Influence of silicon on cadmium availability and cadmium uptake by rice in acid and alkaline paddy soils

Abstract

The aim of this research was to study the inhibiting effects of cadmium uptake into plants by exogenous silicon between acid and alkaline paddy soils, especially, the effects of silicon without the differentiation of pH and accompanying ions. Therefore, without the interference of pH and accompany ions (Na+), the mechanisms of soil chemistry on cadmium (Cd) transformation and accumulation by Si application in the soil-plant system were examined. The acid and alkaline soils for the experiment were collected from the upper soil layer (0~20 cm) from two paddy fields. The Cd solution as CdNO3 and different levels of SiO2 as Na2SiO3 solutions were applied in the potted soils. To eliminate differences of nitrate and sodium among the treatments, 6.8, 6.8, and 1.7 g sodium nitrate was added into the CK, Si0Cd5, and Si90Cd5 treatments, respectively. The soil redox potential and pH during the experiments were determined. After harvest, changes in Cd concentrations in rice grains, stems, and roots were measured. Soil samples in close proximity to the rice roots were collected at harvest. The sequential extraction was employed to estimate the chemical speciation of Cd in the treated soils. The Si applications reduced Cd accumulation in rice plants, thereby increasing the rice yield. With increase of the dosages, the ratios of Cd contents in root, stem, and grain decreased significantly. The Eh and pH of the Si treatments were higher than application of Cd only. In the alkaline soil, Cd contents in the soil solutions of the Si treatments were higher than the Si0Cd5 treatment. In contrast, in the acid soil, the Cd contents in the soil solution of the Si treatments were lower than that Si0Cd5 treatment. In the acid soil, Si application improved the Cd adsorption by soil and alleviated Cd uptake by plants. Therefore, the alkaline soil reduced Cd phytoavailability by forming Si-Cd water-soluble complex, which could not be taken up by rice plants. The Si application significantly increased yield of rice and decreased Cd concentrations in the roots, stems, and grains of rice plants. In the alkaline soil, Cd and Si formed the Si-Cd water-soluble complex, which could not be taken up by the rice plants. In both soils, Si reduced the concentrations of acid extractable Cd and reducible Cd while promoting the formation of oxidable Cd and residual Cd, resulting in low soil Cd bioavailability.