Background and aims <p>Rice is prone to accumulating both inorganic arsenic (iAs) and organic arsenic species, such as dimethylarsinic acid (DMA). DMA is the primary causative agent of rice straighthead disease, a physiological disorder that leads to substantial yield losses. In this study, we investigated whether rice straw-derived biochar with different silicon (Si) contents and Si fertilizer can alleviate rice straighthead disease and decrease DMA accumulation in rice grains.</p> Methods <p>Low- and high-Si biochars were produced from straw of a low-silica rice mutant <i>lsi2</i> and its wild&#xa0;type (WT), respectively, by carbonization at a temperature of 450&#xa0;°C. Pot experiments were conducted to investigate the effects of rice straw-derived biochar and a powdered Si fertilizer on As speciation in soil porewater at different rice growth stages and DMA accumulation in rice grains.</p> Results <p>The Si content of the high- and low-Si biochars differed substantially (127.4&#xa0;g&#xa0;kg⁻¹ for WT and 47.8&#xa0;g&#xa0;kg⁻¹ for <i>lsi2</i> biochar), with approximately 18% Si bioavailability following biochar application. Both biochar and Si fertilizer application alleviated straighthead disease and decreased DMA levels in grain by 24–58.2%, with the high-Si biochar outperforming other treatments.</p> Conclusions <p>The beneficial effects of the biochar on mitigating straighthead disease were primarily attributed to their Si content and the capacity to maintain prolonged Si availability in soil porewater. Additionally, supplementation with Si during the booting stage of rice proved particularly effective in maintaining Si availability, reducing DMA uptake, and alleviating straighthead disease.</p>