Sammendrag
Acid rain may cause soil acidification possibly leading to indirect forest damage. Assessment of
acidification potential of atmospheric deposition is problematic where dry and occult deposition is significant.
Furthermore, uncertainty is enhanced where a substantial part of the potential acidity is represented by
deposition of ammonium (NH4+) since the degree of assimilation and nitrification is not readily available.
Estimates of dry deposition based on deposition velocity are highly uncertain and the models need to be
verified or calibrated by field measurements of total deposition. Total deposition may be monitored under the
forest canopy. The main problem with this approach is the unknown influence of internal bio-cycling.
Moreover, bio-cycling may neutralize much of the acidity by leaching of mainly K+. When the water
percolates down into the rooting zone this K+ is assimilated again and acidity is regenerated. Most
monitoring stations only measure deposition. Lacking measurements of output flux of both NH4+ and NO3-
from the soil one can not assess current net N transformation rates. Assumptions regarding the fate of
ammonium in the soil have strong influence on the estimated acid load. Assuming that all the NH4+ is
nitrified may lead to an overestimation of the acidifying potential.
In parts of the world where dry deposition and ammonium is important special consideration of these factors
must be made when assessing the acidification potential of total atmospheric loading. In China dry and
occult deposition is considerable and often greater than wet deposition. Furthermore, the main part of the
deposited N is in its reduced state (NH4+). The IMPACTS project has monitored the water chemistry as it
moves through watersheds at 5 sites in China. This paper dwells at two important findings in this study. 1)
Potassium leached from the canopy by acid rain is assimilated again upon entering the mineral soil. 2)
Nitrification mainly takes place in forest floor (H- and O-) horizon as NH4+ that escapes this horizon is
efficiently assimilated in the A-horizon. This suggests that the potential acidification capacity of the
deposition may be found in the throughfall and forest floor solution by treating K+ and NH4+, respectively, as
acid cations in a base neutralization calculation (BNC).
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