Rice straw biochar reduces Cd accumulation and promotes Cu accumulation in rice: irrigation regime is the driving factor

Purpose Biochar has become a research hotspot in soil heavy metal pollution remediation. However, there are few studies on the effect of biochar on the heavy metal accumulation in rice under different irrigation regimes, and there is very limited information on the soil heavy metal speciation changes during whole rice growth period. This study aims to clarify the effect and mechanism of biochar on Cd and Cu accumulation in rice grains under different irrigation regimes. Materials and methods The rice straw biochar (0 g kg −1 (CK), 20 g kg −1 (BC20), 40 g kg −1 (BC40), and 60 g kg −1 (BC60)) was applied to the paddy soil (contaminated with Cd and Cu) under three irrigation regimes (flooding irrigation (FI), intermittent irrigation (II), and wet irrigation (WI)), and a rice pot experiment was carried out for two consecutive years. The speciation changes of Cd and Cu in the soil during the key stage of rice growth and the metal content of each part of the rice were analyzed. Results and discussion Biochar promoted the formation of reducible and oxidizable Cd throughout the rice growth period, while the acid-extractable and residual forms were dynamic and affected by irrigation regimes. A significant reduction in acid-extractable Cu content was observed, and biochar mainly converted the acid-extractable Cu to oxidizable Cu, while reducible Cu was sensitive to soil water conditions. As biochar reduced soil available Cd content, BC60 decreased Cd content in brown rice by 50.41%, 70.32%, and 81.52% under FI, II, and WI, respectively, but the Cd content in brown rice was the lowest in FI. Biochar decreased the Cu content in brown rice under FI (maximum 31.2%), while significantly increased Cu content under II (maximum 74.33%) and WI (maximum 49.3%) because biochar increased soil available Cu content after rice heading stage and encouraged its transport from root to shoot. Conclusions Rice straw biochar can be used to control Cd pollution in rice under various irrigation regimes, and the effect is better when combined with flooding irrigation. The application of rice straw biochar to control crop Cu pollution is not suitable for water-saving irrigation.