The interaction of atmospheric sulphur (S) was investigated within the canopies of two boreal forests in Québec, Canada. The net canopy exchange approach, i.e. the difference between S-SO(4) in throughfall and precipitation, suggests high proportion of dry deposition in winter (up to 53%) as compared to summer (1-9%). However, a 3.5‰ decrease in δ(18)O-SO(4) throughfall in summer compared to incident precipitation points towards a much larger proportion of dry deposition during the warm season. We suggest that a significant fraction of dry deposition (about 1.2 kg ha(-1) yr(-1), representing 30-40% of annual wet S deposition) which contributed to the decreased δ(18)O-SO(4) in throughfall was taken up by the canopy. Overall, these results showed that, contrary to what is commonly considered, S interchanges in the canopy could be important in boreal forests with low absolute atmospheric S depositions.
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