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Pathways of dissolved nutrient flux in a deforested Amazonian watershed.
Markewitz, Daniel *,1, Figueiredo, Ricardo2, Schuler, Azeneth3, Moraes, Jorge 3, Dunne, Thomas4, Davidson, Eric5, 1 DB Warnell School of Forest Reources, Athens, GA2 EMBRAPA Amazionia Oriental, Belem, Para, Brazil3 CENA, Piracicaba, Sao Paulo, Brazil4 Bern School of Environmental Science and Management, Santa Barbara, CA, USA5 The Woods Hole Research Center, Woods Hole, MA, USA
ABSTRACT- Over the last four decades many watersheds in eastern Amazonia have been largely converted from primary forests to degraded or managed pasture lands. Some of our early work in streams draining one watershed in this region demonstrated increased cation concentrations in solution during periods of high stream flow, a result contrary to patterns observed in other tropical and temperate streams. We hypothesized that solution losses of nutrients in this largely deforested watershed were derived from surface runoff. To further investigate these pathways of nutrient loss we have instrumented two small watersheds, one in mature forest and another in managed pasture, with surface collectors, soil lysimeters, and groundwater wells. Using a geochemical mixing model approach in which stream water concentrations are approximated based on proportional mixing of surface water, soil lateral flow waters, and groundwater, as well as ancillary stream and well water data from the larger watershed, we infer important pathways of nutrient flow. In the case of Ca, for example, volume weighted mean surface solution concentrations are 45 M while those of groundwater are 14 M, two end points around a stream water concentration of 20 M. Based on measurements for solution cation concentrations we conclude that surface and lateral flow pathways contribute important inputs to stream waters in these deforested tropical watersheds.
Key words: land use change, solution nutrient flux, tropical watershed, stream water chemistry