Factors controlling the acid-neutralizing capacity of Japanese cedar forest watersheds in stands of various ages and topographic characteristics

Acid-neutralizing capacity (ANC) is an important index for streamwater acidification caused by external factors (i.e. chronic acid deposition) and internal factors such as soil acidification due to nitrification. In this study, the influence of forest clear-cutting and subsequent regrowth on internal acidification was investigated in central Japan, where stream pH (near 7.0) and ANC (above 0.1 meq L-1) are high. pH, the concentrations of major cations (Na+, K+, Mg2+ and Ca2+), major anions (NO3-, Cl- and SO42-) and dissolved silica (Si), and ANC were measured in 33 watersheds of various stand ages, during 2002 to 2004. Only NO3- concentration decreased with stand age, whereas pH, ANC, and concentrations of the sum of base cations (BC) and Si were negatively correlated with the minimum elevation of the watershed. The correlation between the BC/Si ratio and minimum elevation suggested that factors contributing to acid neutralization changed at 1100 m above sea level. In watersheds at lower elevations (<= 1100 m), the relatively high contribution of soil water with longer soil contact times should result in higher ANC, and cation exchange reactions should be the dominant process for acid neutralization due to deposition of colluvial soils on the lower slope. In contrast, in higher-elevation watersheds (>= 1100 m), weathered residual soils are thin and the small contribution of deeper groundwater results in lower ANC. These results suggest that the local acid sensitivity is determined by the hydrological and geomorphologic factors generated by steep topography. Copyright (c) 2008 John Wiley & Sons, Ltd.