Dynamics and fate of atmospherically deposited nitrogen in montane tropical forests in the first three years.

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 (NH4 + ) and nitrate (NO3 - ) separately over three 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 studied forests. Total ecosystem retention did not change over the study period, but between three months and three years following deposition 15 N recovery in plants increased from 10% to 19% and 13% to 22% in the primary and secondary forest, 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 NO3 - than 15 NH4 + and the organic soil more 15 NH4 + than 15 NO3 - . Mineral soil did not differ in 15 NH4 + and 15 NO3 - 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 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.