Ott , Neil

Decades of acidic deposition have depleted base cation pools in soil over large parts of eastern north America, including the Muskoka-Haliburton region of central Ontario. This region has also experienced a shift in forest species composition over the past 200 years, favouring sugar maple (Acer saccharum Marsh.) at the expense of species such as white pine (Pinus strobus L.) and eastern hemlock (Tsuga canadensis (L.) Carr.). This shift in species composition may have changed soil chemistry over time due to differences in nutrient and metal inputs in litterfall. An analysis of litterfall and soil chemistry was conducted for five tree species commonly found across central Ontario. Stands were established in the Haliburton Forest & Wild Life Reserve and were dominated by one of balsam fir (Abies balsamea (L.) Mill.), eastern hemlock, white pine, sugar maple, or yellow birch (Betula alleghaniensis Britt.). Analysis of mineral soil oxides suggested that these stands were established on similar parent material. Deciduous dominated stands (maple and birch) had greater litterfall mass compared with conifer stands (fir, hemlock, and pine), generally leading to greater macronutrient inputs to the soil. Elemental cycling through the organic horizons was more rapid in deciduous stands, with base cations having the shortest residence times. This suggests that a change from greater conifer dominance to mixed hardwood forests may lead to more rapid elemental cycling and alter the distribution of elements in soil. Forests in the region are typically mixed and the resulting differences in soil chemistry may influence model predictions of soil recovery from acidification. Laboratory leaching tests indicated that both stand type and the acidity of simulated rainwater inputs influenced soil solution chemistry, with deciduous stands generally having a greater buffering capacity than sites dominated by coniferous species. Changes in soil chemistry were examined for each stand type using the Very Simple Dynamic (VSD) biogeochemical model. Simulations showed that soil base saturation began to increase following lows reached around the year 2000, and similar patterns were observed for all stands. When sulphur (S) and nitrogen (N) deposition were held constant at present rates, soil base saturation recovery (toward pre-1900 levels) was marginal by 2100. With additional deposition reductions, further increases in base saturation were minor at all sites. In conjunction with additional deposition reductions, the elimination of future forest harvesting allowed for the greatest potential for recovery in all stands. Overall, these results suggest that changes in forest cover may influence soil chemistry over time, most notably in the organic soil horizons. However, forecasted recovery from acidification is expected to follow similar patterns among stands, since differences in soil chemistry were less significant in the mineral soil horizons which compose a greater proportion of the soil profile.

Author Keywords: base cation decline, forest harvesting, litterfall, mineral weathering, soil acidification, VSD model