Patterns in and controls over nitrogen loss from wet lowland tropical forests

Background/Question/Methods

It is a widely accepted paradigm that lowland tropical forests on highly weathered soils cycle N in relative excess, with high rates of N trace gas and leaching losses.  However, recent data collected from several notably wet forests (including a lowland rainforest on the Osa Peninsula of Costa Rica that receives more than 5000 mm of rainfall per year) show very different patterns in the major indicators of the N cycle than what is typically seen in drier portions of the biome. This inconsistency forms the basis for an emerging question in tropical ecosystem ecology: Why do very wet lowland forests seem to exhibit a distinctly different nitrogen (N) cycle than their drier counterparts?   
Results/Conclusions
N₂O fluxes (~ 0.8 kg N-N₂O ha-1 y-1), stream N export (~ 3 kg ha-1 y-1) and δ¹⁵N of foliage (avg ~ 0 per mil) in the Osa region are all low relative to drier sites.  In addition, 98% of stream N export is as DON, and foliar N resorption is high.  These data suggest that wet sites may have greater N retention, and perhaps more frequent N limitation of plant growth, than is commonly assumed for lowland forests.  And yet, N inputs from fixation and deposition in the Osa region exceed 10 kg ha-1 y-1 even without accounting for symbiotic N fixation.  Data from¹⁵N-additions, along with rapid consumption of nitrate in surface soils, suggest high N₂ losses may help resolve this apparent discrepancy in N balance.  Finally, several lines of data also suggest that N demand is notably high in these forests, and that significant tree species-level variation in foliar N demand may contribute to strong spatial variation in N inputs, turnover and loss.