Estimating base cation weathering rates and sensitivity to acid deposition for forest soils in Ontario

South-central Ontario receives the highest level of acid deposition in Canada, and a recent assessment has indicated that forest soils are currently in exceedance of the steady-state critical load for acidity. These estimates were obtained using an empirical clay-based weathering model (30--114 eq·ha-l·yr-1), which was shown to underestimate the upper limit of weathering rates based on catchment mass balance (220--1,856 eq·ha-1·yr -1) at 19 forested catchments in south-central Ontario. A new Ontario Weathering Model (OWM) was developed to estimate weathering rates for soils based on simple and widely available soil properties. The model was developed from a linear relationship between soil clay and LOI content and weathering rates estimated with the PROFILE model at 52 study sites. Mineralogical inputs into PROFILE were obtained using a new method for estimating soil mineralogy from total oxide and the Analysis to Mineralogy (A2M) in combination with a PARameter ESTimation (PEST) software. Weathering rates obtained using OWM for the soil rooting zone (252--892 eq·ha-1·yr -1) were more consistent with the catchment mass balance.A subset of 41 sites from the regional critical load study was used in dynamic simulations of the response of soil chemistry to changes in acid deposition. Site-specific weathering estimates obtained using PROFILE and modelled mineralogy were used in simulations in the Model of Acidification of Groundwater In Catchments (MAGIC). Simulations indicate that the exchangeable base cation pool has decreased approximately by 50% and inorganic aluminum (Al3+) in soil solution has doubled since pre-industrial times. Proposed emission reductions are predicted to have little impact on current soil chemistry and about 50% of the study sites will continue to have a Bc/Al ratio below 10. However, the use of a Bc/Al of 10 might not be appropriate as a 50% decline in calcium (Ca2+) and magnesium (Mg2+) levels in soils was predicted at 25% of the study sites where little change in Bc/Al ratios was expected. The decline in Ca2+ and Mg2+ levels below thresholds of base cation limitation in sugar maple foliage suggests further that deposition reductions greater than 25% might be necessary to prevent tree crown damage.Keywords: south-central Ontario, mineral weathering, PROFILE, Clay Model, soil acidification, regional modelling, SSMB, MAGIC.Weathering rates estimated at 116 sites in the region using OWM were applied to critical load calculations with the Steady-State Mass Balance (SSMB). Uncertainty in critical load estimates was addressed by using several established critical chemical criteria combined with modifications in model parameters and deposition sources. The base cation to aluminum (Bc/Al) ratio of 10 predicted the lowest steady-state critical loads and greatest exceedance compared with other chemical limits. Median critical loads estimated using a Bc/Al of 10 and gibbsite solubility of 3,000 m6·eq-2 were 1,977 and 1,426 eq·ha-1·yr -1 assuming 80% or no N retention, respectively. Depending on the acid deposition estimate, up to 39% of the sites were currently exceeded.