CALCULATING CRITICAL S AND N LOADS AND CURRENT EXCEEDANCES FOR UPLANDFORESTS IN SOUTHERN ONTARIO, CANADA

A steady-state mass-balance model was used to calculate critical loads of S and N deposition for maintaining acceptable long-term acidity le vels 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 cat ions 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 gro wing woody biomass of the forest stands; estimates of soil weathering; and mean annual air temperature, precipitation, and evapotranspiratio n. From this, regional isopleth maps were generated to depict the foll owing: (1) current deposition patterns; (2) critical acidification loa ds and their current exceedances (or nonexceedances) for two acidifica tion effects criteria for soil solutions, namely (i) acceptable Al con centrations ([Al](leach,crit) and (ii) acceptable Al to base cation co ncentration ratios ([Al]/[BC](leach,crit)); (3) critical N-eutrophicat ion 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,crit)) set at 0.15 equiv./equiv. This sensitivity to acid pr ecipitation is, as calculated, primarily due to shallow and weathering -resistant soils and soil parent materials (mostly granitic). The midd le portion of the study area is calculated to receive N slightly in ex cess 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 neutraliz ed by the soils and bedrock of this region (predominantly calcareous).