Chemical forms of cadmium in a calcareous soil treated with different levels of phosphorus-containing acidifying agents

Abstract

Calcareous soils with high background cadmium (Cd) levels are widely distributed in south-west China and soil acidity is a major environmental problem. However, little effort has been made to study the changes in chemical speciation as affected by soil acidification other than by acidic rain. In the present study, we investigated the impact of mono-ammonium phosphate (MAP) and phosphoric acid on soil pH and chemical transformation of Cd in calcareous soils. Calcareous soils collected from south-west China were treated with three concentrations (0.1, 0.2 and 0.4 mol kg–1 soil) of MAP and phosphoric acid, and without the addition of acidifying agent, before being incubated at 25 ± 2°C near field capacity moisture content for 60 days. The chemical forms of Cd in the soils were determined by the Tessier sequential extraction scheme. The concentration of Cd in the form bound to iron and manganese oxides (CdFeOx+MnOy) decreased significantly with increasing levels of the two acidifying agents, but the concentration of exchangeable Cd (CdEx) exhibited a significant increase, indicating that the two acidifying agents can effectively promote the transformation of CdFeOx+MnOy to CdEx. MAP may promote the same amount of CdFeOx+MnOy to CdEx as phosphoric acid at the same rate of addition, but the soil pH clearly differed, implying that an increase in water-soluble P with addition of acidifying agent may be one major factor affecting the chemical transformation of Cd. MAP and phosphoric acid should be used carefully as sources of water-soluble phosphorus for calcareous soils with high background concentrations of Cd.