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 availability, pH, effective cation exchange capacity and Ca + Mg:Al ratio, plus tree P nutrition in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand at Glencorse, Scotland have been investigated. The treated trees, grouped according to five height classes, had for four years previously 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 cloudwater composition. The treatments were applied approximately twice a week at the equivalent of 2 mm precipitation per treatment from July to December 1990, and throughout the growing season (May-November) in 1991-92-93. Another group of mixed height class trees, received a double dose of acid mist at each application. Control trees received only the usual precipitation inputs. The acid mist treatments had been found, reported elsewhere, to induce a significant (p < 0.05) reduction in stem diameter growth compared to control trees. Acid-mist treatments significantly (p < 0.01) reduced the water-soluble P content in forest litter 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 labile organic P content (p < 0.01) and P sorption capacity (p < 0.01) of the surface soil however were increased compared to soils under control trees. Using a sensitive root bioassay technique, based on the metabolic uptake of P-32, the acid treatment was shown to have significantly (p < 0.001) increased P stress within the trees, relative to control trees. Acid-mist applied to the tree canopies caused significant(p < 0.01) decreases in the soil pH, and marked changes in the cation exchange complex composition, though total effective cation exchange capacity of the soil showed little variation. However, base saturation was 90% in soils under control trees but decreased significantly under acid-misted trees and very significantly to less than 30% under trees given the double acid dose treatment. Consequently, the (Ca + Mg):AI ratio was reduced markedly from 9.5 +/- 2.9 to 0.4 +/- 0.3. The results from this study therefore support the working hypothesis that acidifying inputs induce P deficiency by slowing the cycling of phosphorus and decreasing P availability within the forest ecosystem. The effects are related to the general effects of acidifying inputs on soil pH and base status. The effects described are considered to be important factors in the forest decline syndrome.