In this study, soil organic P (Po) and low-molecular-weight organic acids (LMWOAs) were quantified under different land uses in order to investigate the effect of N2 fixing tree introduction on phosphorus cycle. Soils were collected from plantations of pure acacia (Ac), pure eucalyptus (Euc) or both species (50/50) and original savannahs (S) that were located in Brazil (low P, high N and high C soil) and in Congo (high P. low N and low C soil). Po and LMWOAs were identified in sodium hydroxide soil extracts with ion chromatography (IC). Phosphate monoesters as AMP and glucose-6-phosphate (G6P) were the main Po forms in both sites. Phytate, ATP and fructose-bisphosphate (FrucbisP) were also present as well as the mineral form of pyrophosphate (PrP). Malate, oxalate and malonate were the major components of LMWOA fraction in the two soils. Citrate was also present at low concentrations. lnterresingly, phytate concentrations were always decreased under acacia plantations. This could result from a better mineralisation or lower inputs into these soils. In mixed stands, the effect of acacia introduction on Po and LMWOAs composition is more pronounced in high P than in low P soil, due to a much better N, fixation. Our results highlight that the introduction of a legume tree is able to strongly modify the composition of Po and LMWOAs in soil even after a first short-time rotation, especially when the rate of N2 fixation is high.