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enzymes as indicated by a reduction of soil phosphatase activity to zero in autoclaved samples. The addition of HgCl 2 did not affect soil phosphatase activity mean phos- phatase activities of non-treated and HgCl 2 -treated soils were 420.8 and 431.5 mg pNP g 21 h 21 , respectively. Addi- tion of HgCl 2 followed by a one-week incubation at 35 WHC markedly inhibited respiration rates in all soils from 60 to 90 inhibition respect to the corresponding non- HgCl 2 -treated soils, although did not reduced it to zero. Thus, in order to ensure that microbial immobilisation of solution P was precluded in the HgCl 2 -treated soils during the 48 h incubation with resins, additional HgCl 2 2500 mg kg 21 of dry soil was added to the corresponding distilled water–resin bag–soil slurries see Section 2. 3.4.2. Net solubilisation, gross mineralisation and immobilisation rates Net P solubilisation rates were higher than gross P miner- alisation and net P immobilisation rates Fig. 4. Net P solubilisation rates were significantly lower … a ˆ : 05† in soils under double-dose acid-misted trees 7.3 mg P g 21 day 21 than in soils under non-acid-misted trees 11.9 mg P g 21 day 21 . Gross P mineralisation rates were more than four times greater in the non-acid-misting treat- ment than in the double-dose acid-misting treatment …P ˆ : 08† : Net P immobilisation rates were very low in all cases. Net changes in soil solution P were all positive and decreased significantly with increasing dose of acid-mist application. Net changes in organic P pools were all nega- tive. Soils under non-acid-misted trees showed the highest net decrease in organic P, whereas soils under double-dose acid-misted trees showed almost no variation in organic P pools. However, the effect of acid-mist dose did not achieved significance at the a ˆ : 05 level.

4. Discussion

Excluding plant P uptake, and P inputs to and outputs from the forest ecosystem, P availability is the net result of inorganic P solubilisation, P immobilisation and organic P mineralisation in the soil. In this study, we have tested the hypothesis that changes in soil phosphatase activity and changes in net P transformation rates resulting in a lower P supply to the soil solution are causal mechanisms for the induction of P deficiency and reduced tree-diameter growth that had been previously reported Sheppard et al., 1995; Carreira et al. 1997 in a sitka-spruce plantation subjected to the application of acidified mist to the forest canopy. We found that potential phosphatase activity mg pNP g 21 dw soil at the soil pH was significantly lower in soils under acid-misted than under non-acid-misted trees. The pattern of enzyme activity inhibition with increas- ing acid-mist dose applied to the forest canopy is even more J.A. Carreira et al. Soil Biology Biochemistry 32 2000 1857–1865 1863 Fig. 5. Relationship between pH and a specific phosphatase activity Pase: labile organic P index, and b P sorption capacity parameters and net P solubilisation rates, in soils of the Glencorse field experiment. P sorption capacity parameters are from Carreira et al. 1997. pronounced when expressed in terms of specific phospha- tase activity P ase :labile P o ratio Fig. 5, indicating that the availability of natural sources of substrate is not acting as a limiting factor for the amount of phosphatase enzymes in the acidified soils. Potential enzyme activity rates measured at saturating substrate concentration as in the phosphatase assay conditions used here are equivalent to the enzyme V max and, thus, can be considered as indicative of the enzyme concentration in the soil Tabatabai, 1994. On the other hand, phosphatase enzyme production is known to be enhanced by low availability of inorganic P in the soil Stevenson, 1986. However, the smaller size of the most- easily available inorganic P pool in soils under acid-misted trees was not paired by a higher potential phosphatase activ- ity in such soils. We also found that soils under acid-misted trees showed lower respiration rates and an attenuated CO 2 release increment in response to