Do the wettest of lowland tropical forests cycle nitrogen in excess?

Background/Question/Methods

Lowland tropical forests on highly weathered soils are thought to cycle nitrogen (N) in relative excess, a notion supported by data showing high rates of N turnover and loss.  Yet, the majority of such data are from the drier end of the tropical rain forest precipitation spectrum, and the few data that exist for the wettest of lowland forests suggest a bit of a paradox:  one where some indices imply high rates of both internal and external N fluxes, while others suggest that wet forests may be more N-constrained than their drier counterparts.  We used both litter and throughfall manipulations and a watershed approach to explore multiple aspects of the N cycle in a lowland tropical forest in southwestern Costa Rica that receives > 5m of precipitation annually, and that has prior evidence of phosphorus (P) limitation. 

Results/Conclusions

Some indices of N cycling were notably high: dissolved N fluxes from litter to soils exceeded 20 kg N ha^-1 y^-1, and soil inorganic N accumulation in resin capsules averaged > 10 μg N d^-1 throughout the rainy season, mostly as nitrate (NO₃^-).  Previous work at the site also demonstrated that free-living N-fixation contributes an additional 10 kg N ha^-1 y^-1. Yet, soil N₂O emissions averaged < 1 kg N ha^-1 y^-1, and stream N export values from this and neighboring sites were all notably low for lowland tropical watersheds.  In addition, extractable NO₃^- pools averaged 7 mg N kg^-1 soil and extractable NO₃^-: NH₄⁺ ratios were < 1, both of which are low compared to other primary lowland forests.  Similarly, foliar δ15N values bracket 0 parts per mil, again well below what is commonly seen in other lowland regions.  Prior data showing exceptionally high rates of litterfall, litter decomposition and soil respiration in this site suggest that high rainfall does not create a redox-induced slowdown of N turnover; rather, high internal demand may contribute to the low N₂O and stream export values.  As well, the low N₂O values do not necessarily mean low denitrification losses; instead, the high rainfall and high soluble C availability in this (and other) wet forests may drive substantial denitrification losses as N₂, which could help maintain chronic N-limitation.  Taken as a whole, these results challenge the assumption that N losses are high from all lowland forests on P-poor soils, highlighting the need to better understand the wettest portions of this globally important biome.