Effects Of Rhizosphere Organic Acids On The Leaching Of Arsenic And Cadmium From Soil And Uptake Of Arsenic And Cadmium By Rice

Under the severe influence of industrial and agricultural activities,soil heavy metal pollution is becoming more and more serious,especially the combined pollution of arsenic(As)and cadmium(Cd).Rice is one of the most important food crops in Southeast Asia and the world.More than half of the world’s population lives on rice.Compared with other crops,rice has higher accumulation capacity of arsenic and cadmium in soil due to its unique growth process,so it is easier to enrich arsenic and cadmium.Research on the application of organic acids in remediation of soil heavy metal pollution has attracted much attention in recent years.Moreover,rice roots can secrete a variety of organic acids,which can react with soil heavy metals such as acidification,complexation and exchange,thereby migrating heavy metals in soil.In this paper,the effects of the types,combinations and concentrations of organic acids on the leaching of arsenic and cadmium from soil were studied by microcosmic experiment system.The effects of organic acids in rhizosphere of rice on absorption of arsenic and cadmium were investigated by pot experiment.The aim of this study was to provide theoretical basis for screening rice varieties with low arsenic and cadmium accumulation and the enrichment of arsenic and cadmium in food chain.The main results are as follows:1.In the experiment to explore the influence of types of organic acids on the leaching of arsenic and cadmium from soil,the results show that the concentrations of arsenic and cadmium dissolved from the soil at the end of the experiment,the control group was 0.25mg/L and 2.5 ug/L,the citric acid treatment group was 1.28 mg/L and 16.6 ug/L,the malic acid treatment group was 1.20 mg/L and 14.1 ug/L,and the oxalic acid treatment group was 0.96 mg/L and 11.7 ug/L,the citric acid + malic acid treatment group was 1.64 mg/L and 29.0 ug/L,the citric acid + oxalic acid treatment group was 1.22 mg/L and 20.6 ug/L,the malic acid + oxalic acid treatment group was 1.33 mg/L and 15.4 ug/L,the citric acid +malic acid + oxalic acid treatment group were 1.37 mg/L and 30.2 ug/L.The concentrations of arsenic and cadmium dissolved in the organic acid treatment group were significantly higher than those in the control group(P<0.05),but the combined organic acid did not show superposition in the process of leaching arsenic and cadmium in the soil.In the experiment to explore the influence of the concentration of organic acids on the leaching of arsenic and cadmium from soil,the results show that the amount of arsenic and cadmium dissolved from the soil is citric acid,malic acid and oxalic acid in descending order,and the same organic acid,with the increase of the concentration,the amount of arsenic and cadmium dissolved from the soil is also more,but this concentration effect gradually weakens with the progress of the reaction.During the experiment,the arsenic dissolved from the soil in the low concentration(0.1 mmol/L and 0.5 mmol/L)organic acid treatment group was 0.9～1.4 times higher than that in the control group,and the cadmium dissolved was 1.0～24.0 times higher than that in the control group.The arsenic dissolved from the soil of the high concentration(5 mmol/L,10 mmo/L,20 mmol/L and 40 mmo/L)organic acid treatment group was 0.1～8.9 times higher than that of the control group,and the cadmium dissolved was 5.5～275.5 times higher than that of the control group.2.Based on the results of microcosmic experiment,the effects of rhizosphere organic acids on the absorption of arsenic and cadmium by rice were studied by pot experiment.The results showed that during rice planting,the secretion of total organic acids in the rhizosphere of the control group ranged from 23.5 to 30.7 mg/L,and the secretion of total organic acids in the rhizosphere under arsenic and cadmium treatment ranged from 30.6 to46.4 mg/L.Under arsenic and cadmium stress,the total organic acids secreted by rice roots increased,and were significantly higher than those in the corresponding control group(P<0.05).During rice planting,compared with the control group,the proportion of organic acids secreted by rhizosphere under arsenic and cadmium stress changed,but the species composition of secreted organic acids remained unchanged.The organic acids secreted by roots are mainly oxalic acid,malic acid,acetic acid and citric acid,as well as a small amount of formic acid and tartaric acid.The total organic acid secretion of rice Fengliangyou4 rhizosphere was 46.36 mg/L,significantly higher than 28.70 mg/L of rice Teyou63,while the contents of available Arsenic and cadmium in rice Fengliangyou4 rhizosphere soil were 1.21 mg/kg and 0.60 mg/kg,respectively,significantly lower than1.44 mg/kg and 0.73 mg/kg of rice Teyou63(P<0.05).The contents of arsenic and cadmium in the underground part were 196.67 mg/kg and 9.31 mg/kg for rice Fengliangyou4,557.55 mg/kg and 25.85 mg/kg for rice Teyou63,respectively.The contents of arsenic and cadmium in the aboveground part were 7.48 mg/kg and 2.66 mg/kg for rice Fengliangyou4,15.49 mg/kg and 4.38 mg/kg for rice Teyou63,respectively,which were significantly lower than rice Teyou63(P<0.05).Through the correlation analysis of the available content of heavy metals in soil,the secretion of organic acids in rhizosphere and the contents of arsenic and cadmium in rice plants,it was found that the content of arsenic in rice stems was significantly positively correlated with the content of arsenic in roots(P<0.01),positively correlated with the content of available arsenic in soil(P<0.05),and negatively correlated with the content of total organic acids in rhizosphere(P<0.01).There was a significant positive correlation between arsenic content in rice roots and soil available arsenic content(P<0.05),a very significant negative correlation with total organic acid content in rhizosphere(P<0.01),and a very significant negative correlation between soil available arsenic content and total organic acid content in rhizosphere(P<0.01).Under arsenic and cadmium stress,the content of cadmium in rice stems was significantly positively correlated with the content of available cadmium in roots and soil,and negatively correlated with the content of total organic acids in rhizosphere(P<0.01).The content of cadmium in rice roots was significantly positively correlated with the content of available cadmium in soil,and negatively correlated with the content of total organic acids in rhizosphere.The content of available cadmium in soil was significantly negatively correlated with the content of total organic acids in rhizosphere(P<0.05).