<p>Biochar from silicon (Si)-rich materials can enhance soil Si availability, but the impact of aging on its Si-releasing behaviors and long-term effectiveness is poorly understood. This study investigated how aging affects Si release from rice husks biochar (RHB), subsequent paddy soil Si availability, and rice Si uptake.&#xa0;Fresh RHB was used as Si-enriched biochar, and two aged variants were prepared through acid treatments with HCl (RHB_HCl) and HNO₃ (RHB_HNO₃). An aqueous dissolution experiment, a soil incubation test, and a rice pot experiment were conducted to evaluate Si release behaviors, soil-available Si, and plant Si accumulation.&#xa0;The aqueous dissolution experiment showed that RHB released the most Si, most likely from surface Si–O polymers, with release inhibited by higher initial aqueous Si concentrations. In contrast, the soil incubation experiment showed that higher soil labile Si concentrations did not hinder Si release, as RHB enhanced plant-available Si (CaCl₂-extactable Si + HOAc-extractable Si) by 24% in high-Si soil and 12.78% in low-Si soil compared to the control without biochar. Aged biochars were less effective, with RHB_HNO₃ showing the weakest Si-releasing capacity. The rice growth pot experiment confirmed fresh RHB application most effectively increased Si concentrations in rice leaves and stems, though grain biomass was not significantly improved.&#xa0;These findings demonstrate that biochar aging significantly reduces the capacity of Si-enriched biochar to supply labile Si. Long-term field trials under natural aging conditions are recommended to further evaluate the persistence of biochar’s effects on soil Si availability and crop performance.</p>