Solute export from forested and partially deforested catchments in the central Amazon

The hydrochemical responses to slash-and-bum agriculture in a small rainforest catchment of the central Amazon were investigated for one year. Disturbances in the partially deforested catchment began in 1987, and during the study a 2-ha plot was cut (July 1989) and burned (October 1989) in preparation for the cultivation of manioc; the partially deforested catchment was approximately 80% deforested at the time of this study. Solute fluxes exported by base flow were estimated from solute concentrations of stream water measured at least once per week. Solute fluxes for storm flow were estimated by measuring streamwater concentrations during two storms. Baseflow runoff represented about 94% of the water outflow from the study basin acid was the dominant pathway of solute export. Total rainfall during the study period was 2754 mm of which 2080 mm was exported from the partially deforested catchment as stream runoff. The ratio of surface runoff to annual rainfall for a similar study conducted in the same catchment while completely forested in 1984 was lower than after the catchment was 80% deforested in 1990 (0.57 versus 0.76), while evapotranspiration (ET) was lower by about a factor of two in 1990 compared to 1984. Particulate removal from the partially deforested catchment was 151 kg ha(-1) yr(-1). Nutrient losses from the partially deforested catchment were higher than those measured when the catchment was undisturbed in 1984 by factors of 1.4, 1.8, and 2.1 for total inorganic nitrogen (TIN), total dissolved nitrogen (TDN), and total nitrogen (TN); and by factors of 4.0, 6.6, and 7.9 for soluble reactive phosphate (PO43-), total dissolved phosphorus (TDP), and total phosphorus (TP), respectively. These data show that deforestation and colonization in upland catchments of the central Amazon alter the hydrochemical balance of streams by decreasing ET, thereby increasing discharge and solute export.

Keywords: Amazon, catchments, deforestation, slash-and-burn agriculture, solutes, , water balance, major ionic solutes, nitrogen-fixation, fresh-water, marine ecosystems, , total phosphorus, floodplain lake, basin, dynamics, rain, succession