| Summary: | 碩士 === 國立中興大學 === 土壤環境科學系所 === 98 === The paddy soils polluted by heavy metals, resulting mainly from inappropriate applications of fertilizers or receiving metal-containing waters during irrigation, have become a serious agricultural problem in Taiwan. Among these metals, Cd attracts much scientific attention because rice, a major food in Taiwan, can uptake greatly this metal and accumulate in the grains. Understanding the chemical form of Cd in soils is relatively important while evaluating its mobility and bioavailability. However, Cd species correlated to rice uptake was less reported, particularly under a flooded condition. This study aims to investigate the changes of the Cd species correlated to rice uptake upon the addition of chemical immobilization agents into a flooded red soil. In the study, a Cd-polluted (27.8 mg kg-1 Cd) red soil (pH 5.3) was collected, followed by adding the phosphate [13.1, 131, and 1310 mg/kg P, (NH4)2HPO4, denoted as 1, 10, and 100P, respectively) and/or lime (CaCO3, 8.3 cmol/kg) into the soil. The amended soil was saturated and incubated for 60 days, and the Cd concentration in soi solutions and Cd speciation in the soil were periodically examined. The summation of Cd concentrations extracted 15 times by a lysimeter showed that the water-soluble Cd could be enhanced by 100 P treatment [from 1.88 mg L-1 (blank) to 3.05 mg L-1 (100P)]. Conversely, the application of CaCO3 led to a decrease of soluble Cd to 0.99 mg L-1. In addition, mixed treatments demonstrated that soluble Cd increased with an increase in (NH4)2HPO4/CaCO3 ratio [from 0.87 mg L-1(Ca + 1P) to 1.94 mg L-1 (Ca + 100P)]. Sequential extraction of chemical-amended soil indicated that 100P treatment would lead to an increase in phytoavailable Cd, increasing in both metal-organic complex-bound and water soluble Cd). On the other hand, CaCO3 treatment decreased phytoavailable Cd; that is, the carbonate-bound, easily reducible metal oxide-bound, and organic-bound Cd increased with CaCO3 addition. Moreover, the phytoavailable Cd increased with an increase in (NH4)2HPO4/CaCO3 ratio in the amendments. It was concluded that Cd species would be varied along with the changes in the soil redox potential. Therefore, understanding the transformations of Cd species under a flooded condition may be helpful in the evaluation of the mobility, bioavailability and potential toxicity of Cd during the rice growth in the paddy soils.
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