Injuries to scots pine mycorrhizas and chemical gradients in forest soil in the environment of a pulp mill in central Finland

The occurrence and condition of Scots pine mycorrhizas were studied at different distances from a pulp mill in Central Finland. The chemical analyses of the soil humus layer in the vicinity of the mill revealed increased levels of ammonium-nitrogen, sulphur and calcium but unaltered concentrations of phosphorus and magnesium. Higher nitrate levels and nitrification were clearly detected at some sites which had recently been limed. Significant decreases in root ramification index and number of living mycorrhizas were found in a zone 0–0.6 km zone surrounding the factory but these parameters increased with increasing distance. Within a 2 km zone around the mill there were abundantCenococcum geophilum andPaxillus involutus-type mycorrhizas while lowered frequencies of several other mycorrhizal types were detected. An ultrastructural study revealed changes in several types of mycorrhizas, the clearest of which were increased tannin deposition in cortical cells, intracellular growth of hyphae in cortical cells and the appearance of electron dense accumulations in the vacuoles of the fungal cells. The ultrastructural changes observed were distributed at least to a distance of 3 km from the mill and occurred in the roots of trees that had only a slight loss of needle mass. Nitrogen deposition is suspected to be the primary cause of root decline but atmospheric S0₂ through the tree crown is also likely to be a contributing factor.