The effects of nitrogen (N) deposition on forests largely depend on its fate after entering the ecosystem. While several studies have addressed the forest fate of N deposition using N-15 tracers, the long-term fate and redistribution of deposited N in tropical forests remains unknown. Here, we applied N-15 tracers to examine the fates of deposited ammonium (NH4+) and nitrate (NO3-) separately over 3 years in a primary and a secondary tropical montane forest in southern China. Three months after N-15 tracer addition, over 60% of N-15 was retained in the forests studied. Total ecosystem retention did not change over the study period, but between 3 months and 3 years following deposition N-15 recovery in plants increased from 10% to 19% and 13% to 22% in the primary and secondary forests, respectively, while N-15 recovery in the organic soil declined from 16% to 2% and 9% to 2%. Mineral soil retained 50% and 35% of N-15 in the primary and secondary forests, with retention being stable over time. The total ecosystem retention of the two N forms did not differ significantly, but plants retained more 15NO3- than 15NH4+ and the organic soil more 15NH4+ than NO3-. Mineral soil did not differ in 15NH4+ and 15NO3- retention. Compared to temperate forests, proportionally more N-15 was distributed to mineral soil and plants in these tropical forests. Overall, our results suggest that atmospherically deposited NH4+ and NO3- is rapidly lost in the short term (months) but thereafter securely retained within the ecosystem, with retained N becoming redistributed to plants and mineral soil from the organic soil. This long-term N retention may benefit tropical montane forest growth and enhance ecosystem carbon sequestration.