Effects of Phosphate and Carbonate Amendments On Bioavailability of Cadmium in a Flooded Taiwan Red Soil.

The applications of lime and phosphate amendments are a common agricultural practice in red soils. However, the influence of this practice on toxicity and bioavailability of Cd in a Cd-polluted red soil remains uncovered. In the study, a Cd-polluted (27.8 mg kg^-1 Cd) red soil (pH 5.2) was collected, followed by adding the phosphate ((NH₄)₂HPO₄, 13.1 to 1310 mg/kg P) and/or lime (CaCO₃, 8.3 cmol/kg) into the soil. The amended soil was saturated and incubated for 60 days, and then the solubility and speciation of Cd in the soil were examined. The results showed that water-soluble Cd (extracted by a lysimeter) increased with an increase in phosphate application, but decreased with the application of lime. While adding phosphate and lime simultaneously, lime could inhibit slightly the release of water-soluble Cd as compared with its counterpart amended with only P. Sequential extraction of P/lime amended Cd-soils showed that P addition would lead to a decrease in exchangeable Cd; however the dissolved and metal-organic complex bounded Cd increased. Since the derived Cd species upon P addition were still phytoavailable, P addition would not decrease the bioavailability of Cd. Conversely, lime addition could decrease Cd bioavailability because the dissolved Cd and exchangeable Cd would be converted to carbonate-bounded Cd, an unusable form of Cd for the plants.