Dynamics and multi-annual fate of atmospherically deposited nitrogen in montane tropical forests

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.