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Water and solute fluxes in small Amazonian forest and pasture watersheds

Christopher Neill, MBL, cneill@mbl.edu (Presenting)
Sonja Germer, U. Potsdam, sgermer@rz.uni-potsdam.de
Alex Krusche, CENA/USP, alex@cena.usp.br
Joaquin Chaves, MBL, jchaves@mbl.edu
Helmut Elsenbeer, U. Potsdam, helsenb@rz.uni-potsdam.de
Sergio Gouveia Neto, CENA/USP, sneto@cena.usp.br
Adriana Castellanos Bonilla, CENA/USP, adrianacaste@gmail.com

Small watersheds provide discrete landscape units in which to address the central LBA question of how changes to land cover influence the function of Amazonian tropical forest ecosystems. We compared water and solute fluxes in small (~1 ha) forest and pasture watersheds at Rancho Grande in Rond˘nia. Streamflow accounted for 1% of rainfall in forest but 18% in pasture, and quickflow dominated stream flows in both forest and pasture. Concentrations of Ca2+, Mg2+, NH4+, NO3- and SO42- in rain and throughfall declined several-fold from the dry to wet seasons and within individual rain events, indicating that dry deposition and aerosol rainout are important regional contributors of solute inputs. In forest, annual input of Ca2+, NH4+, NO3-, SO42- and Cl- exceeded export, export of Na+ exceeded inputs, and input and export of Mg2+ and K+ were balanced. In forest, groundwater was the predominant pathway of export of all solutes except NO3-. More than half of NO3- export from forest occurred in stream quickflows during a small number of rain events. In pasture, annual input of Ca2+, NH4+, NO3- and SO42- exceeded exports, export of Na, K+ and Mg2+ exceeded inputs and inputs and outputs of Cl- were balanced. In pasture, solute export occurred predominantly in quickflow. Export of Cl- and NO3- changed most markedly between land covers. Export of Cl- was 5x greater and K+ export was 2x greater from pasture than forest. Export of NO3- from pasture was 1/3 that of forest. Changes in NO3- concentration between soil solution and groundwater (from 130 to 1 μM) led to losses of 17 kg N ha-1y-1 in forest, more than 3x greater than bulk inorganic N deposition. In pasture, no flowpaths contained NO3- concentrations that could lead to large solution N losses. Predicting watershed solute balances over wider areas depends on: 1) regional solute inputs generated in fragmented landscapes, 2) changes to watershed hydrology that control water flowpaths, and 3) changes to solute chemistry within flowpaths caused by land cover.

Science Theme:  ND (Nutrient Dynamics)

Session:  1B: Chemistry of Streams and Rivers

Presentation Type:  Oral (view presentation (2531 KB))

Abstract ID: 35

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