Remediation and Micro-Ecological Regulation of Cadmium and Arsenic Co-Contaminated Soils by Rotation of High-Biomass Crops and Sedum alfredii Hance: A Field Study

Rotation of high-biomass crops and hyperaccumulators is considered to be an effective, safe and economical method for the remediation of medium-mild heavy metal contaminated soil, but the present studies pay more attention to the removal efficiency rather than changes in soil micro-ecology. In order to explore the remediation effect of hyperaccumulators rotated with high-biomass crops on Cd and As co-contaminated soil, Cd hyperaccumulator ecotype (HE) Sedum alfredii Hance and crops were selected to construct a field experiment, five rotation modes including Sedum alfredii Hance- Oryza sativa L. (SP), Sedum alfredii Hance- Sorghum bicolor (L.) Moench (SS), Sedum alfredii Hance- Zea mays L. (SM), Sedum alfredii Hance- Hibiscus cannabinus L. (SK), Sedum alfredii Hance- Trichosanthes kirilowii Maxim. (ST), and investigated the effects of these modes on the removal efficiency, soil physiochemical properties and micro-ecological effects (soil nutrients, enzyme activities and microbial diversity) through a field experiment. The results showed that total soil Cd from the five rotation modes (SP, SS, SM, SK and ST) decreased by 25.1%, 20.3%, 34.5%, 6.3% and 74.3%, respectively, and total soil As decreased by 42.9%, 19.8%, 39.7%, 39.7% and 45.7%, respectively. The rotation significantly increased soil organic matter by 47.39–82.28%, effectively regulated soil pH value and cation exchange capacity. The rotation modes also significantly increased soil alkali-hydrolysable nitrogen by 9.09–50.91%, but decreased soil available phosphorus and rapidly available potassium. Except for urease, the soil enzyme activities increased overall. The Alpha diversity increased, and soil microbial structure optimized after rotation. ST mode was the most effective remediation mode, which not only reduces the content of Cd and As in the soil, but also effectively regulates the soil micro-ecology. The results from this study have shown that it is feasible to apply Sedum alfredii Hance and the high-biomass rotation method for the remediation of Cd and As co-contaminated soil.