The Distribution Characteristics And Influencing Mechanism Of Bacterial Community In Maize Rhizosphere Soil Under Vanadium Stress

Vanadium（V）is a trace element widely distributed in nature,and its biological activity and toxicity are mainly affected by chemical fraction.Some forms of V are classified as teratogens and potential carcinogens,and their effects on human health are not clear.As a variable element,V has special geochemical properties and complex natural existence forms.However,the migration and transport mechanism of V in soil-plant system is not clear at present,and the interaction between rhizosphere soil microorganisms and environmental factors is not well understood,which limits the accurate identification of its biological toxicity and influencing factors.Research on the migration and transformation of soil-plant system V and its interaction with soil microorganisms can provide an important scientific basis for further understanding the biogeochemical behavior of V and controlling its ecological risks.In this research,Panzhihua Vanadium titanium magnet mining area was selected as the research area,and the soil-maize system under V stress was taken as the research object.Combined with field observation and maize pot experiment,Bureau Community of Reference（BCR）,X-ray absorption near edge structure（XANES）,micro-X-ray fluorescence（μ-XRF）,16S r RNA high-throughput sequencing and metagenomic analysis were used to study the distribution and transport of V in soil-maize system under natural and experimental conditions.The distribution characteristics and influence mechanism of bacterial community in maize rhizosphere soil in Panzhihua area were revealed,as well as the distribution characteristics and influence mechanism of bacterial community in maize rhizosphere soil under artificial simulation V stress.The following insights and conclusions are obtained from the research:（1）The average fraction of residual V in maize rhizosphere soil in Panzhihua area was 83.41%.The alternating wetting and drying promoted the reductive dissolution of iron and manganese（hydrogen）oxides,resulting in the release of reducible V into the soil solution.The content distribution of V in maize organs was as follows:root>leaf>stem>fruit.The enrichment coefficient of maize in Panzhihua region was less than 1,and the transfer coefficient of some maize was more than 1,indicating that maize in Panzhihua region had a strong transport capacity for V.Nemerow pollution index evaluation results showed that V,Cu,Cd and Pb in the soil in Panzhihua area were severely polluted,Cr and Ni were moderately polluted,and Zn were mildly polluted.V has moderate to strong pollution in some areas,indicating that mining activities have caused certain pollution in Panzhihua region.As in soil poses a non-carcinogenic risk to children,and Cr in maize poses a non-carcinogenic risk to adults and children.Cr,Cd and Ni in corn pose a cancer risk to adults and children.Based on the above evaluation results,it is necessary to study the distribution and transport mechanism of V in soil-maize system.（2）The analysis of bacterial community characteristics of maize rhizosphere soil under natural conditions in high V background area showed that the dominant species of soil bacterial community in Panzhihua area were Actinobacteria,Proteobacteria,Chloroflexi,Acidobacteria and Bacteroidetes.Soil physical and chemical properties had an important effect on rhizosphere soil bacterial community.There was a positive correlation between oxidizable fraction of V,carbon and nitrogen metabolism of the bacterial community,and the metabolic function of rhizosphere soil bacterial community was mainly affected by the chemical fraction of soil V.It is worth noting that the residual fraction of V also showed a relatively obvious positive correlation with some bacteria.In this study,the p H value,Eh value,conductivity,water content（WC）,soil organic matter（SOM）,cation exchange capacity（CEC）,total nitrogen（TN）,total phosphorus（TP）,total potassium（TK）,alkali-soluble nitrogen（AN）,available potassium（AK）,available phosphorus（AP）,total vanadium（TV）,and acid-soluble fraction of V（Acid-V）,reducible fraction of V（Red-V）,oxidizable fractions of V（Ox-V）,and residual fractions of V（Res-V）and other 17 environmental factors were determined on rhizosphere soil bacterial community,among which the main influencing factors were:SOM,TP,AP and Acid-V,etc.,which laid a foundation for further revealing the influencing mechanism of bacterial community under V stress.