Abstract:Aiming at the problems of rice safety and quality caused by the activation of toxic heavy metals as a result of soil acidification in paddy soil of southern China, rice straw and husk were selected to produce biochar (reported as RSC and RHC respectively), the effects of different feedstocks biochar on the improvement of acidified soil and the availability of heavy metals were studied. Three biochar application rates (0, 20, 50 g/kg; CK, C1, C2), four soil acidification levels (pH 4.01, 4.25, 4.33, 4.58; L1, L2, L3, L4) were used in this study. The soil pH, total nitrogen, organic matter, available phosphorus, available potassium, and available Cu and Cd contents were determined after incubation for 60 days. Results showed that the RSC improved the pH of acidified soil significantly better than RHC, and the higher the amount of biochar applied, the greater the range increased. The C2 treatment of RSC increased the pH of four acidity levels soils by 0.68, 0.97, 1.29 and 1.71 units, respectively. Two biochar both could increase the contents of total nitrogen, available phosphorus, available potassium, and organic matter in soil. Among them, organic matter was significantly increased in all treatments, especially available potassium, and RSC was superior to RHC on enhancing the nutrients. RHC had no significant effect on soil available Cu content. C2 of RSC could reduce soil available Cu content more effectively than C1 treatment, and reduced the four acidity levels soils by 13.62%, 6.57%, 4.36%, and 7.88%, respectively. Available Cd content in L3 and L4 soils treated with RHC decreased significantly, with a maximum of 13.79% and 19.23% respectively. RSC reduced the maximum content of available Cd in four acidity soils by 20.00%, 25.81%, 20.69%, and 19.23%, respectively. Correlation analysis showed a significant negative correlation between soil pH and available heavy metal content. Rice straw biochar was more effective in improving acidified soils and decreased the availability of heavy metals such as Cu and Cd, and removed the available Cd in contaminated soils better than that of available Cu. Biochar had better effects on the increase of soil pH and available phosphorus content and the decrease of available Cd content in the soil with lower acidification degree, while the decrease of available Cu content was better in the soil with higher acidification degree. Soil pH is the main factor affecting the availability of heavy metals such as Cu and Cd regulated by biochar.