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Nitrogen dynamics in surface and subsurface waters of a montane forest, headwaters of the Amazon, Peru.
Saunders, Thomas*,1, McClain, Michael1, 1 Department of Environmental Studies, Miami, FL, USA
ABSTRACT- We investigated the dynamics of runoff and nitrogen cycling in a pristine first-order catchment in a montane rainforest of the Peruvian Amazon. The study-site is located at 2520 masl in a region characterized by steep hillslopes (∼45 degrees) and over 2,600mm/yr of precipitation. Suction-lysimeters and piezometers were installed along an upland-riparian-stream transect and samples were collected weekly over a 9-month period and analyzed for NO3-, and NH4+. Storm events were sampled for precipitation, throughfall, overland flow, soil-water, groundwater, and stream flow as circumstances permitted. Hydrologic data from a v-notch weir 30m below the site, a weather station within 300 meters distance, and from riparian piezometers demonstrate abrupt storm-induced fluctuations in both hydraulic head and discharge, resulting in a marked pulse in system nitrate concentrations and export through a storm cycle. During baseflow conditions, significant differences (P<0.01) in average NO3- concentrations are found between upland soil-water (27.2±28.3M) and both riparian ground water (4.3±4.3M) and stream water (1.6±2.0M). Ammonium concentrations remained consistent between upland soil-water and riparian groundwater, at 3.6±3.0M and 3.7±4.4M respectively, and drop to 2.7±2.3M in stream water. Preliminary dissolved organic nitrogen data suggest mean values much greater than those reported for total inorganic nitrogen in all compartments. Initial measurements of soil N2O fluxes have been low. Although riparian NH4+ concentrations are generally very low when compared against sites studied in the lowland Amazon and in other tropical catchments, resulting streamwater ammonium concentrations are found to be slightly higher. In contrast, riparian nitrate concentrations in the Amazon headwaters site are generally higher than those found in other tropical riparian zones, with both of these trends partially owing to marked differences in the hydro-geomorphic makeup of the regions studied. Overall patterns of N concentrations within upland/riparian/stream compartments demonstrate that these montane forest catchments are exceptionally efficient in retaining nitrogen.
Key words: amazon, montane, nitrogen, tropical