Deposition of Sulphur and its Influence on Soils in the AOSERP Study Area

In the summers of 1975, 1976, and 1977, several field investigations were carried out to determine the amounts of sulphur deposited in rain, in rain washing off trees, and by direct absorption of SO₂ by soil. The impact of this sulphur deposition on soils in the AOSERP study area was also determined in both field and laboratory experiments. Rain collected at several sites in 1977 was acidic, with some monthly rain samples having pH values below 5.0. The sulphur content of rain was low and there was a gradient of decreasing SO₄ ^2--S deposition in rain with increasing distance from the emission source. Scarcely, any of the sulphur in rain occurred in an acidic form. When instead the rain dripped through jack pine or trembling aspen trees (throughfall and stemflow) more sulphur was deposited. The effect was greater at sites near the emission source as compared to remote sites. Total amount of sulphur in net precipitation (throughfall and stemflow) beneath jack pine and trembling aspen was greater than that in rain. For both jack pine and trembling aspen larger quantities of the cations K⁺, Ca²⁺, and Mg²⁺ were removed from the canopies at sites exposed to SO₂ compared to control sites. Both bare soils and soils with intact lichen cover were found to absorb SO₂ directly from the air. This absorption process amounted to the most important mechanism for removal of sulphur from the atmosphere in the AOSERP study area, particularly in areas close to the emission source. The absorption of SO₂ by soils was also shown to result in increased acidity of the top layer (0 to 1 cm) of some soils. Field experiments showed that sulphate sulphur (applied as K₂SO₄) was mobile in forest soils of the AOSERP study area. Applied sulphuric acid was also found to move quite rapidly through the soils and cause an increase in the acidity of the soil as it moved downward. Ground limestone was an effective method of counteracting the effects of the applied sulphuric acid on soil acidity.