Download effects of acid rain on terrestrial ecosystems

EFFECTS OF ACID RAIN ON TERRESTRIAL ECOSYSTEMS
EFFECTS OF ACID RAIN ON
TERRESTRIAL ECOSYSTEMS
The thirsty earth soaks up the rain, And drinks, and gapes for drink again. The plants such in
the earth and are With constant drinking fresh and fair.
Abraham Cowley from Drinking
Effects on Soil
It was argued by early acid rain researchers that forests in Canada and northeastern U.S.A. are on acidic
soils and therefore acid rain makes no difference to the forests. However, we now know this is not true.
The effects of acid rain on soil is dependent on the behaviour of ions in the soil.
Most of the ions of interest are cations such as Ca2+ and Mg2+ in addition to the cation H+. These ions
are the nutrients needed by plants. There are also toxic ions such as aluminum, lead, mercury, cadium
and other metal ions. These ions are usually not free to migrate in the soil because they are bound to the
negatively charged surface of immobile soil particles. These particles are negative because of the
silicates on the surface of clays and the organic acids on particles of organic matter. The cation-exchange
capacity is the ability of the soil to bind to the positively charged ions but acid rain depletes this capacity.
Acid rain is able to mobilize these ions from the soil in two ways:
1. The H+ ion in the acidic water displaces the other positive ions from their binding sites and
increases the concentration of these ions in the soil water. When the soil particles are no longer
able to bind anymore H+ ions, the concentration of H+ also increases in the soil water.
2. The sulphate and nitrate ions (negatively charged) in the acid rain act as "counterions" which allow
positive ions to be leached from the soil.
Thus, in these two ways, acid rain can unlock the acidity, nutrients and toxic metals that are bound in the
soil.
The toxic metals leached from the soils are very important to the welfare of green plants and the aquatic
ecosystem. Through a series of chemical reactions, the important ions such as K+, Ca2+, Mg2+, and Na+
are leached out and become unavailable to plants as nutrients.
Acid rain can also increase the weathering of silicate minerals in soils. This leads to a loss of mineral
structure and possibly reduced fertility. A possible chemical reaction involving silicates is the following:
2KAlSi3O8 + H+ + 9H2O (Al2Si2O5(OH)4 + 2K+ + 4H4SiO4(orthoclase)
Not all soils are subject to the effects of acid rain. The vulnerable soils are those that are on glaciated
areas of granitic and other highly siliceous bedrock (granite, certain gneisses, quartzite and quartz
sandstone), areas with thick deposits of siliceous sands (such as the sand plains of Denmark and the
Netherlands) and areas of old and highly weathered and leached soils. Soils that are rich in lime are more
resistent to acidity (because they are rich in calcium ions). In fact, lime is the best known buffer and
consequently, is being used increasingly by farmers and foresters. Soils are less vulnerable to acid rain
than surface waters, because they have a buffering capacity (which, sadly, can be depleted by continuous
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EFFECTS OF ACID RAIN ON TERRESTRIAL ECOSYSTEMS
deposition of acid rain). The buffering capacity depends mainly on the underlying bedrock and the
human use and management of the soil.
Effects on Forests
The effects of acid rain on forests are devastating. Acid rain damage was seen in the early 70s when
Germany's famed Black Forest began to decline. The Germans even have a name for the forest decline "Waldsterben". Both coniferous and deciduous trees of all ages showed damage. Acid rain was the only
answer to Waldsterben.
There are numerous acid rain effects on the biology of the forest. There are lesions on the foliage,
nutrient loss from the leaves, a reduced resistance to the environment, pathogens and insects, an
accelerated erosion of the leaf wax, a reduced rate of decomposition of the leaf litter, the inhibition of the
formation of the terminal buds, an increased mortality of seedlings and heavy metal accumulation. There
is also a reduction in tree growth, the dying of useful microorganisms that are symbiotic with tree roots
and the inhibition of the nitrification of ammonium compounds.
Scientists have seen the symptoms of trees under acid rain stress. Trees have physical damage to the
roots and foliage, a reduced canopy cover, crown dieback, a reduced growth rate and finally whole tree
death. Much of the problems are due to the leaching of aluminum from aluminum silicates when the pH
of soil water has reached 4.0. Aluminum damages the root hairs and reduces the uptake of phosphorus
and other nutrients. The tree later dies from starvation and a reduced resistance.
Foliar leaching is also possible. Ammonia and nitrogen may land on the leaf, either wet or dry, and pass
through the semi-permeable membrane on the leaf surface. It will then be incorporated into the leaf cells.
A chemical interaction takes place in the leaves (cation exchange) and potassium, calcium, magnesium
and sulphur are leached out and get washed away from the leaf surface.
Acid rain also has the effect of reducing the rates of decomposition on the forest floor. Soil organisms
(including bacteria) have their respiration rates reduced. There is an increase in ammonia in the soil by
the reduction in mobilization of nutrients (those that are usually released in decomposition) and there is a
decrease in soil nitrate levels because the nitrification process is inhibited.
Researchers have noticed three distinct stages of damage in conifers. First, the nitrates in the acid rain
provide nutrients to the soil and the plants grow rapidly. Secondly, the soil loses its ability to neutralize
the acid, there is a leaching of nutrients and the conifers stop growing. Their needles also begin to turn
yellow. Finally, the aluminum that is released kills the roots and the tree dies due to bacteria, fungi, virus
or the elements.
A study was done on the red spruce of Vermont's Green Mountains. There was definitely damage to the
root hairs and the mycorrhizae (useful microorganisms on root nodules) were missing. The change in the
red spruce caused a change in the whole ecosystem. When a mature spruce dies, it falls to the forest
floor. Seedlings can then become exposed to the fierce winter winds and become uprooted. The wind and
sunlight can dry out the forest floor and increase the stress on the seedlings. With excess sunlight on the
forest floor, ferns can immediately begin to outcompete the seedlings. With no new viable seedlings, the
whole ecosystem cannot repair itself for generations.
There is also damage to other forest vegetation in addition to trees. Acid rain can inhibit the growth of
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EFFECTS OF ACID RAIN ON TERRESTRIAL ECOSYSTEMS
nitrogen fixing algae growing on the surface of the forest floor and fungi become more susceptible to
pathogens because the cuticle and other protective features are damaged. Sometimes other species of
plants become less susceptible to disease because acid rain makes some pathogen less virulent.
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