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- Biogeochemical Consequences of Hydrologic Conditions in a Tropical Montane Rain Forest in Ecuador (2004)
- Tropical montane forests regulate the hydrological cycles of high-elevation areas being an important service for the local population. The objectives of my work were (1) to determine the major hydrological flow paths, (2) to quantify concentrations of dissolved organic C and organic and inorganic forms of N, P, and S, and (3) to identify the major controls of the dissolved C, N, P, and S dynamics in a tropical montane forest in south Ecuador. Three 8-13 ha large microcatchments (MCs) under montane forest at an altitude of 1900-2200 m above sea level were selected. Scientific equipment was installed on five transects, about 20 m long with an altitude range of 10 m. Three unforested sites near the microcatchments were used for rainfall gauging. Within the three monitored years between May 1999 and April 2002, on average 2448 mm of precipitation fell on the study area. The delta 18O of rainfall shows large variations (-12.6 to +2.1 per mil) related to different air-masses. The delta 18O values of throughfall and lateral flow are similar to those in rainfall. Variations in delta 18O values of the soil solution and the stream water are smaller (-9.1 to -3.0 and -5.8 to -8.7 per mil) than those of rainfall, throughfall, and lateral flow. The delta 18O values in stream water increased immediately after an intense rainstorm event to isotope values similar to those of rainfall and lateral flow. This indicated that during elevated rainfall the water flows rapidly in the organic layers to the stream channel paralleling the surface. During this time the water content of the organic layer was higher than of the upper mineral soil. Results from an isotope two-component hydrograph separation for the three microcatchments showed that new rain water ('event water') contributed 44-81 % to the total stormflow runoff during one selected peak discharge. The canopy was a small and the organic layer the major source of DON, DOP, and DOS, which were almost completely retained in the mineral soil. The organically bound forms contributed, on average in all solutions, 54 %, 78 %, and 59 % to the total concentrations of N, P, and S, respectively. The contribution of organically bound forms to total nutrient concentrations was element-specific and differed among the ecosystem fluxes. The portions of DON (23-81 %) were similar and those of DOP (5-100 %) highly variable in all solutions. There were high DOS portions in the aboveground fluxes (51-100 %) and lower portions in the mineral soil (8-47 %). The pH was positively correlated with the DOC and organic N, P, and S concentrations at the scale of pedons. Increasing DOC concentrations in stream water following rainstorms indicated that rainstorms were an important control of the dissolved organic matter dynamics on a regional scale. During the passage of the water through the forest, dissolved inorganic N (DIN) and DIP concentrations increased between the canopy and the mineral soil and strongly decreased in the mineral soil. In contrast, DIS concentrations were highest in the mineral soil and stream water. Thus, the organic layer was the major source for DIN, the canopy for DIP and the mineral soil for DIS. The mineral soil was a sink for DIN and particularly for DIP. Soil drying and rewetting promoted the release of inorganic N. High discharge levels following heavy rainstorm events were associated with pulses of NO3-N and partly also NH4-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively related to runoff conditions. The DIP and DIS concentrations in throughfall and stemflow were negatively correlated to the respective water fluxes, whereas DIS concentrations in the mineral soil solutions of both studied soil depths were positively related to the rainfall volume. DIN and DIP concentrations and fluxes tended to be positively related to the pH of the organic layer. My results demonstrate that nutrient dynamics were mainly controlled by hydrological conditions in the studied steep forested catchments. Furthermore, small ecosystem inputs and outputs indicated tight cycles of dissolved N, P and S in the study forest, except for DIS.