<p><UnorderedList Mark="Bullet"> <ItemContent> <p>High-N addition accelerated soil acidification in tropical montane rainforests.</p> </ItemContent> <ItemContent> <p>Primary rainforest was more sensitive to soil acidification than secondary rainforest.</p> </ItemContent> <ItemContent> <p>Changes in exchangeable Ca²⁺ and H⁺ induced by N input regulated soil acidification.</p> </ItemContent> <ItemContent> <p>P50 or N50P50 addition did not change soil pH or cations in both rainforests.</p> </ItemContent> </UnorderedList></p><p>Tropical forest soils are susceptible to acidification owing to high weathering rates and low buffering capacity. Nutrient additions, particularly nitrogen (N) and phosphorus (P) inputs, can alter soil acidity; however, their long-term effects on the dynamics and underlying mechanisms of soil pH in tropical rainforests are not well understood. Here, we conducted two 13-year N and P fertilization experiments in primary and secondary tropical montane rainforests in Hainan, China. Results showed that long-term high-N addition reduced soil pH, and the effects increased with the rate and duration of N addition in both rainforests. The P-limited primary rainforest was more susceptible to N-induced soil acidification than the N-limited secondary rainforest with higher stand density during the experimental periods. Moreover, the depletion of base cations (primarily Ca²⁺) and the generation of exchangeable H⁺ were the main drivers of N-induced soil acidification. However, low- and medium-N additions, single P addition, and combined N and P addition did not significantly change soil pH or cation concentrations in both forests. These findings suggest that elevated soil N availability induced by long-term fertilization may alter soil cation composition, thus leading to soil acidification and impacting ecosystem functions in tropical forests.</p>