『Abstract
　Critical loads are strongly dependent on the rate of release of base cations from the soil matrix. This study compares five commonly used methods for estimating weathering rates at five acid-sensitive catchments across Nova Scotia, Canada. Three of the methods (Zr Depletion, Clay content, and the PROFILE model) are based on soil profiles and consider only the rooting zone, whereas the two remaining methods (the soil acidification model MAGIC and catchment Mass Balance Deficit) are catchment-based, and account for contributions from all soils within a catchment. Each weathering estimate method resulted in similar values among the five catchments, indicating similar sensitivity to acidic deposition among the study areas. Base cation weathering estimates were very low using the three soil profile-based methods, with rates varying from 3 to 13 mmolc m^-2 a^-1. In contrast, catchment-based methods predicted base cation weathering rates an order of magnitude higher (60 to 155 mmolc m^-2 a^-1), possibly due to spatial heterogeneity of the soil deposits, and contributions from deeper soil (till). Critical load (sulphur and nitrogen) estimates using the profile-based weathering rates indicate that critical loads for forest soils are currently exceeded all catchments by 23 to 61 mmolc m^-2 a^-1. Predicted future reductions in acidic deposition should reduce the magnitude of critical load exceedance, but will not result in the catchments reaching a non-exceeded state.

Keywords: Weathering; Base cations; Acid-sensitive soil; Critical load; Nova Scotia』

1. Introduction
2. Methods
　2.1. Study area
　2.2. Sample collection/preparation
　2.3. Physical parameters
　2.4. Chemical parameters
　　2.4.1. pH measurement
　　2.4.2. Base cation extraction and analysis
　　2.4.3. Weathering estimates
　　2.4.4. Zirconium Depletion method
　　2.4.5. Clay Content method
　　2.4.6. The PROFILE model
　　2.4.7. The MAGIC model
　　2.4.8. Mass balance deficit
　　2.4.9. Critical loads
3. Results and discussion
　3.1. Soil properties
　3.2. Weathering estimates
　3.3. Critical loads
　3.4. Uncertainty
4. Conclusions
Acknowledgements
References

• �@Zirconium Depletion
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• �AClay Content
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• �BPROFILE model
  Warfvinge,P. and Scerdrup,H.(1992): Calculating critical loads of acid deposition with PROFILE - a steady-state soil chemistry model. Water, Air, and Soil Pollution, 63, 119-143.
• �CMAGIC model
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• �DMass Balance Deficit
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• 臨界負荷：Simple Mass Balance (SMB) model
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