Abstract: The forms of sulfur significantly affect the rhizosphere environment of plants. However, the regulatory mechanism of sulfur on the rhizosphere microenvironment of hyperaccumulator plants remains unclear. Therefore, typical hyperaccumulator and tolerant plant species were cultivated in three types of soil with different levels of pollution under adding different sulfur forms (S₀ and SO₄^2-). We measured heavy metal concentrations in soils, organic acid concentrations in the roots, and microbial community structure. The regulatory effects of sulfur on organic acid concentrations and microbial community structure in roots were examined. The results showed that the application of sulfur (S₀) recruited genus Thiobacillus and decreased soil pH, while the application of sulfate (SO₄^2-) recruited genus Caldalkalibacillus, promoting the secretion of succinic acid. In lowly and moderately polluted soils, plants mainly secreted organic acids such as oxalic acid and malic acid, whereas in highly polluted soils, tartaric acid was primarily secreted. Microorganisms such as genus Streptomyces were significantly correlated with organic acid metabolism, which promoted heavy metal absorption by plants. This study provides theoretical and practical guideline for the remediation of heavy metal-contaminated soils.

Key words: heavy metal, hyperaccumulator, rhizospheric microorganisms, root exudate