Estimates of critical loads and exceedances of acidity and nutrient nitrogen for mineral soils in Canada for 2014–2016 average annual sulphur and nitrogen atmospheric deposition

Abstract. The steady-state Simple Mass Balance model was applied to natural and semi-natural terrestrial ecosystems across Canada to produce nation-wide critical loads of acidity (maximum sulphur, CL_maxS; maximum nitrogen, CL_maxN; minimum nitrogen, CL_minN) and nutrient nitrogen (CL_nutN) at 250 m resolution. Parameterization of the model for Canadian ecosystems was considered with attention to the selection of the chemical criterion for damage at a site-specific resolution, with comparison between protection levels of 5 % and 20 % growth reduction (approximating commonly chosen base-cation-to-aluminum ratios of 1 and 10 respectively). Other parameters explored include modelled base cation deposition and site-specific nutrient and base cation uptake estimates based on North American tree chemistry data and tree species and biomass maps. Soil critical loads of nutrient nitrogen were also mapped using the Simple Mass Balance model. Critical loads of acidity were estimated to be low (e.g., below 500 eq^-1 ha yr^-1) for much of the country, particularly above 60° N latitude where base cation weathering rates are low due to cold annual average temperature. Exceedances were mapped relative to annual sulphur and nitrogen deposition averaged over 2014–2016. Results show that under a conservative estimate (5 % protection level), 10 % of Canada’s Protected and Conserved Areas in the study area experienced exceedance of some level of soil critical load of acidity while 70 % experienced exceedance of soil critical load of nutrient nitrogen.