Study On Arsenic Behaviors In Rhizosphere Of Rice And Influence Factors

Over 150 million people worldwide are exposed to elevated levels of arsenic (As) in drinking water in recent years, however food crops, especially rice, can also contribute substantially to human As intake. Rice can accumulate more arsenic than other up-land crops. The rhizosphere is a key interface of soil and plant to regulate arsenic transportation from growth medium to root. Therefore, it is important to explore As behaviors in rhizosphere of rice for reducing As translocation from soil to grain. First, a survey of As contamination in a Pb/Zn mine tailing was carried out to assess the levels of As pollution in this area. After that the hydroponics culture experiment was conduceted to study the dynamic process of iron oxide formation on root surface of rice seedlings and its influence on As absorption in iron plaque and As uptake by roots. Moreover, soil pot experiments with rhizo-bag were set up to investigate the relationship of As and Fe in the interface of soil-plant.We also concerned about the effects of exogenous Si and organic matter on As accumulation in rhizosphere and rice grain.The results showed that the average concentration of As was 328 mg/kg in soil from different sampling sites around Pb/Zn mine tailing. Based on National Soil Environmental Quality Standard of Gradeâ…¢, the As pollution indexes of contaminated paddy fields and woodland were both higher than 3, which showed the contamination grade of the soil reached heavy pollution and the index of normal paddy fields was 2.74.The amounts of iron oxide which was induced by Fe2+ increased with increasing of induction time and Fe2+ concentrations. The As accumulated on root surface with iron plaque ranged from 0.53 g/kg-0.99 g/kg which were higher than the control (without iron plaque) (0.41 g/kg, P<0.05). The formation of iron plaque inhibited markedly arsenic concentrations in rice roots. When the amount of iron plaque was high than 20 g/kg, the inhibition of iron plaque on arsenic uptake by root was reduced. Moreover, iron plaque significantly inhibited translocation of As from root to shoot.The soil pot experiment illustrated that during flooded and non-flooded total arsenic concentrations in soil solution decreased gradually, while total iron concentrations kept stable.The range of pH value and Eh value in soil solution was 7.5-8.3 and 150-200 mV ,respectively. In addition,there was significant positive correlation between As and Fe concentrations in root-soil interface. There was also significant correlation between As and Fe concentrations in root during the cycle of flooded and non-flooded.Growing rice increased pH values in soil solution when soil original pH is less than 7. However, there was a reverse pattern when soil original pH>7. There was no correlation between rhizosphere pH and As concentrations in rhizosphere soil when original soil pH<7.0. However, there was significant positive correlation between rhizosphere pH and As concentrations, and negative correlation between rhizosphere pH and Root-As.The soil pot experiement also showed that the addition of silicon in soil significantly increased total biomass and grain and weights,while addition of silicon and organic matter in soil together reduced root and shoot dry weights of rice at tillering stage and mature stage. Meanwhile, the addition of silicon reduced As concentrations in rice shoot and root at these two growing stages, accounting for 22.8%-60.9% and 52.8%-79.6%.Moreover, the addition of organic matter reduced As concentrations in roots and shoot significantly by 69.5% and 53.8% in roots and by 44.5% and 40.8%, respectively. Arsenic concentrations in shoots and roots were reduced by 68.2%~82.0% and 75.4%~94.1% with the application of silicon and organic matter, respectively.