PARENT SESSION
Oral Session #21: Nutrient cycling: Nitrogen.
Presiding: S. Hart
Monday, August 5. 1:00 PM to 3:45 PM. Grand Ballroom Central, Radisson.

Nitrogen stocks and fluxes in riparian and upland montane forest of the Andean Amazon Basin.

McClain, Michael^*,1, Brandes, Jay², Llerena, Carlos³, Reynel, Carlos³, ¹ Department of Environmental Studies, Miami, FL² Marine Science Institute, Port Aransas, TX³ Facultad de Ciencias Forestales, Lima, Peru

ABSTRACT- Montane forests (700-3500 masl) cover approximately 350,000 km² of the western Amazon Basin, blanketing the eastern slopes of the Andes and separating high elevation puna and paramo ecosystems from expansive lowland rain forests and savannas. Montane forests occur where intense rain falls on steep topography, producing a highly dynamic system of frequent land slides and secondary succession. We have initiated a study of elemental cycling in these forests, with special attention devoted to the role of riparian forests in regulating fluxes of nitrogen from upland forests to adjacent river systems. Our study is focused in two small catchments at approximately 2500 masl in the central Peruvian Amazon. There we monitor hydrological (precipitation, troughfall, stemflow, infiltration, ground water, and stream flows), biological (litter fall), and gas (N₂O) fluxes. We have also investigated floristic composition, above and below ground biomass, and soil properties. Our initial results describe a system characterized by modest biomass but relatively high rates of internal elemental cycling. Aboveground biomass is on the order of 65 Mg ha^-1, which is less than a third of the biomass in mature lowland Amazon forest; annual litterfall, however, is comparable to lowland values at 6.7 T ha^-1 yr^-1 (SD = 2.2). Elemental fluxes in litterfall (N, P, K, Ca, Mg) are also comparable with lowland values. Concentrations of organic N in throughfall are roughly twice those of precipitation, averaging 20 M. In spite of high rates of internal nitrogen cycling, N lost from the system in streamflow is exceedingly low. Streamwater concentrations of inorganic and organic nitrogen are generally below 2 M and 5 M, respectively. Although our emerging nitrogen budget for this forest system must take into account inputs from fixation and outputs through denitirfication and mass wasting, it appears that these forests are effectively retaining nitrogen.

KEY WORDS: montane forest biomass, nutrient retention, nutrient runoff losses, Amazon basin