Potassium (K), an essential macronutrient, its availability is often limited in acidic soils due to its fixation or low solubility. Meanwhile, the solubilized K fraction becomes leached under irrigation, owing to reduced K adsorption capacity of acidic soil, thus affecting soil fertility. Previous studies have demonstrated that biochar exhibits a strong potential to retain nutrients in soil. However, their effect on enhancing the K solubility and its retention capacity, especially under combined application of rice straw biochar (BC) with KSB (potassium solubilizing bacteria), remains uncleared. Therefore, an incubation study was carried out, in which a soil column containing 850-gram soil was divided into topsoil (0–15 cm) and subsoil (15–30 cm) portions. The treatment plan was as CK (without K fertilizer), K (K fertilizer), K+ BC (1 % w/w), K+ KSB inoculum (CFU 10⁷ to 10⁸), and K+ KSB inoculum (CFU 10⁷ to 10⁸) + BC (1 % w/w) with quadruplicate. K fertilizer was added only to the topsoil to assess the K retention effect. Results depicted that under integrated application of BC with KSB, soil pH and organic matter (SOM) contents in topsoil enhanced by 13.5 % and 53.9 % compared to K-treated topsoil. Meanwhile, water-soluble K and available K contents increased by 52.5 % and 49.8 % respectively. Such improvement in soil properties greatly enhanced the soil enzymatic activity and soil bacterial community (Actinomycetota and Bacteroidota) in topsoil. Thus, the overall result depicted that integrated application of BC with KSB notably increased the K bioavailability in topsoil by facilitating the KSB activity to solubilize K. Meanwhile, BC improved the K retention capacity through its enlarged surface area, porous structure, and high cation exchange capacity, thereby reduced the K leaching and enhanced the nutrient availability in acidic soil.