Fates of atmospheric deposited nitrogen in an Asian tropical primary forest

The impacts of increasing nitrogen (N) deposition on forest ecosystems, including on carbon (C) sequestration, largely depend on the extent to which forests are N-limited and so whether and where deposited N is retained within the ecosystem. The 15N tracer method can provide excellent insight into the ecosystem fates of N, but while it has been extensively used in temperate forests it has yet to be sufficiently employed in tropical forests, which are often thought not to be N-limited. Here, we used stable isotope 15NH4+ and 15NO3− tracers applied as solutions to the forest floor to examine the fates of different forms of N in a tropical montane primary forest with low background atmospheric N deposition (6 kg N ha−1 yr−1) in China. We found that a substantial amount of 15N was assimilated by plants over time and significantly more 15N was recovered following 15NO3− addition than following 15NH4+ addition: 7% and 16% of 15N were recovered three months after the respective 15NH4+ and 15NO3− tracer additions and 11% and 29% respectively after one year. In contrast to plants, the organic layer was only an important short-term sink for deposited N: while 21% and 12% of the 15N from 15NH4+ and 15NO3− additions were accumulated in the organic layer after three months, more than half of the retained 15N was lost after one year. Mineral soil was the largest sink for deposited N, and the 15N retained in soil was relatively stable over time for both N forms, with 39% and 32% of the initial 15N input recovered after one year for 15NH4+ and 15NO3− tracer additions, respectively. Overall, the total ecosystem 15N recovery one year after the 15NH4+ and 15NO3− tracer additions was large (60% and 66% respectively), and not significantly different from total recovery after three months, suggesting that a large proportion of deposited N could be retained in the longer term. Based on the measured fate of 15N one year after labeling and the C:N ratios of different plant components, this tropical forest's carbon sequestration efficiency is calculated to be 17 kg C per kg N added, comparable to the values reported for temperate and boreal forests in Europe and North America and indicating substantial N limitation of this tropical forest. Our results suggest that anthropogenic N input in moderate levels may contribute to enhance C sequestration in some tropical forests, without significant long-term loss of N to the environment.