Purpose Enhancing agricultural and environmental benefits of biochar through physical, chemical, or thermal modifications is frequently employed, but information on the concentration, transformation rate, and half-life of available phosphorus (P) in chemically modified-biochar amended P-fertilized calcareous soils is yet lacking. For this reason, we investigated here the effect of citric acid–modified biochar (CAMB) derived from low-ash agricultural wastes (wheat straw) on P transformation and changes in its plant availability indices in a calcareous sandy soil.Methods An incubation experiment comprising three biochar treatments, viz. control (CK, unamended soil), soil amended with 4% CAMB (CAMB1), and soil amended with 8% CAMB (CAMB2), was conducted under laboratory conditions. The soil samples were incubated at 80% soil moisture (v/w) for 9, 18, 27, 36, 46, and 54 d at 25 ± 1 °C, and Olsen-P was meas-ured after each incubation period. Soil pH and EC were also measured after 9 and 54 days of incubation. To describe the release of P and to determine its rate of transformation, first-order equation, parabolic diffusion, power function, and simple Elovich were used.Results Parabolic equation exhibited a good ability to describe the decrease in P release in soil (0.82 R2 and 0.2 SE). Plant available P in CAMB-amended soil was substantially higher than the unamended soil. The lower pH and P transformation rate indices and the release of available P for a more extended period were found in CAMB-amended soils. The half-life of available P in the CAMB-amended soils exceeded 80% over unamended soil. The rate of P diffusion in CAMB-amended soils was lower (0.05–0.06  day−1) than the unamended soil (0.2  day−1).Conclusion Integrated use of citric acid–modified biochar with inorganic P in calcareous sandy soil acts as a slow-release fertilizer, has a low cost, and is a clean alternative to mineral fertilization.