Effects Of Citric Acid On Arsenic Migration And Transformation And Microorganisms In Paddy Soil

In recent years,the safety of agricultural products caused by excessive heavy metals in farmland soil has attracted extensive attention.How to effectively control the pollution of heavy metals in farmland soil and ensure food security has become a hot topic in the field of environmental research.Arsenic(As),a typical toxic metallic element,is one of the main heavy metallic pollutant in paddy fields in China.More and more attention has been paid to the community structure characteristics of indigenous microbes and the community composition of microbes responsible for arsenic biotransformation in arsenic-contaminated soils.As the atypical exudates of rice root,low molecular weight organic acids(LMWOAs)represents an important source of soil organic carbon,which might disturb the soil equilibrium,and further affect arsenic(As)biogeochemical cycling in As-contaminated paddy field.Citric acid(CA)is a representative LMWOAs secreted by rice roots,investigating its effects on the migration and transformation of arsenic and functional microorganisms in paddy soils and their mechanisms can provide a basis for formulating more effective strategies to control the accumulation of As in food crops.In this study,microcosm experiments was carried out by using arsenic polluted paddy soil.Through chemical analysis and high-throughput sequencing technologies,we investigated the effects of citric acid on the migration and transformation of arsenic and functional microorganisms in paddy soil.The main results were as follows:(1)The effects of different doses of citric acid(0.05%,0.1%,0.2% and 0.5%)on arsenic concentration,Fe,Mn and p H in soil porewater were clarified.Citric acid can be fully utilized by microorganisms within 1-10 days.The addition of CA significantly enhanced the mobility of arsenic,and the total concentration of As in pore water increased(1-20 times)with the increasing dose of citric acid.The addition of citric acid also increased the concentration of Fe and Mn in pore water,and the concentration of As was positively correlated with Fe and Mn(R≥0.38,P<0.01).The results of spectral analysis of porewater showed that there were a variety of ionizable functional groups in porewater,such as carboxyl(-COOH),carbonyl(-COO)and hydroxyl(O-H),which play an important role in the release of As in paddy soil.(2)The effects of different doses of CA(0.05%,0.1%,0.2% and 0.5%)on As species in soil porewater was clarified.The application of CA significantly increased arsenate(As(Ⅴ))concentration in porewater,but had no significant effect on the reduction of As(Ⅴ)to arsenite(As(Ⅲ)).The application of CA significantly inhibited the As methylation process,and significantly reduced the concentrations of monomethyl arsenic(MMAs(Ⅴ))and dimethyl arsenic(DMAs(Ⅴ))in porewater.The results of spectroscopic analysis of porewater proved the combination of As,Fe and oxygen atoms(As-Fe-O)and the existence of the ternary complex of As,Fe and organic matter(OM)(As-Fe-OM).The bioavailability of complexed As was reduced,and the As methylation processes was inhibited.(3)The effects of different doses of CA(0.05%,0.1%,0.2% and 0.5%)on the structure of soil microbial community were clarified.The application of CA significantly changed the structure of soil microbial community.At the bacterial phylum level,the application of CA significantly increased the relative abundance of Firmicutes;at the genus level,the application of CA significantly increased the relative abundance of Clostridium＿sensu＿stricto＿12,unclassified＿f＿Clostridiaceae,Caproiciproducens,Desulfitobacterium and Veillonellales-Selenomonadales.Phylogenetic analysis showed that most of the representative OTUs belonged to Clostridium;the results of LEf Se analysis showed that Clostridium and Desulfitobacterium were enriched in 0.5% CA treatment.(4)The relationship between the microbes that significantly increased the application of CA and the conversion of As and Fe was clarified.The results of correlation network analysis show that there was a positive correlation between the concentration of As and Fe in porewater;the abundance of multiple members of Clostridium has a significant positive correlation with Fe,but has nothing to do with the concentration of As;the abundance of Desulfitobacterium was a significant positive correlation with the concentration of As and Fe in porewater.In addition,the addition of CA also increased the relative abundance of Veillonellaceae,Bacillus,Desulfitobacterium,and Hydrogenispora,which are associated with the reduction of As and Fe and the degradation of CA,respectively.