in Cd mobilization.The whole genome analysis showed that strain DY-8 was considered to belong to species Alcaligenes faecalis.DY-8 had complete siderophore and ammonia biosynthesis pathway.Siderophore and ammonia was speculated to be two important matters for Cd removal from soil.Meanwhile,the production of ammonia plays an important role in improving soil acidification.In addition,strain DY-8 also had a metabolic pathway of sulfide production,which can rapidly metabolize cysteine to produce Cd S nanoparticle materials.Therefore,strain DY-8 has potential values in solving Cd pollution in soil and water,and soil acidification.