A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Control of stream water cations by surface soil processes and land use effectson the exchange of nutrients between terrestrial and aquatic ecosystems in theEastern AmazoniaDaniel MarkewitzThe University of GeorgiaRicardo de O. FigueiredoInstituto de Pesquisa Ambiental da AmazôniaEric A. DavidsonThe Woods Hole Research CenterAlex V. Krusche, Reynaldo L. Victoria, Jorge M. Moraes, and Azeneth E. SchulerCentro de Energia Nuclear na Agricultura – USPThomas DunneUniversity of Califorina, Santa BarbaraAddress of corresponding author:Daniel B. Warnell School of Forest ResourcesThe University of GeorgiaAthens, GA 30605 - USAEmail: DMARKE@smokey.forestry.uga.eduWhile land use change is known to affect cycling of carbon and plant nutrients interrestrial ecosystems, the effect on small streams draining altered landscapes is less wellknown. We present results from a 10,000-hectare watershed on highly weathered soil inthe Eastern Amazon where a positive correlation between stream water solute concentrationand discharge was observed. Base flow stream water had low concentrations of Ca +2 , Mg +2 ,and K + , because the primary minerals had already been extensively weathered in theseHaplustox. During the wet season, however, these same cations were leached from surfacecharged exchange sites in upper soil horizons and were transported to the stream withbiogenically derived bicarbonate. This near surface process, as opposed to deep soilmineral weathering, was entirely responsible for the generation of streamwater alkalinity.This observed pattern in stream chemistry is contrary to the seasonal patterns widelyrecognized in temperate ecosystems with less strongly weathered soils. Both forest andpasture surface soils of this tropical landscape contain exchangeable cations and bothproduce large amounts of CO 2 and HCO 3 - during the wet season. Enrichment of the cattlepasture soils with cations from forest clearing and burning twenty years previously may befurther enhancing wet season leaching of cations. To elucidate more fully this exchange ofnutrients from terrestrial to aquatic systems, we are measuring and modeling the flowpathsof soil water and nutrients along forest and pasture hillslopes of this watershed.