Critical loads and H⁺ budgets of forest soils affected by air pollution from oil sands mining in Alberta, Canada

We investigated the critical load (CL) and exceedance (EX) of sulfur (S) deposition, temporal changes in soil chemistry, and H⁺ budget of soils in plots dominated by Pinus banksiana (jack pine) or Populus tremuloides (trembling aspen, aspen) in two acid-sensitive watersheds to assess the risk of soil acidification by S emissions from oil sands mining in the Athabasca oil sands region (AOSR), Canada. The CLs and EXs were determined by two methods: one was based on bulk deposition and the other based on total deposition (as a sum of bulk deposition and interception deposition). The CLs ranged from 223 to 711 mol_c ha^-1 yr^-1 based on bulk deposition. Those values were similar to that obtained based on total deposition. However, EXs based on bulk deposition were significantly lower (p < 0.001) than those based on total deposition due to the relative increase of SO42- concentrations in interception deposition, indicating that EXs based on bulk deposition only could underestimate the risk of soil acidification in the AOSR. The S deposition did not exceed CLs in the long-term for both methods. The pH in the forest floor increased and available SO42- (as the sum of soluble and adsorbed SO42-) in the forest floor and surface mineral soils increased in both jack pine and aspen stands between 2005 and 2010. The H⁺ budget ranged from -289 to -130 mol_c ha^-1 yr^-1 in jack pine stands and from -510 to -371 mol_c ha^-1 yr^-1 in aspen stands. Our results suggest that 1) soils in the studied forest stands have recovered from acidification based on the increasing soil pH over time and the negative H⁺ budget, and 2) the risk of soil acidification should be assessed by CL and EX calculated based on total deposition.