Sources of weathering-derived solutes in two granitic catchments with contrasting forest growth

Chemical weathering is one of the controlling factors for landform evolution. Here, we present deductions about the location and rate of chemical weathering within catchments based on chemical observations of soil water, groundwater, springs. and streams in two granitic catchments with contrasting forest growth conditions (forested and non-vegetated) and soil development. The two catchments are located in the Tanakami Mountains of central Japan, and share the same climate. In both catchments, SiO2 and Na+ are the dominant weathering-derived solutes in the first-order stream. Using hydrological evidence, the contribution to the SiO2 and Na+ fluxes from soil and bedrock were partitioned. Despite the difference in mean soil depth, net fluxes of SiO2 and Na+ from the soil profile do not differ much between the two catchments, suggesting low levels of easily weatherable minerals in the forest soil. Net SiO2 and Na+ fluxes from bedrock are about three times greater in the forested catchment than in the bare catchment, and these differences contribute largely to the difference in net fluxes from the catchments. Net fluxes of SiO2 and Na+ from the forested catchment are about twice those from the bare catchment. Part of this increase may be related to the greater proton production in forested catchment. Consequently, about 60 to 70% of the net SiO2 and Na+ fluxes from the non-vegetated catchment, and 70 to 90% from the forested catchment, are derived from bedrock, suggesting that the contribution of bedrock to the export of these weathering products increased with forest growth and soil formation. Copyright (C) 2004 John Wiley Sons, Ltd.