Reasons for the Low Availability of As in a Paddy Soil with High As Content.

The arsenic (As) content in the soils of Guandu Plain, Taiwan, can be as high as approximately 500 mg kg^-1 but the concentration of As in the rice grains harvested in this area is relatively low. To understand the reasons of low As availability in Guandu soils, submerging incubation experiment was conducted for a Guandu soil and the chemical compositions in the soil solutions were monitored as a function of time. The results showed that the soil pH increased from 6.03-6.72 in the beginning 20 days and gradually reached a constant value (i.e., 6.97). Conversely, redox potential decreased from 249 to -234 with increasing submerging time. Arsenic sequential extraction predominantly associated with amorphous and crystalline Fe oxides, which accounted for 34.7-63.8% of the total As content. During the incubation, the As/Fe concentration ratios in the soil solutions were significantly lower than that of the soil. Therefore, it was presumed that a large fraction of As released into soil solution was re-adsorbed by the soil. This was confirmed by the high adsorption capacities of As(V) and As(III) of the soil, obtained by an additional adsorption experiment for the soil. Meanwhile, the formation of Fe oxides on rice root and in rice rhizosphere further inhibited the uptake of As by the rice root. These processes resulted in the low uptake rate of As by rice roots.