Biochar reduced soil extractable Cd but increased its accumulation in rice (Oryza sativa L.) cultivated on contaminated soils

PurposeThis study focused on the effects and mechanisms of biochar amendment to Cd-contaminated soil on the uptake and translocation of Cd by rice under flooding conditions.Materials and methodsPot and batch experiments were conducted using Cd-contaminated soil collected from a field near an ore mining area and a cultivar of Oryza sativa ssp. indica. Biochar derived from rice straw under anaerobic conditions at 500 degrees C for 2h was mixed with the soil at the rate of 0, 2.5, and 5%.Results and discussionThe application of 5% biochar reduced CaCl2-extractable soil Cd by 34% but increased Cd concentration in brown rice by 451%. Biochar amendment decreased water-soluble Fe2+ in soils and formation of Fe plaques on roots and weakened the Fe2+-Cd2+ competition at adsorption sites on the root surface. Biochar increased water-soluble Cd in the soil and consequently Cd uptake by rice roots by releasing water-soluble Cl-. Biochar application also reduced the proportion of cell wall-bound Cd in the root, which caused easier Cd translocation from the cortex to the stele in the root and up to the shoot.ConclusionsRice straw biochar (with high concentration of water-soluble Cl-) reduced CaCl2-extractable soil Cd but increased Cd concentration in rice under flooding condition.