Effects of phosphate addition on immobilization and phytoavailability of Cd in soil and the chemical form and translocation of Cd within rice plants

Abstract

In this work the effects of phosphate (P) on the pH, Cd phytoavailability, and Cd distribution in rice plants was examined in a pot experiment and field experiments using soil exhibiting a high (> 60 mg Cd/kg soil) Cd concentration for the pot experiment and high (> 60 mg Cd/kg soil) and low (< 0.5 mg/kg) Cd concentration for the field experiment. The soil was treated with four P levels, 0 (control), 50, 200, and 1000 mg P/kg soil in the form of a commercial P-fertilizer (0-52-34; 52% P2O5 and 34% K2O). Cd immobilization in soil was evaluated by a chemical fractionation scheme for both the pot and the field experiments. Additionally, the effects of various P concentrations (0-1000 mg/L) on the chemical forms and subcellular fractionation of Cd within rice plants were examined. The addition of P increased the pH and Cd adsorption by the soil and decreased the Cd concentration in the soluble and exchangeable Cd fraction. There was an increase in dry matter yield with increasing P addition for the pot as well as for the field experiment. Results obtained for plant uptake for the pot experiment were different compared to the field experiment. Due to high Cd concentrations present in soil, plants showed severe Cd toxicity symptoms. Cd uptake by the whole rice plants was not altered with increasing P addition. This was different for the field experiment as with increasing P application the Cd concentration in plants decreased. With increasing growth stage the Cd concentration in plants was decreased being more pronounced for higher levels of P application ascribed to a reduced Cd uptake, translocation, and as a result of the dilution effect of the plant’s Cd with increasing biomass. The chemical forms of Cd within the rice plants were altered after Paddition and the uptake was decreased. The subcellular fractionation showed a redistribution of the plant’s Cd to the immobile cell wall fraction with higher levels of Paddition.