Saturday, September 7, 2013

The Effects of Acid Rain on Forest Nutrient Status

The effects of acidic atmospheric inputs on forest nutrient status must be assessed within the context of natural, internal acid production by carbonic and organic acids as well as the nutrient inputs and drains by management practices such as harvesting, fire, and fertilization. In all cases the anion associated with acid inputs must be mobile in the soil if leaching is to occur; immobilization of anions can effectively prevent from leaching. Soil acidification will occur only if the often substantial buffering capacity of the soil in question is exceeded by acid inputs and weathering from primary minerals is insufficient to offset losses by leaching. Such circumstances are rare but certainly could occur in theory, at least, given sufficiently large acid inputs on poorly buffered soils. Soils most sensitive to change are thought to be those of moderately acid pH and low cation exchange capacity. Neither very acid soils nor neutral, highly buffered soils are sensitive to acidification by acid rain.

Leaching of Nutrients

Hydrogen ions from the acid in acid rain replace the nutrient elements in the soil. For every unit of acid added to the soil, an equivalent amount of nutrient elements is removed. As a result, more nutrients get leached from the soil than arrive from weathering of rocks or precipitation

Extremely high acid inputs, acid rain can cause temporary increases in nitrogen mineralization and nitrification in soils. While temporary increases in N availability can cause increased forest growth in N-deficient forests, increased Al availability can cause toxic reactions in tree roots. Little is known about tree aluminum toxicity levels as yet, however. It must be emphasized that assessment of acid rain effects is a problem of quantification. Given sufficiently high inputs on sensitive sites, negative effects of acid rain must occur, as is true of inputs of any substance, including H2O. Acid rain inputs of sufficient magnitude to cause acute effects, such as growth increase due to N mobilization or growth decrease due to Al mobilization, are apparently very rare under ambient field conditions. Long-term effects on forest nutrient status can be either beneficial or adverse, depending on site nutrient status and amount of atmospheric inputs.


The effect of acid rain on the environment depends greatly on the ability of soils to neutralize the acid. Limestone and other rocks and soils containing calcium carbonate are most effective. Acids react with calcium carbonate to produce neutral compounds and carbon dioxide. Mature forest soils are also able to neutralize the acids in rain. Such soils are acidic. Krug (1983) reports that mature soils in New England or Norwegian forests have a pH of 3.8, and they may contain as much acid as would fall in 1000 years of rain at 1m/year with a pH of 4.3. These soils are highly resistant to acid rain.

In contrast, thin alpine soils lacking carbonates and  acid and overlying granite rocks are not able to neutralize acid rain. The same is true for disturbed forests where forest fires and logging have reduced the organic material in the soil, which also make the soil more susceptible to damage by acid rain.

Influence of Acid Rain on Tropical Rainforests

Tropical rainforests are the biggest forest area in the world. They constitute 6% of the whole area of the Earth and they have great significance in water and heat balance on our planet. What is more, these forests are home for the half of the species of plants and animals we are familiar with. They are a treasure trove of nature which acid rain threatens.

The main danger to rainforests is human's wasteful economy. The forest are cut own and burnt. The processes are mainly connected with industrialization and accordingly they carry acid rain threat which is local (burning - carbon dioxide, industry - sulphur dioxide and nitrogen oxides). Furthermore, omnipresent and very mobile cloud and air pollution also do not omit the tropics. Even rain acidified to a very small degree whose whole total yearly fall is from 1000 to 2000 mm, destroys ever-green tress whose vegetation period lasts a whole year.

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