（3）The results of artificial simulated V stress pot experiment showed that the p H value of soil pore water decreased with the growth cycle,and the p H value of maize group was lower than that of maize group,indicating that organic acids and amino acids produced by rhizosphere secretions could affect the p H value of soil pore water.The Dissolved Organic Carbon（DOC）value of maize rhizosphere soil decreased gradually,indicating that with the development of growth cycle,maize rhizosphere and microbial community gradually absorbed and consumed soil Carbon,resulting in the gradual decline of DOC content.With the increase of the addition of V,the content of V in soil pore water gradually increased by an order of magnitude.The content of V in non-maize group was higher than that in maize group,indicating that maize absorbed soil V.Phreeqc simulation results show that the chemical forms of V in pore water are abundant,and V mainly exists in the form of pentavalent oxygen anion,among which H₂VO₄^-accounts for the largest proportion.（4）The physicochemical properties of rhizosphere soil（TN,TP,SOM and CEC）did not change significantly with the growth cycle.Alkaline phosphatase（AKP）showed little change with the growth cycle.β-glucosidase（β-GC）decreased gradually with the development of growth cycle.The contents of Urease（URE）and Catalase（CAT）increased gradually with the growth cycle.With the increase of V concentration,the ratio of Acid-V gradually increases,the ratio of Red-V gradually increases,and the ratio of Res-V gradually decreases.The proportion of Res-V in the non-maize group was higher than that in the maize group,and the proportion of Acid-V in the non-maize group was lower than that in the maize group.（5）With the increase of V concentration,Malondialdehyde（MDA）in maize leaves increased,indicating that the degree of lipid peroxidation in maize cells gradually increased.The overall distribution characteristics of V in different organs of maize were as follows:root>leaf>stem>fruit.The transfer coefficient and enrichment coefficient of maize organs were all less than 1,and at maturity stage>flowering stage>jointing stage.When V stress content is 0,50,100,200 mg·kg^-1,respectively,the content of V in maize organs also showed an order of magnitude increase.（6）Combined with synchrotron radiation XANES analysis,the results showed that the Fe elements in maize root were mainly composed of Fe₂O₃and Fe₃O₄.Fe in soil is mainly composed of Lepidorcrocite and Fe₂O₃,which indicates that Fe exists in trivalent form in soil.The results ofμ-XRF analysis showed that V and Fe were significantly enriched in roots,and the high value area of the two highly coincided,indicating the cooperative absorption of V and Fe in plant roots.It indicated that the redox or precipitation reaction may have occurred in maize roots,and some Fe²⁺reduced V⁵⁺to V⁴⁺or V³⁺,including VO²⁺,V（OH）²⁺and V（OH）₃（s）,which attach to the root.（7）At the class level,the initial values of rhizosphere soil are similar under different V concentration gradient stress.The relative abundance of Proteobacteria,Actinobacteria,Chloroflexi,Firmicutes and Cyanobacteria are in order.Soil physical and chemical properties were positively or negatively correlated with phylum,and the bioavailability of TV and V was closely correlated with various bacterial communities.The co-occurrence network analysis showed that with the increase of V addition,the number of edges increased,and the positive correlation between most genera became more and more significant.This fits well with the"pressure gradient"hypothesis,suggesting that a hostile toxic environment can strongly support facilitation between microbial species.（8）With the change of maize growth cycle,TN,p H,TP and TV were the main environmental influencing factors of maize rhizosphere soil bacterial community at each stage.The chemical fraction of V and maize rhizosphere exudates also affected the evolution of bacterial community structure.In general,bioavailable V（Acid-V,Red-V,Ox-V）was strongly correlated with the bacterial community,while Res-V was weakly correlated with the bacterial community.The bioavailability of Res-V could not be observed within the experimental period because the response of bacterial communities was not balanced,owning to the short period of the pot experiment compared with the natural conditions in the field.Through observation and artificial natural system simulation experiment of the soil-maize system V distribution characteristics in the soil under different concentrations of V,pollution assessment and the process of migration,exploring V of rhizosphere soil bacterial community,the response and adaptation mechanisms for the study of the system can provide the theory basis for V biogeochemical behavior,reduce the risk of ecological environment and has very critical application prospect.