The Enrichment Mechanisms Of Arsenic In Water-Soil-Rice System In Typical High Arsenic Groundwater Distribution Area Of The Jianghan Plain

Arsenic（As）is one of the important pollutants that threaten human health.Rice,as an important food crop in the world,is more prone to arsenic accumulation than other crops because it grows in a reduced environment of flooding.Therefore,arsenic pollution in rice is a serious threat to human health.The groundwater is often used as an important irrigation water source in areas with high arsenic groundwater,and previous studies have focused on the source,migration and transformation of arsenic in groundwater and the safety of drinking water.However,the enrichment mechanism of arsenic in water-soil-rice system caused by irrigation with high arsenic content in groundwater is still unclear.The Jianghan Plain（JHP）,located in the middle reaches of the Yangtze River,has been known as the"home of fish and rice"since ancient times and is an important commodity grain base in China.Rice has always been an important crop in Jianghan Plain,accounting for 5%～10%of the total production of the country.It is found that groundwater with high arsenic content is widely distributed in JHP and is used as agricultural water for paddy field irrigation.Therefore,taking jianghan Plain as a typical study area,it is of great significance to explore the biogeochemical behavior of arsenic in water-soil-rice system under the influence of high-arsenic groundwater irrigation,and to provide corresponding countermeasures and suggestions for agricultural production activities.In view of the above research background,this article revolves the high arsenic groundwater irrigation water and soil and rice system transformation of arsenic in the migration and enrichment mechanism of the key scientific problems,to as the typical research area of JHP,the integrated use of field investigation and indoor cultivation monitoring technology,on the basis of find out the distribution high arsenic groundwater and water chemical characteristics of in JHP,systematic assessment of arsenic pollution in paddy fields in JHP;pot experiment was used to reveal the migration,transformation and enrichment mechanism of arsenic in water-soil-rice system under high arsenic groundwater irrigation.The effect and mechanism of groundwater irrigation with high arsenic content on rice growth under different fertilizer application were investigated.Therefore,it provides the basis for regional scientific rice planting and escorts food security.The main results are as follows:1.Occurrence characteristics of arsenic in water-soil-rice system in Jianghan Plain.Based on the investigation of arsenic concentrations in different media in the water-soil-rice system in Jianghan Plain,the present situation and health risks of arsenic pollution in the water-soil-rice system in the rice growing area of Jianghan Plain were systematically evaluated in combination with the physical and chemical indexes of water,soil and rice.Arsenic concentration of diving in paddy fields ranged from 0.02 to42.88μg/L,with an average concentration of 3.08μg/L.Arsenic concentration in soil pore water ranged from 0 to 41.77μg/L,with an average of 11.64μg/L.Arsenic concentrations in paddy soils ranged from 10.20 mg/kg to 21.90 mg/kg,with an average of 16.50 mg/kg,which were in line with the secondary standard of Soil Environmental Quality Standard（GB15618-1995）（25 mg/kg）.Arsenic concentration in rice samples ranged from 0.13 mg/kg to 0.95 mg/kg,with an average concentration of 0.45 mg/kg.34.15%of the samples exceeded the national standard（GB2762-2017）limit of 0.5mg/kg.Based on the analysis of geographic accumulation index（Igeo）,health risk index（THQ）and carcinogenic risk index（CR）,arsenic pollution levels in diving and soil in paddy fields were relatively low,but arsenic content in rice was more harmful to human health and should be considered as a dangerous element in agricultural ecosystems.The absorption of arsenic in soil is low,mainly in the combination of iron and aluminum oxides,which is also a potential form of rice absorption and utilization.Based on the correlation between soil DGT-AS concentration and arsenic concentration in rice,it can be seen that the sampling points in the region can be divided into two categories:Group1,y=0.34x+0.21,R2=0.70;Group2,y=1.09x+0.23,R2=0.60.Combined with the investigation,it can be seen that the main reason for arsenic enrichment in rice in Group2 is the existence of high-arsenic groundwater irrigation.Rice plants near large surface water systems such as Tongshun River and Dongjing River are in a more reductive environment,and their bioenrichment coefficient（BCF）value is higher,which makes rice more likely to enrich arsenic.Therefore,it is recommended that rice be planted as far as possible from large surface water systems,especially the confluence of multiple rivers.In addition,rice should not be in the state of long-term continuous