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Impacts of Acid Rain on Soils

Introduction

Soil is the basis of wealth upon which all land-based life depends.

The damage that occurs to ecosystems from acidic deposition is dependent on the buffering ability of that ecosystem. This buffering ability is dependent on a number of factors, the two major ones being soil chemistry and the inherent ecosystem sensitivity to acidification. Indirect damage to ecosystems is largely caused by changes in the soil chemistry. Increasing soil acidity can affect micro-organisms which break down organic matter into nutrient form for plants to take up. Increasing soil acidity also allows aluminium (a common constituent of soil minerals) to come into solution. In its free organic form, aluminium is toxic to plant roots and can lock up phosphate, thereby reducing the concentrations of this important plant nutrient.

What Effect Does the Soil and Underlying Bedrock Have on Acid Rain?

Soils containing calcium and limestone are more able to neutralise sulphuric and nitric acid depositions than a thin layer of sand or gravel with a granite base.

If the soil is rich in limestone or if the underlying bedrock is either composed of limestone or marble, then the acid rain may be neutralised. This is because limestone and marble are more alkaline (basic) and produce a higher pH when dissolved in water. The higher pH of these materials dissolved in water offsets or buffers the acidity of the rainwater producing a more neutral pH.

Acid Sensitive Areas

In regions where the soil is not rich in limestone or if the bedrock is not composed of limestone or marble, then no neutralising effect takes place, and the acid rainwater accumulates in the bodies of water in the area. This applies to much of the north-eastern United States where the bedrock is typically composed of granite. Granite has no neutralising effect on acid rainwater. Therefore over time more and more acid precipitation accumulates in lakes and ponds.

The water bodies most susceptible to change due to acid precipitation are those whose catchments have shallow soil cover and poorly weathering bedrock, for example granite and quartzite. These soil types are characterised by the absence of carbonates that could neutralise acidity. The run-off water from such areas is less buffered than from areas such as limestone catchments, with an adequate level of carbonate. Such catchments and waters are termed acid-sensitive (poorly buffered), and can suffer serious ecological damage due to artificially acidified precipitation from air masses downwind of major emissions.

Notable high-risk areas in Canada and the United States are the Canadian Shield, the Adirondack Mountains, the Laurentians, the Appalachians, and the Green Mountains of Vermont. These areas are vulnerable because of their high elevations, small watersheds, and naturally acidic soils. Different types of bedrock contain variable amounts of alkaline chemicals. Regions with bedrock containing less alkali have a lower capacity for reducing acidity, and thus are more sensitive to acid deposition.

Effects of soil on vegetation

When acid rain falls, it can affect forests as well as lakes and rivers. To grow, trees need healthy soil to develop in. Acid rain is absorbed into the soil making it virtually impossible for these trees to survive. As a result of this, trees are more susceptible to viruses, fungi and insect pests.

Long-term changes in the chemistry of some sensitive soils may have already occurred as a result of acid rain. As acid rain moves through the soils, it can strip away vital plant nutrients through chemical reactions, thus posing a potential threat to future forest productivity.

Poisonous metals such as aluminium, cadmium and mercury, are leached from soils through reacting with acids. This happens because these metals are bound to the soil under normal conditions, but the added dissolving action of hydrogen ions causes rocks and small-bound soil particles to break down.

Plant life in areas where acid rain is common may grow more slowly or die as a result of soil acidification. In the Green Mountains of Vermont and the White Mountains of New Hampshire in the United States 50% of the red spruce have died in the past 25 years. There has also been noted a reduced amount of growth in existing trees as measured by the size of growth rings of the trees in these areas.

These effects occur because acid rain leaches many of the existing soil nutrients from the soil. The number of micro-organisms present in the soil also decreases as the soil becomes more acidic. This further depletes the amount of nutrients available to plant life because the micro-organisms play an important role in releasing nutrients from decaying organic material. In addition, the roots of plants trying to survive in acidic soil may be damaged directly by the acids present. Finally, if the plant life does not die from these effects, then it may be weakened enough so that it will be more susceptible to disease or other harsh environmental influences like cold winters or high winds.

Critical Loads

Environmental response to pollutants depends on many factors. Some regions cope with acidification better than others, having larger 'critical loads'. Critical load refers to the greatest assault that an ecological system can withstand before showing measurable degradation.

Scientists determine critical load by examining rock and soil type, land use and rainfall. If soil is fertile with a pH greater than 4.5, and rainfall is relatively low, the critical load will be high. The terrain can withstand moderately large additions of acidity without undue suffering. Conversely, in low pH soils, acidification mobilises toxic aluminium ions. If coniferous forests predominate, or if land is devoted to rough grazing, the result is a low critical load. Even minor acid deposition is undesirable.

There are very few long-term UK monitoring studies of soil acidification and none of soil biota. Chemical data are available from a few specific sites, from a small number of regional studies and from three national studies. From the limited information available, the National Expert Group on Transboundary Pollution has concluded that there is evidence that acid deposition has resulted in widespread acidification of acid sensitive soils in the UK. Further critical loads modelling research suggests that soil recovery from acidification may take many years or even decades.