Wednesday, August 4, 2010 - 3:40 PM

COS 67-7: Patterns in and controls over nitrogen loss from wet lowland tropical forests

Alan R. Townsend, University of Colorado, Boulder, William R. Wieder, Institute of Arctic and Alpine Research, University of Colorado at Boulder, Philip G. Taylor, Institute of Arctic and Alpine Research, University of Colorado at Boulder, Cory C. Cleveland, University of Montana, and Gregory P. Asner, Carnegie Institution.

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.