increases in the moisture content of the soils. Soil respiration can be used as a measure of the activity of soil microbes Zibilske, 1994. All this suggests that soil acidification induced by acid-mist- ing treatments have affected the role of phosphatase enzymes in the soil P cycling not only by pH-induced changes in their activity but also by lowering the rate of enzyme production by the microbial biomass. Changes in phosphatase activity can result in changes on P mineralisation only if P mineralisation is limited by an insufficient quantity of phosphatase otherwise, the potential decrease in the rate of P mineralisation by a lower phospha- tase activity might be compensated by a higher amount of phosphatase enzyme. This explains why some studies have shown no correlation between phosphatase activity and net P mineralisation rates e.g. Trasar et al., 1991, whereas others have shown otherwise e.g. Harrison, 1982; this study. We found that gross P mineralisation rates showed a decreasing trend with increasing acid-mist dose, that was not compensated by changes in the P immobilisation rates. These findings support the hypothesis that the combined effect of soil acidification on the activity and size of the phosphatase enzyme pool, and on the microbial activity, may affect P availability by slowing the supply of P to the soil solution resulting from the net balance between P mineralisation and immobilisation. Changes in the net balance between P fixation to and P solubilisation from the mineral phase in soils may also alter the supply of P to the soil solution. We found that net P chemical solubilisation rates were significantly higher in non-acid-misted than in acidified soils. This is consistent with previous results which showed an increase in P sorp- tion-fixing capacity in soils under acid-misted trees, as shown by the Langmuir’s adsorption maxima and the phos- phorus buffering capacity Carreira et al., 1997 Fig. 5. Increases in soil P sorption capacity induced by increasing soil acidification in areas where the initial soil pH was already slightly acid the Glencorse soils had a mean pH value of 5.09 has been found elsewhere Pare´ and Bernier, 1989b. The overall combination of changes in P transfor- mation rates reported here demonstrates that acidic-mist inputs may induce a decrease in soil P availability through the synergic effect of soil acidification in slowing the rate of processes involved in both the geochemical and the biochemical P cycle. The appearance of P deficiencies in forest stands receiv- ing high ‘natural’ acidifying atmospheric inputs has been reported both in North America and Europe. Bernier and Brazeau 1988 reported P deficiencies in sugar maple stands in the Quebec area. Mohren et al. 1986 and Houdijk and Roelofs 1993 found generalised P deficiencies in forest stands in the Netherlands, especially in Douglas fir stands. Harrison et al. 1999 reported a general link between ‘thin’ canopy condition and phosphorus stress in beech, Scots pine and Sitka spruce stands growing on a range of comparable soils in the UK. Several causal mechanisms were proposed to explain the association between the appearance of P deficiencies in forest trees and high atmospheric acidifying inputs Fig. 1. On the one hand, soil acidification is hypothesised to cause i increase in the inorganic P sorption capacity of the soils and concomitant decreases in P solubility Van Breemen et al., 1983; Pare´ and Bernier, 1989b; and ii decreases in the rate of organic P mineralisation Harrison, 1982; Pare´ and Bernier, 1989a. On the other hand, the induction of N saturation associated to high N-containing, atmospheric, acidic inputs, may enhance the tree demand for other nutri- ents especially P, potentially resulting in nutritional imbal- ances Nihlgard, 1985; Binkley et al., 1989. However, the establishment of clear linkages between observed P defi- ciency symptoms and any of the proposed causal mechan- isms has been difficult in the above-cited, field-based studies describing ‘natural’ variability. In a field-manipulation experiment at Glencorse Scot- land, Sheppard et al. 1995 found that application