Organic nitrogen loss dominates nitrogen export from a wet lowland tropical forest watershed

Background/Question/Methods

Many lowland tropical forests are thought to cycle N in relative abundance, and as such, have comparatively large soil nitrate pools and thus a high potential for inorganic N losses from the ecosystem.  In some tropical sites, significant anion exchange capacity in deep soils can “trap” nitrate prior to stream export, but many of these sites still display high net nitrification, high nitrous oxide fluxes, and periods of relatively high nitrate export. These studies, however, have generally been conducted in drier portions of the biome, receiving < 2500 mm precipitation y^-1 and underlain with highly weathered soils with notably low P availability. Yet, up to 20% of tropical forests reside on moderately fertile soils and receive upwards of 2.5 meters of rain per annum. In such forests, there is an emerging view that N does not cycle in excess of biological demand. This raises the question: does nitrate dominate hydrologic N loss from lowland forests in wet tropical forests? To address this issue, we examined N losses on a weekly basis for one year from a tropical watershed located on the Osa Peninsula, Costa Rica, which receives 5000 mm of rainfall y^-1.

Results/Conclusions

We estimate total hydrologic N losses from this watershed to be ~8 kg N ha^-1 y^-1. Organic forms of N comprise nearly all of this flux: dissolved organic N (DON) was 2.8 kg N ha^-1 y^-1, and high rainfall in the late wet season drove an erosional flux of particulate organic N (PON) of ~5 kg N ha^-1 y^-1, which constituted the largest flux of N from the watershed.  DON tracked dissolved organic carbon concentration throughout the year (r² = 0.78). Stream nitrate and ammonium concentrations were similar and remained low across seasons, averaging 10 mg N/L. A broader survey of surface waters (n = 56) across the region suggests these patterns are broadly distributed. The patterns of dissolved N loss mimic those observed in unpolluted temperate forests, where most N loss occurs as a leak of DON independent of biological demand. The combination of significant DON and PON losses could impose a significant constraint on N accumulation in wet tropical forests, especially those with high erosion rates.  The high organic N losses also provide one mechanistic explanation for data suggesting that wetter tropical forests do not cycle N in excess of biological demand.