Ja. Carreira et al., Changes in soil phosphatase activity and P transformation rates induced byapplication of N- and S-containing acid-mist to a forest canopy, SOIL BIOL B, 32(13), 2000, pp. 1857-1865
The indirect effects of acid-misting applied to tree canopies, on soil P av
ailability, phosphatase activity and P mineralisation rates in a Sitka spru
ce (Picea sitchensis (Bong.) Carr) stand have been investigated. The treate
d trees, grouped according to five height classes, had for four years previ
ously 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 deposi
tion. Another group of mixed height-class trees, received a double dose of
acid mist. Control - no acid-misted-trees received only the usual precipita
tion inputs. The acid mist treatments had previously been found to induce a
significant reduction in stem diameter growth, but no canopy visible injur
y symptoms were observed. Root bioassays had demonstrated the induction of
P nutritional stress in acid-misted versus no acid-misted trees. Soil chemi
cal 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 he
re on changes in the organic P subcycle (phosphatase activity and gross min
eralisation rates). We used a method which applies independent treatments t
o soil samples (control, HgCl2-addition, HgCl2-addition + autoclaving) to s
eparate mineral solubilisation, organic mineralisation and immobilisation o
f 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 mu
g 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 patter
n 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 th
e suggested effect of N saturation enhancing demand for P by the trees, may
have induced P deficiency and reduced tree growth. The effects described a
re considered to be important factors in the forest decline syndrome. (C) 2
000 Elsevier Science Ltd. All rights reserved.