of acid-mist to a mature Sitka spruce did not result in any visible injury symptoms to the tree canopy, yet it resulted in a highly significant reduction in stem area increment. The use of root bioassays, a technique based on the metabolic uptake of 32 P by roots which provides an integrated measurement of the balance between the nutrient demand of the tree and its supply from the soil Harrison and Helli- well, 1979, demonstrated that acid-misted trees were under P nutritional stress Carreira et al., 1997. Subsequent research has clearly linked such nutritional stress to marked changes in the patterns of P availability and P cycling induced by soil acidification. Acid-mist applied to the tree canopies induced the acidification of surface soil and litter, as demonstrated by significant decreases in soil and litter pH, and increases in the contribution of acidic cations both to the cation exchange complex and to the litter and soil solution chemistry Carreira et al., 1997. In addition, acid- mist treatments significantly reduced the concentration of inorganic phosphorus in the forest litter and soil leachates; and significantly increased the P sorption capacity of the soils Carreira et al., 1997. We have reported here lower J.A. Carreira et al. Soil Biology Biochemistry 32 2000 1857–1865 1864 net P solubilisation and gross P mineralisation rates, and no change in P immobilisation rates, in the acidified soils. Thus, the combined effect of soil acidification on P trans- formation rates leads to a decrease in the P supply to the soil solution and a lower availability of P in soils under acid- misted, P-deficient trees. However, the possibility that the P stress detected on acid-mist treated trees also results, at least partially, from enhanced demand of P by the trees due to the presence of N in the applied acid-mist cannot be fully discounted. Acknowledgements We wish to thank L. Sheppard, A. Crossley, N. Cape and F. Harvey I.T.E., Bush, Scotland for providing access to the Glencorse Field Experiment, as well as information about the experimental design, results and general conclu- sions from their previous studies at the site. This research was made possible by a bursary ref. EV5V-CT-94-5229 for J. Carreira from the ENVIRONMENT programme of the Commission of the European Communities. References Bernier, B., Brazeau, M., 1988. Nutrient deficiency symptoms associated with sugar maple dieback and decline in the Quebec Appalachians. Canadian Journal of Forest Research 18, 762–767. Binkley, D., Driscoll, C.T., Allen, H.L., Schoeneberger, P., McAvoy, D., 1989. Acidic Deposition and Forest Soils: Context and Case Studies of the Southeastern United States. Springer, New York. Carreira, J.A., Harrison, A.F., Sheppard, L.J., Woods, C., 1997. Reduced soil P availability in a Sitka spruce Picea sitchensis Bong. Carr plantation induced by applied acid-mist: significance in forest decline. Forest Ecology and Management 92, 153–166. Crossley, A., Wilson, D.B., Sheppard, L.J., Leith, I.D. Cape, J.N. 1991. Effects of acid mist on mature Sitka spruce, Interim report, Department of the Environment, UK, 29pp. Foy, C.D., Chaney, R.L., White, M.C., 1978. The phisiology of metal toxicity. Annual Reviews Plant Physiology 29, 511–566. Harrison, A.F., 1982. Labile organic phosphorus mineralization in relation- ship to soil properties. Soil Biology Biochemistry 14, 343–351. Harrison, A.F. 1989. Phosphorus distribution and cycling in European forest ecosystems. In: Tiessen, H. Ed., Phosphorus Cycles in Terres- trial and Aquatic Ecosystems. Regional Workshop I: Europe, SCOPE- UNEP, pp. 42–76. Harrison, A.F., Bocock, K.L., 1981. Estimation of soil bulk-density from loss-on-ignition. Journal of Applied Ecology 18, 919–927. Harrison, A.F., Helliwell, D.R., 1979. A bioassay for comparing phos- phorus availability in soils. Journal of Applied Ecology 16, 497–505. Harrison, A.F., Carreira, J.A., Poskitt, J.M., Robertson, S.M.C., Smith, R., Hall, J., Hornung, M., Lindley, D.K., 1999. Impacts of acidifying pollu- tant inputs on forest canopy condition in the UK: possible role of P limitations. Forestry 72, 