Geographical variation of the acid buffering of vegetated catchments: Factors determining the bicarbonate leaching

Abstract

In order to understand the determining factors of the geographical variation of buffering in the world's vegetated catchments, this study examined the HCO3- leaching of 107 experimental watersheds selected from regions ranging from tropical rain forests to humid continental or subpolar forests including several temperate grassland. With consideration of the soil development and the factors controlling it, the influences of geological, climatological, and topographical conditions on discharge HCO3- concentrations and fluxes are examined. The HCO3- discharge fluxes from catchments were correlated with climatological indexes. The increase of annual mean temperature reflected the increase in fluxes, although this tendency was not that significant in the precipitation-flux relationships. Notable differences in these correlations were not found among three major groups of bedrock types; igneous, metamorphic, and sedimentary groups, with exceptions being noted at the sites having materially HCO3- rich calcite, dolomite and volcanic ash. There were, however exceptionally high flux sites whose fluxes could not be predicted by the empirical model when considering only temperature and precipitation controls. These sites belong to the mountainous part of the Japanese archipelago that is orogenically active and have in common steep hillslopes. Positive correlations were found between relief ratio, which was calculated by dividing elevation difference between highest and lowest points in the catchment by the distance between those two points, and fluxes in igneous and sedimentary groups including these high- HCO3- flux sites. These facts suggest that it is a fundamental requirement to consider the status of the soil forming when discussing the global variations in the buffering of watersheds. The differences in geomorphologic conditions controlling soil development are also just as important as the climatological effects.