Acid-sensitivity in the western boreal forest of Canada, and the importance of atmospheric base cation deposition for preventing soil acidification

Colin Whitfield1, Shaun Watmough2
1 Geography, University of Saskatchewan
2 Environmental and Resource Studies, Trent University

O 1.7 in Long term trends in the functioning of ecosystems

14.07.2014, 15:55-16:15, H18

Industrial activities associated with extraction of oil sands in western regions of Canada’s boreal forest have resulted in greatly elevated emissions of sulphur (S) and nitrogen (N), prompting concerns over the potential for acidification.  Assessments of terrestrial and aquatic systems downwind of these emissions sources indicate low critical loads of acidity.  Sandy, quartz dominated soils are common in the region and have very low base cation weathering rates; the median rate of release of base cations through weathering estimated using PROFILE at 63 forest plots was 17 mmolc m–2 yr–1.  Throughfall deposition of S and N was as high as 360 mmolc m–2 yr–1 within 20 km of the main industrial centre, although deposition declined rapidly with distance from the industrial activities.  Base cation deposition however, mostly exceeded the combined inputs of S and N in throughfall, particularly during the summer months.  The potential for soil acidification at a forested monitoring site close (<3 km) to the largest mine was assessed at the plot-scale using the dynamic acidification model, MAGIC (Model of Acidification of Groundwater in Catchments).  Despite very low base cation weathering rates (~6 mmolc m–2 yr–1) and high (250 ~mmolc m–2 yr–1) acid (S + N) deposition at this site, soil base saturation and soil solution pH and molar Ca:Al ratio were predicted to increase in the future assuming acid and base cation deposition constant at current rates.  This work shows that despite extremely low soil base cation weathering rates in the region, the risk of soil acidification is mitigated to a large extent by high base cation deposition.  In contrast to S emissions, base cation deposition is derived from fugitive dust sources associated with extensive land clearing activities and is poorly quantified for regional modelling studies.

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last modified 2014-06-19