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Abstract Conditioning alkaline soil with acidic phosphorus-rich biochar might contribute to improving soil phosphorous (P) availability and waste utilization efficiency. In this study, acidic phosphorus-rich biochar was prepared using halophyte biochar (HBC) modified by H3PO4 (P-HBC) and H4P2O7 (PA-HBC). The P-containing groups and P fractions in the HBCs were characterized, and the effects of biochar on the P fractions in alkaline soil were examined with incubation experiments. The results showed that the pH of P-HBC (3.31) and PA-HBC (2.17) was significantly reduced compared with HBC, and the total P contents rose to 4.66 g·kg− 1 and 5.24 g·kg− 1, respectively. The spectral characterization confirmed the loading of P-containing and acidic functional groups in the HBCs after modification. The application of P-HBC and PA-HBC in alkaline soils decreased the soil pH and facilitated the transformation of stable P into active fractions. Overall, acidic phosphorus-rich biochar can be employed to increase P activity in alkaline soil. Graphical Abstract