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Fertilizer management can alter the availability and biological cycle of soil phosphorus (P). The objective of this study was to explore the relationship among soil P availability, organic phosphorus (Po) mineralization and soil bacteria characteristics under organic and inorganic fertilization in saline-alkali paddy soil. Four treatments were designed, namely no fertilizer (Control), mineral fertilizer (NPK), mineral fertilizer with rice straw (NPKS) or rice straw-derived biochar (NPKB). Compared with NPK treatment, straw and biochar addition greatly increased P uptake by rice while decreased soil organic P/total P ratio. The contents of soil available P (AP) and alkaline phosphatase (ALP) in NPKS treatment were higher than those in NPKB treatment. This was most likely due to the higher abundance of Streptomyces in NPKS treatment than that in NPKB treatment, which was known as a potential Po mineralizer. NPKS and NPKB treatments significantly increased diversity and abundance of phoD -harboring bacteria but have no significant effect on the total bacteria. Based on SEM analysis, the composition of phoD -harboring bacteria community was more important in ALP activity than the increased phoD -harboring bacterial abundance and diversity. Soil dissolved organic carbon (DOC) content and DOC: AP ratio were the key factors affecting the formation of total bacterial and phoD -harboring bacterial community, which were significantly increased in NPKS treatment. Compared with biochar application, straw was more efficient in increasing the abundance of some phoD -harboring bacteria, which preferred carbohydrate-rich environments, thereby improving the availability of P and its potential biological transformation in saline-alkali paddy soil. [ABSTRACT FROM AUTHOR]