Inorganic and organic nitrogen losses from a montane tropical forest in Las Alturas, Costa Rica

Background/Question/Methods

The relative nitrogen (N) richness of lowland tropical forests is evidenced in their hydrologic losses, which are dominated by inorganic N forms (DIN: nitrate (NO₃^-) and ammonium (NH₄⁺)).  In contrast, unpolluted temperate forests, which are thought to be N limited, show losses dominated by dissolved organic nitrogen (DON).  Although montane tropical forests are thought to be N limited as well, the hydrologic losses from these systems are relatively unexplored. In this context, we asked whether N losses were dominated by DIN or DON in a montane tropical forest in southern Costa Rica.  We measured concentrations of dissolved N in stream water, soil solution, and precipitation in a montane forest watershed (>1300 m alt., <18°C m.a.t.) for a full water year in the Talamanca Range in southern Costa Rica.  We monitored discharge and stream chemistry, analyzing DIN and DON at different times, and flow regimes, throughout the year.

Results/Conclusions

We predicted that, similar to N limited, unpolluted temperate forests, DON export would be a major pathway of N loss from this watershed.  We expected that inorganic N (DIN) losses would be low both in comparison to DON and to DIN losses from lowland tropical forests where N is relatively abundant.  Surprisingly, median NO₃^-, NH₄⁺ (n=50) and DON (n=17) stream water concentrations were 8.5, 5.0 and 0.2 umol L^-1, and yearly losses were 3.7, 1.9, and 0.5 kg ha^-1 yr^-1, respectively.  Although DIN concentrations at this site were 20-30% of those reported for lowland Amazonian sites, yearly NO₃^- and NH₄⁺ losses were much higher.  This high N export likely is the product of high water yields in stream runoff (80% of precipitation) due to the interplay of high precipitation (4 m yr^-1) and steep topography. Soil water from 30 and 100 cm beneath the soil and sampled during the rainy season had concentrations of NO₃^- and NH₄⁺ < 50 and 10 umol L^-1, respectively.  This is considerably lower in nitrate than in lowland Amazonian forests, which suggests these forests are not as N rich.  However, the DIN/DON ratio (11) indicates that this site is not experiencing N limitation that is maintained by slow, but uncontrollable, losses of DON. Furthermore, these data suggest that, contrary to expectation, this montane forest site may be relatively N rich.