『Abstract
　Base cation (BC) concentrations of rain, throughfall, percolation from leaf litter, and soil solution were periodically measured in two forests: Kannondai (red pine stand on volcanic soil) and Yasato (deciduous stands on granitic soil). Calculation of a BC budget gave the rate of BC release from soils; the BCs originated from mineral weathering and cation exchange. Weathering rates under field conditions were estimated from the Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) of water and soil samples. Isotope ratios decreased in the order rain＞throughfall＞percolation＞soil solution. Clay and silt had extremely high isotope ratios; this suggests that the sandy fraction, whose isotope ratio was smaller than that of the soil solution, was the main contributor to mineral weathering. estimated BC weathering rates (kmolc・ha^-1y^-1) were 1.16 for Ca and 0.57 for Mg at Kannondai, and 0.82 for Ca and 0.51 for Mg at Yasato. The unexpected high weathering rate of granitic soil in Yasao was due to the wide coverage of the original parent material by volcanic ash. The contribution of cation exchange derived by subtraction was a little smaller than the weathering rates and was similar to the values estimated from a dynamic model that we developed.

Keywords: mineral weathering; neutralizing capacity; Sr isotope ratio; BC budget; dynamic model』

1. Introduction
2. Materials and methods
　2.1. Field measurement
　2.2. Estimation of the weathering rates and base cation budget
　2.3. Estimation of soil chemical changes with a dynamic model
3. Results and discussion
　3.1. Ion budget estimation from field measurements
　3.2. Weathering rate estimation based on Sr isotope ratios
　3.3. Estimation of changes in exchangeable cations with a dynamic model
4. Conclusions
Acknowledgements
References