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[Display omitted] The effects of P addition on the soil available P, ACP activity, AM diversity and microbial N: P ratio in two tropical forests. AP, available P. ACP, acid phosphatase; AM, arbuscular mycorrhiza; +, increase; -, decline; ×, no effect. • Plants and microorganisms accessed P mainly by releasing hydrions and organic acids. • Plants and microorganisms accessed less P by root interception and mineralization. • Forest succession would not aggravate microbial P limitation. • Inorganic P addition inhibited the transformation and absorption of organic P. Increasing atmospheric nitrogen (N) and phosphorus (P) deposition affect soil nutrient availability and biogeochemical cycles, yet the impacts on the regulatory processes of soil P in P-poor tropical rainforests are not well understood. Based on a ten-year N and P addition experiment in primary and secondary rainforests in Hainan, China, we investigated the effects of N and P additions on four soil P fractions, acid phosphatase (ACP) activity, kinetic characteristics and arbuscular mycorrhizal (AM) diversity. Our results showed that P addition significantly increased soil available P (AP), but had no significant effects on microbial P and microbial N:P ratio in both rainforests. Late-successional primary rainforest had higher microbial P and lower N:P ratio compared to early-successional secondary rainforest, suggesting that forest succession would not aggravate microbial P limitation. Plants and microorganisms accessed most inorganic P by releasing hydrion and organic acids to mobilize soil P, whereas inorganic P accessed by root interception and organic P readily mineralized by ACP and phytase enzymes were relatively low. In the primary rainforest, ACP and kinetic parameters were significantly increased under low N addition, while P addition remarkedly declined ACP activity and AM diversity in both rainforests. Additionally, CaCl 2 extractable P, citrate extractable P, and HCl extractable P in the two forests were significantly positively associated with soil AP, but negatively associated with ACP activity, kinetic parameters and AM diversity. Therefore, the transformations of organic P were inhibited after the exogenous addition of inorganic P. Overall, our study evaluates the soil P acquisition strategies and regulatory processes in tropical rainforests with different successional stages, which can be used to predict the effects of long-term N and P deposition on soil P cycling in P-poor tropical rainforests. [ABSTRACT FROM AUTHOR]