Changes in Cd Species in a Taiwan Red Soil as Influenced by the Addition of Phosphate Fertilizer.

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 attention because rice, a major food in Taiwan, can uptake greatly this metal and accumulate in the grains. In the study, the decreases in the bio-availability and mobility of Cd in a Cd-contaminated Tao-Yuan red soil (27.8 Cd mg/kg) are evaluated by adding various amounts of phosphate (i.e., (NH₄)₂PO₄, 13.1 to 13110 mg/kg P), a fertilizer commonly used in red soil. We expected that P would not only promote the plant growth but also decrease Cd availability to plants due to the formation of Cd-P precipitate. Batch results indicate that Cd release increased with an increase in ionic strength from 0.001M to 1M but with a decrease at pH from 8 to 1. P addition would decrease Cd release, and the release decreased with an increase in P application. The metal-organic complex-bound dominated Cd species in the red soil followed by exchangeable, easily reducible metal oxide-bound, carbonate-bound, and organic-bound Cd. The exchangeable Cd would convert to metal-organic complex-bound Cd upon the P addition; however, no significant change in other Cd species was observed. The conversion in Cd species is unclear in the current stage, which merits further study to clarify this issue.