REDUCED SOIL P AVAILABILITY IN A SITKA SPRUCE (PICEA-SITCHENSIS (BONG) CARR) PLANTATION INDUCED BY APPLIED ACID-MIST - SIGNIFICANCE IN FOREST DECLINE

The effects of acid-misting applied to tree canopies, on soil P availa bility, pH, effective cation exchange capacity and Ca + Mg:Al ratio, p lus tree P nutrition in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand at Glencorse, Scotland have been investigated. The treated tree s, grouped according to five height classes, had for four years previo usly received acid mist consisting of a mixture of HSO4 and NH4NO3 (pH 2.5) at concentrations of 3.2 mM H+ and 1.6 mM each of NH4+ NO3- and SO42-, with and without soda-glass ballotini, thus simulating cloudwat er composition. The treatments were applied approximately twice a week at the equivalent of 2 mm precipitation per treatment from July to De cember 1990, and throughout the growing season (May-November) in 1991- 92-93. Another group of mixed height class trees, received a double do se of acid mist at each application. Control trees received only the u sual precipitation inputs. The acid mist treatments had been found, re ported elsewhere, to induce a significant (p < 0.05) reduction in stem diameter growth compared to control trees. Acid-mist treatments signi ficantly (p < 0.01) reduced the water-soluble P content in forest litt er as well as the labile inorganic P (resin-extractable) content (p < 0.05) and phosphate concentrations in equilibrium soil solution (p < 0 .01) in the surface soil, compared to soils under control trees. The l abile organic P content (p < 0.01) and P sorption capacity (p < 0.01) of the surface soil however were increased compared to soils under con trol trees. Using a sensitive root bioassay technique, based on the me tabolic uptake of P-32, the acid treatment was shown to have significa ntly (p < 0.001) increased P stress within the trees, relative to cont rol trees. Acid-mist applied to the tree canopies caused significant(p < 0.01) decreases in the soil pH, and marked changes in the cation ex change complex composition, though total effective cation exchange cap acity of the soil showed little variation. However, base saturation wa s 90% in soils under control trees but decreased significantly under a cid-misted trees and very significantly to less than 30% under trees g iven the double acid dose treatment. Consequently, the (Ca + Mg):AI ra tio was reduced markedly from 9.5 +/- 2.9 to 0.4 +/- 0.3. The results from this study therefore support the working hypothesis that acidifyi ng inputs induce P deficiency by slowing the cycling of phosphorus and decreasing P availability within the forest ecosystem. The effects ar e related to the general effects of acidifying inputs on soil pH and b ase status. The effects described are considered to be important facto rs in the forest decline syndrome.