Calculating critical S and N loads and current exceedances for upland forests in southern Ontario, Canada

Abstract

A steady-state mass-balance model was used to calculate critical loads of S and N deposition for maintaining acceptable long-term acidity levels within upland forests in southern Ontario. Preliminary estimates about critical S and N loads were obtained using existing information about soils, vegetation and atmospheric ion deposition from 12 forest sites, all located within provincial parks or conservation areas. The following were considered: wet atmospheric deposition of all major cations and anions; availability for plant uptake of N, Ca, Mg, and K in the rooting space of each soil; nutrient uptake and storage in the growing woody biomass of the forest stands; estimates of soil weathering; and mean annual air temperature, precipitation, and evapotranspiration. From this, regional isopleth maps were generated to depict the following: (1) current deposition patterns; (2) critical acidification loads and their current exceedances (or nonexceedances) for two acidification effects criteria for soil solutions, namely (i) acceptable Al concentrations ([Al]leach,crit) and (ii) acceptable Al to base cation concentration ratios ([Al]/[BC]leach,crit); (3) critical N-eutrophication loads and their current exceedances for acceptable levels of NO3-N concentrations in soil solutions ([NO3-N]leach,crit). It was found that the northern part of the study area (part of the Canadian Shield) is currently subjected to atmospheric S and N deposition in excess of critical loads, with [Al]leach,crit set at 0.02 mequiv./L or [Al]/[BC]leach,crirt set at 0.15 equiv./equiv. This sensitivity to acid precipitation is, as calculated, primarily due to shallow and weathering-resistant soils and soil parent materials (mostly granitic). The middle portion of the study area is calculated to receive N slightly in excess of the N eutrophication limit, when [NO3-N]leach,crit is set at 0.1 mequiv./L. Considerable co-deposition of base cations (Ca, Mg, K) in the middle and southern part of the study area alleviates some of the atmospheric acidification stress. This stress is further neutralized by the soils and bedrock of this region (predominantly calcareous).