Ammonium polyphosphate (APP) is currently being increasingly used as source of phosphorus (P) fertilizer worldwide and has exhibited positive effects on soil P availability in calcareous soils. However, the mechanisms of APP-induced abiotic and biotic transformations of soil P remain elusive. Influences of APP application on P transformation in a calcareous soil were investigated in comparison to monoammonium phosphate (MAP). The application of APP significantly increased soil Olsen-P by 61.3% relative to MAP application. Specifically, APP application (i) significantly increased biologically-based P corresponding to P extracted by CaCl2, citric acid, and enzyme; (ii) increased labile Ca-P proportion (NaHCO3 extractable P) while decreased stable Ca-P proportion (H2SO4 extractable P); (iii) decreased organic P (Po) content. In APP-treated soil, the decrease in pH, crystal Fe and Al oxides together with the increase in organic C and complex Fe oxides contributed to the P sorption decrease and P desorption/dissolution increase. Meanwhile, APP significantly increased alkaline phosphatase activity and phoD gene abundance, which contributed to Po mineralization increase. Additionally, APP changed β-diversity and increased the relative abundance of Rhodobacterales and Myxococcales harboring pqqC gene, indicating the contribution of P solubilizing bacteria to inorganic P (Pi) dissolution. Redundancy analysis showed that soil pH can explain 70.4, 53.8, and 55.1% of the total variances observed in the composition of BBP, Pi and soil P sorption–desorption parameters, respectively. This suggests that the change of soil pH from APP application in this calcareous soil altered soil abiotic and biotic P transformation processes, promoting soil P availability. [ABSTRACT FROM AUTHOR]