Temporal Dynamics of Arsenic Availability in Guandu and Pinchen Soils during Rice Cultivation

• Main Authors: Puu-Tai Yang, 楊圃臺
• Other Authors: Shan-Li Wang
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/jtwxbs

碩士 === 國立臺灣大學 === 農業化學研究所 === 105 === The soils at Guandu plain have been known for high As concentration since 2008. When comparing to other As contaminated soils, As in Guandu soils (Gd) seems to be less available and relatively less As was transported from soil to rice grain, which was attributed to the accumulation of As by iron plaque on rice root in previous studies. However, the transportation of As from soil to rice roots at different rice growing stages has not been fully understood. To elucidate how As transports from soil solid phase to rice roots, pot experiment was carried out in an environment-controlled greenhouse with nature light source. For the purpose of comparison, the experiment was also conducted for Pinchen soil (Pc) with a high Fe content. The As concentrations of Gd and As-spiked Pc soils were 321 and 88.0 mg kg-1, respectively. The soils were submerged during rice cultivation. Soil pH and Eh were measured every 10-20 days. Soil solutions, soil, core samples and rice plants were also collected at certain time intervals during the experiments. The results showed that the higher contents of soil organic matter and amorphous iron (hydr)oxides in Gd soil led to a rapid decrease in Eh and the release of Fe in the early stage of submergence, while the releasing rate of arsenic was slower due to the re-adsorption by iron (hydr)oxides in the soil. Nevertheless, a higher ratio of arsenic was released into soil solution compared to the counterpart of Pc soil because there was up to 52% of total arsenic content sorbed on amorphous Fe (hydr)oxides in Gd soil. When Fe(II) is released and moved to the surface of rice roots, Fe(II) would be oxidized to Fe(III), which subsequently precipitates in the rhizosphere and rice root surface due to radial oxygen loss (ROL) from rice roots. In this study, lepidocrocite was the main Fe species in iron plaques of both soils, which suggested that ROL from rice roots under flooded condition was a gradual process. Because of higher ratio of As(V) and DMA in Pc soil solution, arsenic was sorbed much readily onto iron plaques in Pc soil than in Gd soil. Spatial barriers in rice rhizosphere may also limit oxidation and transportation of arsenic from soil solution to rice roots, which then lowered the amount of arsenic accumulated in rice shoot.