<p>Biochar is extensively used in environmental remediation, but the effects of aging on the immobilization of cadmium (Cd) in contaminated soils treated with biochars containing various endogenous Cd is unclear. This study investigated the effects of simulated acid rain (SAR) aging on the bioavailability of Cd in soil amended with two rice straw biochars (RSBs, marked as RS1 and RS2) and two sludge biochars (SSBs, marked as SS1 and SS2) under flooding condition. Results showed that SSBs had higher specific surface area, ash, silicate contents, and more functional groups than those of RSBs. The total Cd in the biochars followed the order of RS1 &gt; RS2 &gt; SS1 &gt; SS2. SSBs applications significantly decreased soil CaCl₂-extractable and acid-soluble Cd by 51.3%-78.6% and 4.51%-5.44% after a 30-day incubation with 70% water holding capacity than the control. SAR aging significantly decreased the soluble, CaCl₂-extractable, and acid-soluble Cd in all treatments compared with those before aging. This may be due to the decreased effects from the formation of insoluble Cd sulfide and the adsorption of Cd by Fe–Mn (oxidro)oxides under continuous flooding condition was outweighed the increase in soluble Cd caused by SAR accumulation. Moreover, the soluble, CaCl₂-extractable, and acid-soluble Cd in RSBs treated soils were significantly higher than those of SSBs after SAR aging. The study indicated that the RSBs with high endogenous Cd posed greater environmental risks, thus the long-term effects of biochar application in soil need further study.</p>