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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 http://www.scar.utoronto.ca/~weather/maryp/Effects/terrest.html (1 of 3) [6/4/2002 5:56:51 PM] 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 http://www.scar.utoronto.ca/~weather/maryp/Effects/terrest.html (2 of 3) [6/4/2002 5:56:51 PM] 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. http://www.scar.utoronto.ca/~weather/maryp/Effects/terrest.html (3 of 3) [6/4/2002 5:56:51 PM]