Nitrogen Cycling in Wet Tropical Forests

It is commonly assumed that nitrogen (N) cycles in excess of biological demand in many lowland tropical forests. Evidence supporting this generalization includes: high foliar N concentrations, elevated foliar and soil δ15N values, large soil NO3- pools, elevated NO3-: NH4+ ratios, and high rates of N loss in both soluble and gaseous forms. Unfortunately, the vast majority of these data come from forests occupying relatively dry portions of the biome (i.e., forests receiving < 2500 mm annual precipitation), with very few from wetter lowland sites. However, recent analyses from a wet tropical forest site (i.e., > 5000 mm y-1) on the Osa Peninsula in SW Costa Rica suggest that the paradigm of "N excess" may not hold across the full spectrum of precipitation that characterizes the tropical forest biome. Most previous studies in wet tropical forests have focused on montane ecosystems where low temperatures and heavy precipitation create low soil redox conditions, which slow litter decomposition and N mineralization. In contrast, soils in the lowland forests of the Osa Peninsula are well-drained and remain aerated despite high rainfall, which accelerates decomposition and nutrient regeneration processes. Specifically, results from a 15N pool dilution experiment show high rates of gross N mineralization (9.5 ± 1.8 μg N g soil-1 d-1, mean ± 1 SE) and nitrification (2.6 ± 0.4 μg N g soil-1 d-1). Though NH4 and NO3 supply is high, net mineralization accounts for only 2% of this flux, suggesting limited accumulation of inorganic N in excess of biological demand. Additionally, we observed relatively high rates of dissimilatory nitrate reduction to ammonium (DNRA, 0.5 ± 0.1 μg N g soil-1 d-1), a mechanism that conserves inorganic N within the ecosystem. These patterns compliment other data that indicate relatively conservative N cycling in these very wet forests: relatively low δ15N values in foliage (bracketing 0 %) and soils (+4 %); small inorganic N pools dominated by NH4+; and, low losses to nitrous oxide (< 1 kg N2O-N ha-1 y-1) and dissolved N (0.4 kg NO3 -N ha-1 y-1), which is dominated by organic forms of N. Despite the apparent efficiency of biological N retention of N, intense rainfall does drives a large export of particulate organic N in the late wet season (5 kg PON ha-1 y-1). Collectively, these data support a more nuanced conceptual model of how N cycles in tropical lowlands: forests receiving > 2500 mm y-1 may exhibit comparatively high biological N demand that reduces potential N losses to atmospheric and aquatic realms.