Effect of ambient SO2 levels on S fractions in Pinus sylvestris foliage growing in the subarctic

The total S and SO₄‐S concentrations of Scots pine (Pinus sylvestris L.) needles growing at a distance of 10–80 km from the metallurgical complex in Monchegorsk in subarctic Russia were measured, and organic S was calculated as the difference between them, in order to study the impact of SO₂ (3–28 μg m^?3 a^?1) on foliar S fractions. Total S concentrations of currentand previous‐year needles collected in April 1991 and July 1992 amounted to 658–2548 ppm, SO₄‐S concentrations to 99–1297 ppm and organic S concentrations to 447–1599 ppm. The SO₄‐S concentrations and the S/N ratios (0.032–0.113 on a gram atom basis) show that S in excess of the growth requirements of trees was entering the needles at all the five sites studied. Foliar K and Ca, but not Mg, increased with increasing SQ₄‐S. It is hypothesized that exposure to SO₂ concentrations of ≥2 μg m^?3 as a growing season mean together with pollution episodes of ～ 100 μg m^?3 h^?1 result in an oxidative stress high enough to damage Scots pine needles, which derive SO₂ effectively from the atmosphere, and that cellular acidification is a secondary effect.