367–377. Hedley, M.J., Stewart, J-W.B., Chauhan, B.S., 1982. Chamges in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Science Society of America Journal 46, 970–976. Hendershot, W.H., Duquette, M., 1986. A simple barium chloride method for determining cation exchange capacity and exchangeable cations. Soil Science Society of America Journal 50, 605–608. Houdijk, A.L., Roelofs, J.G., 1993. The effects of atmospheric nitrogen deposition and soil chemistry on the nutritional status of Pseudotsuga menziesii , Pinus nigra and Pinus sylvestris. Environmental Pollution 80, 79–84. Hu¨ttl, R.F., 1988. Forest decline and nutritional disturbances. In: Cole, D.W., Gessel, S.P. Eds.. Forest Site Evaluation and Long-term Productivity, University of Washington Press, Seattle, pp. 180–186. John, M.K., 1970. Colorimetric determination of phosphorus in soil and plant materials with ascorbic acid. Soil Science 109, 214–220. Kazda, M., 1990. Indications of unbalanced nitrogen nutrition of Norway spruce stands. Plant and Soil 128, 97–101. McLean, E.O., et al., 1982. Soil pH and Lime Requirement. In: Page, A.L., et al. Eds.. Methods of Soil Analysis, Part 2, Chemical and Micro- biological Properties, 2nd ed. Book series no 9, Soil Science Society of America, Madison, MI, pp. 199–223. Mohren, G.M., Van der Burg, J., Burger, F.W., 1986. Phosphorus defi- ciency induced by nitrogen input in Douglas fir in the Netherlands. Plant and Soil 95, 191–200. Nihlgard, B., 1985. The ammonium hypothesis: an additional explanation to the forest dieback in Europe. Ambio 14, 2–8. Pare´, D., Bernier, B., 1989a. Origin of the phosphorus deficiency observed in declining sugar maple stands in the Quebec Appalachians. Canadian Journal of Forest Research 19, 24–34. Pare´, D., Bernier, B., 1989b. Phosphorus-fixing potential of Ah and H horizons subjected to acidification. Canadian Journal of Forest Research 19, 132–134. Schulze, E.D., 1989. Air pollution and forest decline in a spruce Picea abies forest. Science 244, 776–783. Sheppard, L.J., Leith, I.D., Cape, J.N., 1994. Effects of acid mist on mature grafts of Sitka spruce. Part I. Frost hardiness and foliar nutrient concen- trations. Environmental Pollution 85, 229–238. Sheppard, L.J., Crossley, A., Harvey, F.J., Cape, J.N., Fowler, D. 1995. Long term effects of field exposure to acid mist on the performance of a single Sitka spruce clone. Final report, Department of the Environment, UK, 75pp. Stevenson, F.J., 1986. Cycles of Soil: Carbon, Nitrogen, Phosphorus, Sulfur, Micronutrients. Wiley, New York 380pp. Tabatabai, M.A., 1994. Soil Enzymes. In: Weaver, R.W. Ed.. Methods of Soil Analysis, Part 2, Microbiological and Biochemical Properties, Book Series no 5, Soil Science Society of America, Madison, WI, pp. 775–833. Tomlinson, G.H., 1990. Effects of acid deposition on the forests of Europe and North America. CRC Press, Boca Raton, FL 281pp. Trasar, M.C., Gil, F., Carballas, T., 1991. Liming and the phosphatase activity and mineralization of phosphorus in an andic soil. Soil Biology Biochemistry 23, 209–215. Ulrich, B., Mayer, R., Khanna, P.K., 1980. Chemical changes due to acid precipitation in a loess-derived soil in central Europe. Soil Science 130, 193–199. Van Breemen, N., Mulder, J., Driscoll, C.T., 1983. Acidification and alka- linization of soils. Plant and Soil 75, 283–308. Van Dick, H.F.G., Roelofs, J.G.M., 1988. Effects of excessive ammonium deposition on the nutritional status and condition of pine needles. Physiologia Plantarum 73, 494–501. Wolf, D.C., Skipper, H.D., 1994. Soil Sterilization. In: Weaver, R.W. Ed.. Methods of Soil Analysis, Part 2, Microbiological and Biochemical Properties, Soil Science of America, Madison, WI, pp. 41–49. Zibilske, L.M., 1994. Carbon Mineralization. In: Weaver, R.W. Ed.. Methods of Soil Analysis, Part 2, Microbiological and Biochemical Properties, Book series no 5, Soil Science of America, Madison, WI, pp. 835–863. Zou, X., Binkley, D., Doxtader, K.G., 1992. A new method for estimating gross phosphorus mineralisation and immobilization rates in soils. Plant and Soil 147, 243–250. J.A. Carreira et al. Soil Biology Biochemistry 32 2000 1857–1865 1865