Chlorine weaken the immobilization of Cd in soil-rice systems by biochar

Rice (Oryza sativa L.) was cultivated in a Cd-contaminated soils with rice straw biochar (BC) and water-washed rice straw biochar (W-BC) were applied to investigate the underlying mechanisms and possible reasons for biochar's weakening effects on the immobilization of Cd in soil-rice system. The results indicated that W-BC reduced the Cd concentration in pore water as well as in the roots and shoots of rice by 26.24%, 53.23% and 62.47% respectively. On the contrary, there was an increase in Cd contents by 50.27% in pore water, 2.32% in the roots, and 12.80% in the shoots of rice under BC treatment. Furthermore, Cd content in rice shoot was significantly and positively correlated with Cl^- addition to the soil (P < 0.01). This phenomenon could be attributed to several combined effects: (1) the increase of Cl^- in the soil decreased the soil pH, enhanced the dissolved organic carbon in soil pore water and increased the complexes of Cd²⁺ and Cl^-, resulting in the release of Cd from solid phase into solution phase, (2) the chloride in the soil increased the uptake of CdCl⁺ instead of Cd²⁺ by the roots, thereby causing an increase of Cd in rice tissues. These results demonstrate for the first time that biochar with high chloride content could weaken its immobilization effects on soil Cd and even enhance Cd uptake by rice.