Regulation Of Arsenic Content In Soil And Radish By Selenium And Its Effect On Bioaccessibility Of Arsenic

Arsenic（As）is a typically toxic and pathogenic element,its pollution has become one of the global major environmental problems,especially in Southeast Asia.The As pollution of vegetables is one of the main sources of intake of As for humans.Selenium（Se）is an essential nutrient for human beings,which plays an important physiological and biochemical activities in human body and can be absorbed through the food chain.The mitigating effect of Se on As toxicity is now a hotspot on the current agricultural environment and biological interdisciplinary research.The current research focuses on the physiological response of plants and the total amount of elements analysis of Se to alleviate As stress.However,little has done upon migration and transformation of elemental species and their regulation in soil-vegetable-human gastrointestinal environmental pathway.In this paper,Selenite（Se（IV））,selenate（Se（VI））,Se-yeast（Se-Y）and Se-malt（Se-M）were applied through the pot experiment to study the effects of them on the simulated As polluted soil.The physiological and biochemical items,microbial biomass,total As contents and its species in soil and radish were analyzed,and the in vitro simulation method was used to study the regulating effect of different forms of exogenous Se fertilizers on As.The aim of this study was to explore the effect of Se on the migration and transformation of As species and their bioaccessibilities during the growth period of radish and then entering the human gastrointestinal environment as food.These can provide some evidences for restoration of As pollution and ecological health risk assessment,combining elemental speciation analysis with biochemical indicators and microbial characteristics when comprehensively explore the regulation characteristics of Se on As pollution.The main research results are as follows:（1）Under the condition of As-contaminated soil,Se-Y and Se-M treatment group significantly improved the growth of radish,while the Se（IV）and Se（VI）treatment were just the opposite and Se（VI）treatment,in particular,showed obvious inhibition on the growth of radish root.Different forms of exogenous Se and its concentration on As regulation showed obvious response characteristics in chlorophyll and antioxidant index of the growing radish.The application of organic Se can enhance the stress resistance of radish.The chlorophyll content,POD and SOD activities,proline and soluble protein contents of organic Se group were higher than those of inorganic Se group,while MDA content was lower.In addition,application of Se（VI）showed stress and toxicological effects on radish.（2）After adding different exogenous Se into the As-contaminated soil,the catalase activity,except for the 24Se level of the organic Se group,was decreased in comparsion of the control group;The organic Se treatment significantly increased the activity of urease,while the urease activity of the inorganic Se treatment firstly decreased and then decreased,and was lower than the organic Se treatment.The inorganic Se treatment generally reduced the activity of dehydrogenase,while the organic Se was the reverse;The sucrase activity of the inorganic Se treatment firstly increased and then decreased,and increased gradually for the organic Se treatment,but both were higher than the control;The FDA hydrolase and dehydrogenase activities of the organic Se treatment were generally higher than the corresponding inorganic Se treatment.Correlation analysis showed that there was a significant positive correlation between the soil enzyme activities and most microorganisms,which indicated that they responsed to the exogenous Se in good agreement.（3）The organic Se treatment increased the abundance of microorganisms in the soil and improved the soil microbial community structure,while the inorganic Se treatment reduced the abundance of microorganisms in the soil and showed inhibition on microorganisms.With increasing of Se concentration,the total Phosphor Lipid Fatty Acid（PLFA）microbial content of Se-Y and Se-M treatments increased gradually,while the Se（IV）treatment decreased gradually and the Se（VI）treatment also increased.The total PLFA in the inorganic Se treatment was lower than the control,while it was higher than the control group at the high Se level(>12 mg kg^-1)for the organic Se treatment and was lower than the control at the low Se level(<3mg kg^-1).Compared with the control,the application of organic Se significantly increased the abundances of Gram-positive bacteria,Gram-negative bacteria and fungi,while the inorganic Se inhibited bacteria and fungi.After application of inorganic Se and Se-M,the abundance of actinomycetes was lower than that of the control,however,it showed a gradual increase for Se-Y treatment.Various exogenous Se treatment increased the F/B（fungus/bacteria）and F/A（fungus/actinomycetes）in soil.Inorganic Se increased the A/B（actinomycetes/bacteria）,while organic Se was the opposite.Correlation analysis showed that the Se sources,levels and their interactions had significant effects on the soil microbial PLFA contents and their community structure（P<0.01）.（4）The As species contents of soil in each treatment group were:residual As>iron As>calcium As>aluminum As>exchangeable As.The addition of organic Se can significantly reduced the residual As and iron As contents.The redundant analysis of these four exogenous Se-treated soils showed that the exchangeable As contributed the most to the soil microbes and enzyme activities.（5）The As contents in radish roots of Se-M,Se-Y and Se（IV）treatments were lower than the control,which inhibited the absorption of As by radish.The As contents of radish in Se-M and Se-Y treatments decreased gradually with the increase of Se concentration,while they decreased first and then increased for the Se（IV）treatment.The average contents of organic As in the radish of each Se treatments were in an order as follows:Se-M>Se（IV）>Se-Y,and the percentages of organic As in total As were:Se-M>Se-Y>Se（IV）.The order of migration coefficient of As from soil to radish was TF_soil-root>TF_soil-shoot>TF_soil-leave,which decreased from bottom to top.Two-way analysis of variance showed that the different Se sources,Se levels and their interaction factors had significant effects on As contents and enzyme activities in the radish tissues（P<0.01）.（6）The As contents of gastric phase and intestinal phase were less than the control after different exogenous Se treatments,and the antagonism of the organic Se on As was more obvious than that of inorganic Se.The As content of gastric phase in each treatment group was also less than that of the small intestinal phase.The content of As（III）was lower than As（V）.The percentage of inorganic As in the treatment of different exogenous Se varied from 43.97 to 78.57%,indicating that Se can mutually affect the conversion between the As inorganic and organic forms.The bioaccessibility of As in the gastric and intestinal phases were both in the following order:Se（IV）>Se-Y>Se-M.It can be seen that addition of the organic Se greatly reduced the bioaccessibility of As when comparing to the inorganic Se.The As contents of radish treated by Se（IV）,Se-Y and Se-M were significantly positively correlated with the bioaccessibility of As in the gastric and small intestine（P<0.01）.Two-way analysis of variance showed that the addition of different exogenous Se,treatment levels and their interaction had significant effects on the bioaccessibility of the gastric and intestinal phases（P<0.01）.These results above showed that the addition of exogenous Se-Y and Se-M to soil can effectively reduce the As contents in the upper soil,improve the microbial community structure in soil,inhibit the absorption of As in radish,alleviate the toxicological effects of As,and reducing the bioaccessibility of As in radish compared in comparison to those inorganic Se.