Effect Of Humic Acid,Fulvic Acid And Citric Acid On Arsenic Methylation In Paddy Soils And Its Molecular Biological Mechanism

Arsenic contamination in rice fields leads to excessive accumulation of arsenic in rice grains,which have significant impacts on the safety of agricultural goods and human health.Recent studies found that excessive arsenic methylation in paddy soil quickly contributes to the straighthead disease in rice,and the paddy soil's arsenic methylation efficiency can be enhanced differently following the addition of various organic materials.Besides,arsenic methylation is a significant driving force for the volatilization of arsenic from paddy soil to the atmosphere.However,which organic constituents in rice fields dominate arsenic methylation?What is the biological mechanism?It is not yet clear.Therefore,two humic substances such as humic acid,fulvic acid,and citric acid,a representative organic acid in secretion from rice root,were selected for this article.The effects of adding humic acid,fulvic acid,and citric acid on the cycle of arsenic methylation in paddy soil and its associated molecular biological mechanisms exposed by performing arsenic volatilization capture tests and rice culture test at the seedling stage and molecular biological analysis.The aim of the study is to provide the scientific framework to minimize the incidence of rice straighthead disease,ensure the safety of rice production,and clarifying the microbiological mechanism of the biogeochemical arsenic cycle.The main results are as follows:?1?Through static time culture test with the arsenic volatilization capture device,it was found that the content of total arsenic and methyl-As in soil solution increased at different levels after adding humic acid,fulvic acid,and citric acid,and they increased significantly with the increase of the concentration.The methyl-As content increased by approximately 4080.5%and 4727.3%compared to CK at 13 weeks,with1%concentration of fulvic acid,and citric acid,respectively.The citric acid and 1%fulvic acid treatments stimulated the effect of arsenic methylation in the soil more significantly.The chemical structure of the citric acid with more Carbonyl C and less Aromatic C might be an important reason for promoting arsenic methylation.In each treatment,the content of arsenic volatilization increased significantly with incubation time.The content of arsenic volatilization in paddy soil was the highest at 1%concentration of humic acid treatment.The main species of arsenic volatilization are AsH₃,Me₂AsH,and TMAs.In soil solution,the content of total arsenic,organic arsenic,and inorganic arsenic positively correlated with soil pH.At each treatment,there was a significantly positive correlation between the content of total arsenic and total iron in the soil solution?P<0.01?.Increasing the pH and decreasing the Eh of soil facilitated reduction and dissolution of iron-adsorbed arsenic following the application of humic acid,fulvic acid,and citric acid,and this may be an important reason for increasing the arsenic content of the solution.?2?Through the arsM gene and 16S rRNA qPCR amplification and high-throughput sequencing of soil samples at a 1%concentration and CK,the soil arsM gene and 16S rRNA copy numbers for humic acid,fulvic acid,and citric acid treatment were found to be lower than control at different degree,and the abundance of both was significantly lower than CK for citric acid treatment.The increase of arsenic concentration in soil solution at each treatment may be a key reason for a decline in abundance of entire soil bacteria and arsM microorganisms.The application of humic acid,fulvic acid,and citric acid has significantly changed the diversity of total bacteria and arsM microorganisms in paddy soil.Based on the change in the top 20 abundant arsM microorganisms in each treatment,the relative abundance of some arsM microorganisms such as Sphaerobacter?14.8%?,Stackebrandtia?7.2%?,Amycolatopsis?8.6%?and unknown bacteria?16.7%?increased significantly in CA-1treatment compared to control.The relative abundances of Apiotrichum?4.4%?,Microbacterium?5.2%?,and Microlunatus?6.8%?in FA-1treatment were significantly higher than CK.Citric acid treatment eventually increases the efficiency of arsenic methylation by increasing the abundance of microorganisms with a more substantial arsenic methylation capacity.?3?Through simulate rice root zone cultivation at seedling stage,it was found that the total arsenic content and inorganic arsenic content in the soil pore water decreased slightly after adding three organic acids.The organic arsenic content,MMAs content and TMAsO content of 0.1%fulvic acid treatment increased compared to CK.The content of DMAs in the three organic acid treatments was lower than that in the control,while the methyl-As proportion increased slightly compared to control.The concentration of total arsenic and inorganic arsenic in soil solution of rice root area at 0.1%concentration of humic acid,fulvic acid and citric acid treatments were all lower than control,and the content and proportion of methyl-As were higher than control.At humic acid and fulvic acid treatments,the contents of inorganic arsenic,MMAs,and DMAs in rice roots were significantly higher than that of CK?P<0.05?.Although the inorganic arsenic and each methyl-As species in roots treated with 0.1%citric acid.Fulvic acid treatment significantly increased the content of inorganic arsenic in rice stems and leaves?P<0.05?,but there was no significant difference at MMAs and DMAs.Each treatment changed the Eh and pH of the cultivation system to varying degrees after cultivation.