Objective   To investigate the effect of biochar made from rice straws on cadmium (Cd) immobilization in different types of soil under flooding.

  Method   An in-lab experiment on 3 Cd-added different types of soil with or without a 5% addition of biochar made from rice straws was conducted under varied durations of flooding. The types of soils used were yellow soil, paddy soil derived from quaternary red clay, and brown soil.

  Result   The biochar reduced pH and redox potential (Eh) but increased conductivity of the soils on the first day after flooding. As the flooding persisted, Eh in the soils reduced continuously, but the rate declined in the presence of the biochar. During the initial stage of flooding, the Cd contents in the soils were highest in the yellow soil at 272.5 μg·L⁻¹ followed by the paddy soil at 23.48 μg·L⁻¹, while the brown soil at 1.44 μg·L⁻¹ being the lowest. The Cd reductions by 31.66% in the yellow soil, 75.04% in the paddy soil, and 66.67% in the brown soil were attributed to the added biochar. Under prolonged flooding, the Cd in the yellow and paddy soils gradually decreased even without the biochar addition. In 30d, the reductions were 89.34% on the yellow soil and 76.53% on the paddy soil. In comparison, the addition of the biochar brought about 85.41% Cd reduction on the yellow soil and 37.03% on the paddy soil. After 30d of flooding, the biochar out-performed control with the CaCl₂-Cd contents in the yellow, paddy, and brown soils lowered by 17.3%, 56.3%, and 12.4%, respectively.

  Conclusion  By adding 5% of the rice straw biochar, the water-soluble Cd in the 3 different types of soil could be significantly reduced. Prolonged flooding made the effect less pronounced, and the biochar immobilization of Cd appeared more effective in the paddy soil than the other two soil types.