Report - ICP Forests

54 3. Intensive Monitoring
3.3 Soil solution chemistry and its trends
3.3.1 Introduction
Effects of atmospheric deposition need to be regarded with respect to the receptor. Most
important effects on soil solution (and as such on solid soil phase) are a change of buffer range
as a result of acid deposition and mobilisation of potentially toxic elements as well as nutrient
imbalances and resulting nutrient deficiencies. These soil and soil solution mediated processes
affect vegetation in terms of reduced growth resulting from impaired nutrient uptake, fine root
dieback and general stress reactions of the vegetation like excessive flowering (Koch and
Matzner, 1993; Løkke et al., 1996; Sverdrup and Warfvinge, 1993).
An important tool to describe risks of atmospheric pollution is the calculation of critical loads
and their exceedances, aiming at the protection of forest ecosystems from harmful effects on
forest structure or function (Augustin et al., 2005). The critical load concept is accepted as the
basis for air pollution abatement strategies, in order to reduce or prevent damage to the
functioning and vitality of forest ecosystems caused by transboundary acidic deposition (Løkke
et al., 1996).
Critical loads are defined as “a quantitative estimate of an exposure to one or more pollutants
below which significant harmful effects on specified sensitive elements of the environment do
not occur according to present knowledge” (Spranger et al., 2004). Critical load calculations
balance the depositions to which the ecosystem is exposed with its capacity to buffer the input,
take up or remove it from the system without harmful effects within or outside the system
(Spranger et al., 2004). Critical loads can be derived empirically or can be calculated by means
of a simple mass balance or dynamic models. The so-called Simple Mass Balance (SMB) model
can be considered a standard model for calculating critical loads for terrestrial ecosystems under
CLRTAP (Spranger et al., 2004). Critical loads can be calculated for acidifying substances,
eutrophying N compounds and heavy metals.
Model-based approaches for calculating critical loads aim at linking deposition of air pollution
with its biological effects to the ecosystem. As the biological effects often are of complex
nature, chemical criteria are mostly used to simplify the modelling. This calls for appropriate
(soil) chemical criteria with proven (empirical) relationships to biological effects. For theses
chemical criteria values have to be defined that mark the threshold below which harmful effects
on the specified biological indicator are not expected (Spranger et al., 2004). Exceedances of
critical limits do not necessarily result in instant dieback of trees or ecosystems but do illustrate
an enhanced risk for trees to be more susceptible to additional stressors. Exceedances may result
in a loss of assimilation area, growth reductions and nutrient imbalances (Augustin et al., 2005).
Consequently critical loads are a function of the chosen chemical threshold values (critical
limits) applied within the model (Hall et al., 2010).
This chapter presents pH and basic cation to aluminium ratio in the soil solution of a broad share
of Level II plots and evaluates these parameters against well documented critical limits. The
results presented in the following sub-chapters are a first analysis. The temporal analysis of
BC/Al ratio and pH values is not sufficient to describe the changes in soil solution chemistry.
Concentrations of BC, Al and main anions (NO 3 , SO 4 ) need to be investigated in follow-up
studies.