Effects Of Typical Low Molecular Weight Organic Acids Secreted By Rice Roots On Arsenic Migration And Transformation In Soil-rice System

Arsenic（As）is a toxic metalloid element.Long-term exposure to a small amount can cause chronic poisoning in the human body,increasing the risk of cancer,cardiovascular,nervous system,kidney and respiratory diseases.The potential harm caused by soil As pollution has spread worldwide,and about 150 million people worldwide are threatened by arsenic pollution.Rice（Oryza sativa L.）is more likely to accumulate As due to its growth conditions and unique physiological characteristics.Due to the anaerobic reduction state of the paddy soil environment under flooding conditions,it is beneficial to enhance the bioavailability of As and the absorption of As by rice plants.Therefore,As pollution in paddy soil has attracted wide and close attention.Low molecular weight organic acids source of soil organic carbon,play an important role in the biochemical cycle of soil-rice system.Moreover,due to the high concentration of LMWOA in rice rhizosphere environment（compared with non-rhizosphere soil environment）,it will have a more direct and far-reaching impact on the environmental fate of As in rhizosphere soil.Therefore,it is of great significance to explore the mechanism of LMWOA-mediated As transformation and migration in rhizosphere soil to reduce arsenic accumulation in rice and ensure food safety.In this thesis,the secretion of LMWOA in rice rhizosphere under arsenic stress was studied.On this basis,acetic acid（AA）,oxalic acid（OA）and citric acid（CA）were selected for soil microcosm experiments and pot experiments.The main conclusions are as follows:1、2mg/L sodium arsenite was added and LMWOA secretion from roots of 7 rice varieties（Liangyou 8106（LY8106）,Fengliangyou 4（FLY4）,Yinuo 19（YN19）,Wuyujing3（WYG3）,Teyou 63（TY63）,Nanjing 5055（NJ5055）and Zhenuo（ZZN））in the middle and lower reaches of the Yangtze River was monitored.To explore the LMWOA secretion strategies of different rice varieties in response to arsenic stress.It was found that all six rice species,ZZN,YN19,WYG3,TY63,LY8106 and NJ5055,increased the total secretion of LMWOA as well as the appearance of AA secretion under arsenic stress.In comparison,total LMWOA secretion in FLY4 rice decreased by 61%and no increase in the type of LMWOA secreted was observed.The above results indicated that the secretion of LMWOA varied among rice cultivars in response to arsenic stress.Most rice varieties increased the total secretion of LMWOA and the secretion of new LMWOA,such as formic acid,acetic acid,citric acid and malic acid.2、Exogenous application of AA,OA and CA was conducted to monitor the As morphological changes in pore water at the 6th hour,2nd,7th,14th,21st and 28th days of culture in a simulated rhizosphere environment,so As to explore the mechanism of exogenous LMWOA mediating the transformation and migration of As in rhizosphere soil.The experimental results showed that the addition of AA and OA inhibited the release of As in soil,and the concentration of As in soil porewater decreased by 42%after 21 days of culture.AA hindered the activation of As in soil by promoting the formation of secondary minerals,while OA fixed As in soil by increasing the formation of secondary minerals and the activity of arsenic oxidizing functional microorganisms（aio A relative abundance increased by 24%）.At the same time,OA also increased the relative abundance of As（III）methylation functional genes（ars M）in the soil and significantly increased the percentage of organic arsenic in the soil,with a 25%increase in the percentage of DMA in the soil after 21 days of incubation.The addition of CA greatly accelerates the release of As from the soil solid phase through chemical and microbial action.LEf Se analysis of soil functional bacteria revealed the possibility that bacteria such as Burkholderia,Magnetospirillum,and Mycobacterium were involved in As reduction or methylation in the rhizosphere of paddy soils.In addition,sterilization experiments also revealed that the main pathway of AA and OA mediating soil As release process was microbial action,while the As mobilization effect under CA treatment was the result of the combined action of chemistry and microorganisms.3、The effects of exogenous addition of AA,OA and CA on As migration in soil-rice system were investigated by pot experiment.The As content in soil porewater on days 1,5,12 and 19 after transplanting rice was monitored,as well as the growth indexes and As content of rice plants after 19 days of culture were monitored.The results showed that AA and CA inhibited the root development of rice plants by 65.28%and 53.89%,respectively.The application of AA and OA significantly reduced the As content in the rhizosphere pore water of rice at the middle stage of culture,which decreased by 35.57%and 77.01%,respectively,on the 12th day of culture.In CA treatment,As content in underground and above-ground parts of rice increased by 1.28%and 70.24%,respectively,and the As migration coefficient(TF_As)in the soil-rice system was increased.These results suggest that different LMWOA may affect the migration of As in the soil-rice system by acting on the growth and development of rice plants and mediating As transformation in rice rhizosphere soil.In this study,hydroponic experiments,soil microcosm experiments and pot experiments were conducted to explore the secretion of organic acids in roots of different rice varieties under arsenic stress,the chemical-microbial mechanisms of AA,OA and CA mediated As migration and transformation in rhizosphere soil,and the effects of AA,OA and CA on As migration in the soil-rice system.The results of this study provide a theoretical basis for understanding the migration and enrichment of As in soil-rice system.