Sulfur enhances cadmium bioaccumulation in Cichorium intybus by altering soil properties, heavy metal availability and microbial community in contaminated alkaline soil.

Cadmium (Cd) contamination seriously threatens the soil health and food safety. Combination of amendment and accumulator plant is a green and effective technique to improve phytoremediation of Cd-contaminated alkaline soil. In this study, a potting experiment was conducted to investigate the effect of sulfur on Cd phytoextraction by Cichorium intybus (chicory). Soil chemical and microbial properties were determined to reveal the mechanism of sulfur-assisting Cd phytoremediation by chicory. Soil pH decreased from 7.77 to the lowest 7.30 with sulfur addition (0.6, 0.9 and 1.2 g kg^-1, LS, MS and HS treatment); Electric conductivity, sulfate anion and available cadmium concentration increased gradually with increasing sulfur doses. Cd concentration of shoot and root significantly increased from 1.47 to 4.43 mg kg^-1, 6.15 to 20.16 mg kg^-1 by sulfur treatment relative to CK, which were attributed to increased available Cd concentration induced by decreased pH. Sulfur treatments significantly increased the Cd bioconcentration factor by 64.1%, 118.6%, 201.0% for shoot, 76.3%, 145.6% and 227.7% for root under LS, MS and HS relative to CK treatment, respectively (P < 0.05). However, only MS treatment significantly improved the Cd removal efficiency by 82.9% in comparison of CK treatment (P < 0.05). Microbial community diversity measured by 16SrRNA showed that Thiobacillus and Actinobacteria were the key and dominant strains of soil microbial communities after sulfur addition, which played a pivotal role in the process of sulfur oxidation involved in decrease of soil pH and the transformation of Cd forms. Correlation analysis and path analysis by structural equation model indicated that soil sulfate anion and Thiobacillus directly affected Cd removal efficiency by chicory in Cd-contaminated alkaline soil. This suggests that combination of sulfur and chicory may provide a way to promote Cd bioaccumulation for phytoremediation of Cd-contaminated alkaline soil.