Environmental Engineering Reference
In-Depth Information
This reduction in bacterial activity, and the
associated disruption of nutrient supply, may be
sufficient to offset the benefits that some soils
receive from acid rain in the form of extra
nitrogen compounds (LaBastille 1981).
Changes in soil chemistry, initiated by acid
rain, also lead to nutrient depletion. In a
normally fertile soil, nutrients—such as calcium,
potassium and magnesium—are present in the
form of positively charged microscopic particles
(cations) bonded by way of their electrical
charge to clay and humus particles or other soil
colloids. They can be removed from there by
plants, as required, and are normally replaced
by additional cations released into the soil by
mineral weathering (Steila 1976). As acidic
solutions pass through the soil, hydrogen ions
replace the basic nutrient cations, which are
then leached out of the soil in solution with
sulphate and nitrate anions (Fernandez 1985).
Regular acid-induced leaching of this type, leads
to reduced soil fertility, and consequently affects
plant growth. Needle yellowing in coniferous
trees, for example, has been attributed to the
removal of magnesium from acidified forest
soils (Ulrich 1989).
The mobilization of toxic metals, such as
aluminium, cadmium, zinc, mercury, lead, copper
and iron, is another feature which accompanies
soil acidification (Fernandez 1985). Most of these
metals are derived from bedrock or soil minerals
by natural weathering, but there is evidence from
some areas, such as the northeastern United
States, that atmospheric loading is also important
(Johnson and Siccama 1983). Acid rain liberates
the metals in ionic form, and they are carried in
solution into groundwater, lakes and streams, or
absorbed by plants (Park 1987). The detrimental
effects of these toxic metals, such as aluminium,
on the aquatic environment, are well-
documented. Their impact on the terrestrial
environment is less clear, however. Some metals,
such as iron, are essential for growth, and only
become toxic at higher concentrations; other
metals may be present in amounts normally
considered toxic, yet the vegetation is
undamaged, presumably because it has adapted
to the higher concentrations (Park 1987). The
main claims that metals have initiated vegetation
damage have come from western Germany,
where high levels of aluminium in soils have been
blamed, in part, for the forest decline in that area
(Fernandez 1985). The aluminium restricts the
development of the fine root systems of the trees
to the upper part of the soil profile. If the topsoil
dries out, the surviving deep roots are unable to
supply enough water to meet the needs of the
trees. Water stress occurs and growth rates
decline (Ulrich 1989). Elsewhere, there is no clear
evidence that tree growth has been impaired by
toxic metal mobilization.
The damage attributed to acid rain is both
visible and invisible. In some cases, the impact is
only apparent after detailed observation and
measurement. For example, a survey of annual
rings in a mixed spruce, fir and birch forest
exposed to acid rain in Vermont, revealed a
progressive reduction in growth rates between
1965 and 1979 (Johnson and Siccama 1983).
Between 1965 and 1983, in the same general
area, there was also a 25 per cent decline in the
above-ground biomass of natural sugar maple
forest (Norton 1985). Physical damage to the
fine root systems is another element common to
trees in areas subject to acid rain (Tomlinson
1985).
Most of these symptoms can only be
recognized following careful and systematic
survey, but there are other effects which are more
directly obvious, and which have received most
public attention. They can be grouped together
under the general term 'tree dieback', which
describes the gradual wasting of the tree, inwards
from the outermost tips of its branches.
Dieback has been likened to the premature
arrival of autumn (Norton 1985). On deciduous
trees, the leaves on the outermost branches begin
to turn yellow or red in mid-summer; they dry
out and eventually fall, well ahead of schedule.
These branches will fail to leaf-out in the spring.
In succeeding years, the problem will spread from
the crown until the entire tree is devoid of foliage,
and takes on a skeletal appearance, even in
summer (Norton 1985). Coniferous trees react
