Estimating mineral surface area and base cation weathering rates of Spodosols under forest in British Columbia, Canada

Abstract

Soil mineral surface area is regarded as a key uncertainty in the estimation of base cation weathering rates, yet is rarely measured. Acidification studies rely heavily on pedotransfer functions (PTFs) that use widely available soil data to estimate mineral surface area. This study examined the relationship between soil properties and mineral surface area in soils (n = 25) from Kitimat, British Columbia, an area that is receiving elevated sulphur (S) deposition due to recent modernization of an aluminum (Al) smelter. Mineral surface area was measured on bulk soil samples using BET (Brunaeur, Emmett and Teller) gas-adsorption. Previously published particle size-based PTFs were a poor predictor of surface area in Kitimat soils (R2 between 0.42 and 0.66). Instead, mineral surface area was best predicted using a regionally-specific PTF (R2 = 0.81), which used particle size as well as the concentration of kaolinite, the most abundant clay mineral in the region. Surface area values estimated using the regionally-specific PTF were applied to the PROFILE model to calculate weathering rates for critical load estimates. These estimates predicted that none of the sites received S deposition in exceedance of their critical load for acidity. However, as surface area is largely related to kaolinite content (a mineral that does not largely contribute to weathering rates), the applicability of using surface area functions for weathering rates is questionable. Further, the texture-based PTF developed for Kitimat did not provide accurate estimates of measured surface area for other soils in Canada, particularly at surface area values exceeding 2.5 m2 g−1.