The effect of biochar and hematite on cadmium availability in a paddy soil under flooded and drained conditions

Flooding and drainage cycles affect the biological and chemical characteristics of the soil in paddy fields and play an important role in cadmium (Cd) availability. In this study, the effect of biochar (B), hematite (H), and their simultaneous application (HB) on pH, Eh, and calcium chloride extractable Cd was investigated in a Cd-contaminated soil during a flooding and drainage period. Also, the Cd fractionation at the end of the incubation period was studied. The pH value increased during the flooding period and decreased with the beginning of the drainage period. The Eh value decreased from the beginning of the flooding and showed an increasing trend during the drainage period. Calcium chloride extractable Cd decreased during the flooding period, while at the beginning of the drainage increased with a slight slope and followed a decreasing trend up to the end of the drainage period. Cd concentration reduced at the end of the flooding period in the B, H and HB treatments compared to the control by 16.8%, 20.5%, and 25.7%, respectively. Also, the reduction in Cd concentration compared to the control treatment at the end of the drainage period was 18.2, 23.1 and 28.2% respectively. HB treatment reduced the Cd availability by reducing the soluble and exchangeable fractions and increasing organic, iron, and manganese oxides fractions. The results indicated that the application of amendments can reduce Cd availability in paddy fields and its uptake by the rice plant.