Study On The Effects Of Iron-containing Minerals In Paddy Soil On Soil Microbial And Arsenic Speciation Transformation

Arsenic（As）in cultivated soil not only affects the growth of rice,but also enters the human body through contaminated rice,which is harmful to human health.As one of the main carriers for adsorption or co-precipitation of arsenic（As）in iron-rich soils in southern China,iron-containing minerals can be reduced and dissolved by chemical or biological processes during rice growth under the flooded state,resulting in the release of As.Under the condition of drainage of paddy soil,the reduction and dissolution of iron-bearing minerals form new mineral fixed arsenic,which affects the occurrence form and biological availability of arsenic in soil.Therefore,it is of great significance to understand the effects of changes in iron-containing minerals during rice growth cycle on the transformation of arsenic forms and related functional microorganisms for the prevention and control of arsenic accumulation in rice.In this paper,through the microcosmic waterlogging anaerobic cultivation method and the indoor simulation experiment of waterlogging-drainage reduction-oxidation alternate cultivation method,taking the iron-rich paddy soil in South China as the research object,using wet chemistry method,XRD,high-throughput sequencing,real-time fluorescence quantitative analysis and other analysis techniques,We investigated the effects of the transformation of iron-arsenic minerals（scorodite）and iron-containing minerals（ferrihydrite）on the morphological transformation of As and related microorganisms,to provide theoretical basis and reference for the remediation technology of arsenic contamination in paddy soil.Key findings were as follows:（1）The effects of iron-containing minerals on soil microorganisms and arsenic transformation under anaerobic conditions were clarified.Indigenous microorganisms in soil can cause reductive dissolution of scorodite under anaerobic conditions.The major microorganisms involved in the anaerobic reduction of Fe（III）and As（V）were Anaeromyxobacter,Dechloromonas,Geothrix,Geobacter,Ideonella,and Zoogloea.The humus model AQDS increased the relative abundance of dominant species,such as Dechloromonas,Geobacter,Ideonella,and Zoogloea,but did not alter biodiversity and community structure during scorodite reduction.Among these organisms,Geobacter showed the largest increase and was the predominant Fe（III）and As（V）-reducing bacteria in AQDS and scorodite culture samples.AQDS promoted the initial reductive dissolution rate of scorodite,resulting in higher concentrations of dissolved Fe and As in the AQDS treatment than in the treatment without AQDS addition.More than 80%of the As（V）released by scorodite is converted to As（III）by microbial reduction.The addition of AQDS resulted in significantly greater increases in arr A gene abundance and putative arr A sequences（mainly Geobacter）,than in the treatment without AQDS.Therefore,the addition of AQDS promotes the release and reduction of Fe（III）and As（V）from arsenic-containing iron minerals,thereby increasing the mobility and activity of As,leading to an increased environmental risk of As.（2）The effects of iron-bearing minerals on soil microorganisms and arsenic transformation under alternate reduction-oxidation conditions of watered-drainage were studied.The water-drainage water management model had significant effects on arsenic speciation.Under reduction conditions,arsenic is reduced to the more toxic and mobile As（III）,while under oxidation conditions,the reduction products are oxidized to As（V）,which adsorbs on the surface of iron oxides or soil particles and reduces their activity.By comparing the changes of arsenic in the sterilization group,it was found that arsenic reduction was mainly driven by microbial action,while arsenic oxidation was caused by both chemical and biological action.The arr A gene expression was favored by the flooding stage,while the aio A gene expression was favored by the draining stage.There was a significant positive correlation between arr A gene copy number and dissolved As（III）during arsenic reduction.In the process of arsenic oxidation,there was also a significant positive correlation between dissolved As（V）and aio A gene copy number,indicating that the expression of arsenic functional genes plays a key role in the process of arsenic morphological transformation.Firmicutes,Proteobacteria,Acidobacteria,Chloroflexi,Bacteroidetes,Actinobacteria,Planctomycetes,and Verrucomicrobia are waterfall-drainage processes The dominant microorganisms in Proteobacteria.At the phylum level,the dominant microbial community was basically unchanged,but the relative abundance changed significantly among the treatments,indicating that the addition of arsenic and iron oxides affected the composition of related functional microorganisms.For example,the concentration of dissolved Fe（Total）and Eh were positively correlated with the relative abundance of Chloroflexi and Bacteroidetes,and the concentration of dissolved As and As _Adwere negatively correlated with Firmicutes.Based on the prediction of high-throughput data,the major microorganisms involved in REDOX processes during waterlogging and drainage reactions were Geobacter,Tumebacillus,Rhodoplanes,Desulfobacca,and unclassified Betaproteobacterium.The results showed that arsenic speciation and its migration and transformation process could be effectively regulated by controlling water management,soil physical and chemical properties,and exogenous materials（such as ferrihydrite）addition.