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 ¹⁵ N tracers, the long-term fate and redistribution of deposited N in tropical forests remains unknown. Here, we applied ¹⁵ N tracers to examine the fates of deposited ammonium ( NH 4 + ) and nitrate ( NO 3 - ) separately over 3 years in a primary and a secondary tropical montane forest in southern China. Three months after ¹⁵ N tracer addition, over 60% of ¹⁵ N 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 ¹⁵ N recovery in plants increased from 10% to 19% and 13% to 22% in the primary and secondary forests, respectively, while ¹⁵ N recovery in the organic soil declined from 16% to 2% and 9% to 2%. Mineral soil retained 50% and 35% of ¹⁵ N 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 15 NO 3 - than 15 NH 4 + and the organic soil more 15 NH 4 + than NO 3 - . Mineral soil did not differ in 15 NH 4 + and 15 NO 3 - retention. Compared to temperate forests, proportionally more ¹⁵ N was distributed to mineral soil and plants in these tropical forests. Overall, our results suggest that atmospherically deposited NH 4 + and NO 3 - 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.

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