SOLUTE EXPORT FROM FORESTED AND PARTIALLY DEFORESTED CATCHMENTS IN THE CENTRAL AMAZON

The hydrochemical responses to slash-and-bum agriculture in a small ra inforest catchment of the central Amazon were investigated for one yea r. Disturbances in the partially deforested catchment began in 1987, a nd during the study a 2-ha plot was cut (July 1989) and burned (Octobe r 1989) in preparation for the cultivation of manioc; the partially de forested catchment was approximately 80% deforested at the time of thi s study. Solute fluxes exported by base flow were estimated from solut e concentrations of stream water measured at least once per week. Solu te fluxes for storm flow were estimated by measuring streamwater conce ntrations during two storms. Baseflow runoff represented about 94% of the water outflow from the study basin acid was the dominant pathway o f 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 s imilar 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 los ses from the partially deforested catchment were higher than those mea sured when the catchment was undisturbed in 1984 by factors of 1.4, 1. 8, and 2.1 for total inorganic nitrogen (TIN), total dissolved nitroge n (TDN), and total nitrogen (TN); and by factors of 4.0, 6.6, and 7.9 for soluble reactive phosphate (PO43-), total dissolved phosphorus (TD P), and total phosphorus (TP), respectively. These data show that defo restation and colonization in upland catchments of the central Amazon alter the hydrochemical balance of streams by decreasing ET, thereby i ncreasing discharge and solute export.