flooding as far as possible,and water resources should be rationally managed.When pumping groundwater with high arsenic content for agricultural irrigation,groundwater should be pre-treated and arsenic reduced before irrigation.2.The mechanisms of high-arsenic groundwater irrigation on arsenic enrichment in water-soil-rice system.Through pot culture experiments,continuous sampling and monitoring of overwater,pore water,rhizosphere soil and rice were carried out to identify the main periods of arsenic absorption by rice,and reveal the influence mechanism of arsenic enrichment in water-soil-rice system by groundwater irrigation with high arsenic content.The physicochemical properties of overlying water in rice pot were mainly related to irrigation water,and the physicochemical properties of soil pore water were affected by both irrigation water and rice cultivation.The hydrochemical type of overlying water and soil pore water is HCO₃-Ca type,and the content of Ca²⁺,Mg²⁺and HCO₃^-in soil pore water is obviously higher than that in overlying water.Groundwater irrigation with high arsenic content enhanced reductive dissolution of iron oxides in soil,and arsenic content in soil pore water increased obviously at heading stage and filling stage.Exogenous arsenic input and reductive environment can significantly increase the content of adsorbed arsenic in soil,enhance the bioavailability of arsenic in soil,and enhance the absorption of arsenic by rice.With the growth of rice,the ability of iron film on rice root surface to absorb arsenic gradually increased,and the accumulation of arsenic in both aboveground and underground parts of rice also gradually increased.Arsenic uptake by rice roots was mainly from tillering stage to heading stage,and intermittent irrigation and drainage could effectively reduce arsenic concentration in soil pore water,thus reducing arsenic accumulation in rice.The main form of arsenic in rice is As（III）.Applying organic fertilizer can promote soil microbial activities,enhance reductive dissolution of iron oxide（hydroxide）,promote absorption of arsenic by rice,and enrich rice with As（V）.3.The mechanisms of different fertilizers on arsenic enrichment in water-soil-rice system.The effects of different fertilizers（organic fertilizer,inorganic fertilizer and bio-organic fertilizer）on arsenic transfer and transformation in water-soil-rice system were studied by pot culture experiments,and the effects of exogenous arsenic input on the system under high arsenic groundwater irrigation were explored.The effect of fertilizer application on the physical and chemical properties of overlying water in rice pot was limited,and there was no obvious change after 15 days of fertilization.The physical and chemical properties of soil pore water are affected by the properties of fertilizer and irrigation water:the concentration of sulfate radical is the highest under the application of organic fertilizer,while the content of chloride ion is the highest under the application of inorganic fertilizer.Under the condition of biological organic fertilizer application,rhizosphere soil mineral dissolve the reducibility of maximum intensity,soil pore water arsenic concentration is highest,the highest accumulation of arsenic in rice root,but as a result of biological organic fertilizer cultivation of arsenic in rice transport ability is weak,leading to the upper（straw and rice）rice arsenic content in the lowest.At the tillering stage,the concentration of arsenic in pore water was higher when organic fertilizer was applied than inorganic fertilizer.At heading and flowering stage and filling and maturity stage,the reductive dissolution of iron minerals in rhizosphere soil was stronger in pot with inorganic fertilizer,resulting in higher arsenic concentration in pore water.When high-arsenic groundwater irrigation is used,arsenic concentration in water,soil and rice will be increased to varying degrees,and the total arsenic concentration in rice will exceed the standard to varying degrees,among which the arsenic concentration in potted rice with inorganic fertilizer has the greatest influence.As organic matter can form complex with arsenic,high-arsenic groundwater irrigation has the greatest effect on the arsenic content in pore water of inorganic fertilizer pot with the lowest organic matter content,and the soil arsenic content in organic fertilizer pot with the highest organic matter content increases the most.In the process of rice cultivation,if surface water with low arsenic content is used for irrigation,inorganic fertilizer can reduce arsenic accumulation in rice.When high arsenic groundwater is used for irrigation,organic fertilizer can effectively reduce arsenic accumulation in rice.The DGT concentration of arsenic in rice should not exceed 0.85μg/L when irrigated with surface water and 0.25μg/L when irrigated with high-arsenic groundwater if the arsenic concentration in rice at harvest is to meet the national standard.