Critical loads for alkalization in terrestrial ecosystems

Critical loads are a risk assessment approach that has supported large decreases in atmospheric acidic deposition globally. In Canada, SO_x emissions fell by approximately 70 % between 1990 and 2021, whereas total particulate matter (TPM) emissions increased by about 40 %, mostly after 2010. Base cations are a major component of TPM, and critical load models consider base cation deposition as beneficial to ecosystems insomuch as it reduces the risk of acidification. However, close to point sources, high levels of alkaline dust deposition have altered soil chemistry and caused an undesirable shift in ecosystem state; something that critical loads are designed to prevent. In this study, the simple mass balance model (SMB) was modified with the objective of preventing base cation accumulation in soil above an acceptable threshold. The concept was applied to a forested site close to large emission sources of sulphur, nitrogen, and base cations in the Oil Sands region of Alberta, Canada. At this site, base cation leaching measured at 25 cm was approximately three times higher than estimated background leaching and exceeded combined SO₄ + NO₃ leaching. The critical load for alkalization was exceeded under each scenario considered in this study, although the exceedance was marginal if all N in current deposition was assumed to leach from soil. While this framework can easily be applied to regional and national critical load efforts, the main uncertainties of the proposed approach include base cation deposition estimates, assumptions regarding the behavior of N in soil, the selection of an appropriate Alk_le(crit) and the long-term immobilization of deposited base cations in soil.