| Arsenic(As) is a metalloid mineral widely found in nature and is recognized as the class I carcinogen by the International Agency for Research on Cancer(IARC).With the rapid development of industrial and agricultural activities,the unreasonable discharge of industrial"three wastes",the arsenic-containing pesticides,the excessive application of chemical fertilizers,and the irrigation of farmland sewage have caused the accumulation of arsenic in farmland soils which damaged the soil ecological environment,caused soil degradation,threatened the normal reproduction activities of soil animals and microbes;As in soil can be absorbed by crops which will be harmful to the human body and animal health through the food chain;As can also infiltrate into groundwater,contaminating it and causing arsenic poisoning.Pteris vittata is the world’s first discovered arsenic super accumulating plants and currently widely used in the remediation of arsenic-contaminated soil.Differences in soil moisture conditions can cause arsenic morphology,valence transformation,affect soil microbial activity and structure,and thus influence the remediation effect of pteris vittata.Therefore,the effects of pteris vittata on the remediation of soil arsenic pollution under different water conditions were studied and the mechanism was discussed to understand the biological effects of water on arsenic.In this paper,a pot experiment was conducted with pteris vittata under 50%,100%,150%maximum field water holding capacity(WHC)and Alternate Wetting and Drying(AWD)conditions in low-concentration arsenic-contaminated farmland soil to research the remediation effects.The effects of different water strips on the remediation efficiency of pteris vittata were studied by analyzing the changes of total arsenic and arsenic forms in the soil and plants;The 16S rRNA and Biolog-ECO microplate methods were used to study the different water conditions on the rhizosphere of pteris vittata;The relationship between microbial population changes and the relationship between microbial and arsenic valence transitions were revealed.The soil environmental reference values based on human health risks were predicted.The main findings are as follows:(1)There were no significant differences in total arsenic in soil under different water conditions,but there were significant differences in arsenic morphology between different treatments.Under 50%and 100%WHC conditions,soil arsenic mainly exists as(Ⅴ),while 150%WHC treatment group coexists as(ⅲ)and as(Ⅴ),and the first two samples of AWD treatment group are the As(Ⅴ)form exists,and the third sample is the coexistence state of(ⅲ)and(Ⅴ).The remediation efficiency of pteris vittata at the final harvest was 2.12%～5.96%.(2)High-throughput sequencing of 16S rRNA showed that soil water content had a significant effect on the microbial community structure in rhizosphere.The number of OTU in soil samples increased significantly.The dominant population in rhizosphere soil was Proteobacteria,which is also the main microbial population participating in the circulation of ferric,sulfide,and arsenic.Its relative abundance increased with the increase of soil water content,and the highest abundance was found in the AWD treatment group(57.69%);There are many species involved in the horizontal community composition.The oxidative reduction bacteria,Bacillus and Pseudomonas,have the function of oxidative detoxification,and have high abundance in all treatments.Lysobacteria are a kind of oxidizing bacteria existing in non-submerged soil,which can reduce the concentration of AS in soil pore water.Lysobacteria has higher abundance in the 150%and 50%WHC treatment group.The microorganisms of the Geobacter genus can participate in the reduction of the Fe,S and As in different ecological environment systems.(3)Results by Biolog-ECO showed that rhizosphere microorganisms had the highest utilization rate of carbohydrate carbon sources,followed by carboxylic acid and amino acid carbon sources;Amine carbon sources had the lowest utilization rate;The order of microbial utilization of carbon sources in the four groups was 150%WHC>50%WHC>100%WHC,WAD.Increasing soil moisture will lead to the decrease of functional diversity and uniformity of the microbial community in the rhizosphere of pteris vittata.The dominance does not change significantly.However,in the AWD treatment group,the dominance of microorganisms has increased.It is speculated that the alternating wet and dry soil environment is not conducive to certain environmentally sensitive bacterial populations.(4)Based on human health risk,the soil value of farmland soil environment is calculated and calculated using the acceptable carcinogenic risk level of 10-5 and the risk quotient of 1.When the acceptable carcinogenic risk level is 10-5,the soil environmental reference value is 2.95 mg kg-1.The soil environmental reference value derived from the risk quotient of 1 is 13.19 mg kg-1. |
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