Phosphorus (P) acquisition strategies of plants to adapt to P limitation in primary forest ecosystems of karst regions remain unclear. Root phosphatase and arbuscular mycorrhizal (AM) fungi, involved in two common P acquisition strategies, were measured in two N₂-fixing plants (Albiziaodoratissima and Cajanuscajan) growing in shrubland and primary forest soils with and without soil AM inoculation. Both plants cultivated in primary forest soils had lower AM colonisation and N₂ fixation rates but higher root acid phosphatase activity in the rhizosphere when compared with those in shrubland soils. Plants in shrubland soils predominantly exploit P resources by stimulating AM colonisation of roots, but do so by enhancing root phosphatase activity in primary forest soils. AM colonisation in both N₂-fixing plants was positively correlated with N₂ fixation rates but negatively correlated with root acid phosphatase activity. Soil available P content was higher in both N₂-fixing plants under soil inoculation with AM fungi than in the treatment without fungal inoculation; root and shoot P content did not vary significantly between treatments. AM fungi and phosphatase enzymes increase N₂-fixing plants’ capacity to obtain soil P, thus contributing to the decrease in soil P limitation in karst ecosystems.