Changes in soil phosphatase activity and P transformation rates induced by application of N- and S-containing acid-mist to a forest canopy

The indirect effects of acid-misting applied to tree canopies, on soil P availability, phosphatase activity and P mineralisation rates in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand 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 H2SO4 and NH4NO3 (pH 2.5) at concentrations of 3.2 mM H+ and 1.6 mM each of NH4+, NO3- and SO42-. These simulate cloud water composition in uplands affected by acidic deposition. Another group of mixed height-class trees, received a double dose of acid mist. Control - no acid-misted-trees received only the usual precipitation inputs. The acid mist treatments had previously been found to induce a significant reduction in stem diameter growth, but no canopy visible injury symptoms were observed. Root bioassays had demonstrated the induction of P nutritional stress in acid-misted versus no acid-misted trees. Soil chemical analysis showed that acidifying inputs induced changes in the inorganic P subcycle by: (i) increasing acidity and P sorption capacity of the soils; and (ii) decreasing the concentration of labile inorganic P. We report here on changes in the organic P subcycle (phosphatase activity and gross mineralisation rates). We used a method which applies independent treatments to soil samples (control, HgCl2-addition, HgCl2-addition + autoclaving) to separate mineral solubilisation, organic mineralisation and immobilisation of solution P. Soil phosphatase activity was significantly lower under acid-misted trees (263.9 mug pNP h(-1) g(-1)) than under control trees (382.0 mug pNP h(-1) g(-1)). Soils beneath double-dose acid-misted trees showed a 42% reduction in the labile inorganic P pool-size, and a 33% increase in the labile organic P pool size, compared to soils under control trees. A pattern of decreasing net P solubilisation and gross P mineralisation rates with increasing acid-mist dose-level was also found. Thus, the combined effects of acid-misting on the inorganic and organic P subcycles, in addition to the suggested effect of N saturation enhancing demand for P by the trees, may have induced P deficiency and reduced tree growth. The effects described are considered to be important factors in the forest decline syndrome. (C) 2000 Elsevier Science Ltd. All rights reserved.