Abstract: In order to understand the effects of pH on Cd speciation in soil solution,  anodic stripping voltammetry (ASV) and Visual MINTEQ 3.0 model were developed to characterize the speciation of Cd in rhizosphere soil solutions of Cdhyperaccumulating ecotype (HE, Sedum alfredii Hance) and non-hyperaccumulating ecotype (NHE, Sedum sarmentosum Bunge) under different soil pH conditions (4.0, 5.5, 7.0 and 8.5). The model simulation results demonstrated that the proportions of Cd²⁺ in soil solutions of both two plants decreased with the increase of pH. Cd²⁺ fraction in rhizosphere solution varied between 46.1%-3.2% for HE, and 60.9%-9.9% for NHE. Cd-DOM complexes were the dominant Cd speciation when pH≥5.5. Cd²⁺ fractions in rhizosphere solution were always greater for NHE than for HE, but Cd-DOM complexes were the reverse. In addition, results measured by ASV showed that labile Cd complexes varied from 5.5% (pH 8.5) to 79.6% (pH 4.0) for HE, and between 11.8%-86.7% for NHE. Moreover, the fractions of labile Cd measured by ASV were significantly correlated with those calculated by the model (R²=0.80); the results verified the suitability of both the experimental and model methods.

Key words: species biodiversity, canopy coverage, evergreen broadleaved forest, canopy vertical structure