Our objective was to evaluate the relative contribution of physicochemical (diffusion) and biological (mineralization) processes to the supply of ionic P (iP) in solution and potential P availability to plants in a low P sorbing forest soil. To this end, we quantified the gross amount of diffusive iP (ionic P species that can be transferred from the solid phase to the soil solution due to a gradient of concentration, F DIFF), P remineralization (gross mineralization of microbial P, F REM), and gross mineralization of P in dead soil organic matter (F MDSOM) using isotopic dilution techniques during a long-term (154 d) incubation experiment. Initial pools of P in dead soil organic matter and of microbial P represented high proportions (77 and 17%, respectively) of total P (31 μg g−1). The F MDSOM value (1.0 μg g−1) was lower than the F DIFF value (1.2 μg g−1) during the 154-d period of incubation. The F MDSOM and F DIFF values were quantitatively very low compared with F REM (13.7 μg g−1). The F REM value contributed largely to the total pool of isotopically exchangeable iP (89%), suggesting that microorganisms play a crucial role in P availability and cycling. Net organic P mineralization (F MDSOM + F REM – P immobilization by microorganisms) caused a large increase (340%) in readily available P (iP in solution). Extrapolated to longer time scales (1 yr or more), F MDSOM appeared to be higher than F DIFF since F DIFF rapidly reaches its theoretical maximum value. We conclude that, in our low P sorbing sandy forest soil, inorganic P sorbed to the solid phase represented a small but rapidly available pool, and P in dead soil organic matter a larger but slowly available pool. Our work showed that the relative contribution of physicochemical or biological processes to plant available P depends on the length of the observation period. Limits of the isotopic dilution approaches to quantify gross or net organic P mineralization are discussed.