Mechanisms of reduced Cd accumulation in rice by straw biochar: The role of iron plaque

Although biochar has been increasingly explored for cadmium (Cd) imobilization in paddy soils, the pathways by which biochar modulates sulfur and iron transformation under redox fluctuations during rice growth to diminish soil Cd bioavailability and enhance the Cd adsorption of iron plaque (IP) await in-depth exploration. Accordingly, a pot experiment was conducted with Cd-contaminated paddy soil. Rape and rice straw biochars, pyrolyzed at 450 o C (RALB, RLB) and 800 o C (RAHB, RHB) were applied at rates of 0.5% (BC 0.5 ) and 1% (BC 1 ). The results revealed that biochar reduced the grain Cd and root Cd levels by 22.2~65.5% and 18.2~66.7%, particularly in the RALB 1 and RLB 1 treatments when compared to control (CK). Structural equation modeling revealed that soil CaCl₂-extractable Cd, along with iron plaque, were pivotal factors affecting Cd accumulation in rice. Biochar effectively decreased soil CaCl₂-extractable Cd, and RALB 1 and RLB 1 treatments exhibited the lowest Cd levels with an average reduction of 35.6% relative to the CK. The RLB 1 treatment exhibited the highest increase (42.5%) in Cd associated with the Fe/Mn oxide fraction in soil at the heading stage, as well as a 5-fold increase in Cd within the IP at the tillering stage. RALB 1 treatment elevating plaque sulfur content by 3.3–5.2 times and minimizing Cd translocation from iron plaque to rice roots. In conclusion, biochar mediates sulfur-iron dynamics to suppress soil Cd mobility and strengthen the barrier function of iron plaque, thereby reducing Cd accumulation in rice.