SO2‐dependent cation competition and compartmentalization in Norway spruce needles

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Abstract
Ion contents in needles from Norway spruce trees [Picea abies (L.) Karst.] growing in Würzburg and in the SO2‐polluted Erzgebirge mountains were analysed to quantify cations which accumulate together with sulphate. In Würzburg there was a positive correlation of potassium (0.680 ± 0.300 Eq Eq−1 SO4−2), magnesium (0.415 ± 0.111 Eq Eq−1 SO4−2) and zinc (0.059 ± 0.006 Eq Eq−1 SO42−). In the Erzgebirge, potassium was also the stoichiometrically most important cation (0–887 ± 0–180 Eq K+ Eq−1 SO42−). All other correlations examined were weak or statistically non‐significant. At both sites the calcium content of spruce needles did not depend on the sulphate content. The lack of a role for Ca2+ in neutralizing sulphate is a consequence of the presence of free oxalic acid in needles. Soluble oxalic acid precipitates Ca2+, which thereby becomes unavailable as a counterion for SO42−. The activity coefficients of Ca2+ and oxalate2−, and the solubility product of Ca‐oxalate, were determined from in vivo data. It is concluded that the chronic accumulation of atmospheric sulphate in spruce needle vacuoles depletes available potassium and thereby strongly interferes with spruce growth and canopy turnover. This leads to impaired spruce vitality, even at sites where acute SO2 disease symptoms are absent.