Correlation Between Acid Rain and Nitrogen Fixation


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The atmosphere is the sphere of gases that surrounds the Earth while the geosphere is the sphere of all the rocks and minerals on the Earth. Both are the subsystems that make up the planet. According to [cite], the atmosphere is a protective shield that nurtures Earth’s life and protects it from the hostile outer space climate. It is the source for plant photosynthesis of carbon dioxide and respiration of oxygen. This supplies the nitrogen that nitrogen-fixing bacteria and ammonia-producing industrial plants use to produce nitrogen that is chemically bound, an integral component of life mole while the geosphere is the region of the planet that human beings live on and from which they derive much of their food, minerals, and fuel. Once thought of having an almost infinite buffering ability against human interference. The geosphere is now well known to be very fragile and hazardous to human activity as it was mined each year in the extraction of billions of tons of minerals and coals.

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Concisely, many chemical substances can be found in both atmosphere and geosphere in a way that the atmosphere consists of more free-moving chemical particles as most of them are in the form of gasses molecules. The major substance in the atmosphere is nitrogen with a percentage of 78. 09% followed by oxygen with 20.95%, 0.93% of argon, and 0.04% of carbon dioxide. On the other hand, the geosphere which is composed of three layers – earth crust, mantle, and core – consists of eight important elements that made up the Earth crust which are oxygen, silicon, aluminum, iron, calcium, sodium potassium, and magnesium.

According to the Report of the Swedish Preparatory Committee for the U.N Conference on the Human Environment (Engstrom, 1971), acid precipitation has affected the N2 fixation in the soil. Nitrogen is a major limiting factor in forests in which a decrease of N2 fixation will cause damage to the forest ecosystem.

Low pH of the acid rain will inhibit the nitrification process which will results in conservation of nitrogen since nitrate is relatively soluble and readily lost to flow out or groundwater in some ecosystem (Likens et al,1970)

The objective of this research paper is to discuss the effects of acid rain on the nitrogen fixation process that takes place in the soil in forests of Coniferous, Western Washington.

Nitrogen fixation was measured by using the reduction of acetylene method in which this method is based on the fact that the nitrogenase enzyme is the reducing catalyst. The enzyme will act on acetylene and reduce it to ethylene. The rate of ethylene production is equal to the rate of N2 fixation.

Samples were placed in 20ml of culture tubes with a stopper. The tubes were then evacuated by hand using a 50cc syringe and flushed with a gas mixture of 10% acetylene, 20% oxygen, and 70&argon. Acetylene was produced from reacting calcium carbide with water and scrubbing the resulting gas with concentrated sulphuric acid. Both Oxygen and argon used were in industrial welding grade.

At the end of the incubation period, ethylene produced was measured by using gas chromatography. Ethylene concentration was determined using acetylene as an internal standard and ethylene production was calculated by subtracting the concentration of ethylene impurity in acetylene. The calculated rate of N2 fixation was based on the theoretical ratio of 3moles ethene produced per 1 mole of N2 fixed.

Two modifications were made for some samples. Firstly, 0.05% carbon dioxide was added to the gas mixture for some of the epiphyte samples. Next, 2cc of acetylene was injected into stoppered tubes. The first modification gives out a higher rate of ethylene production compared to the second modification. However, none of these modifications were significant.

Soil plots were isolated for experimental treatment by sinking plastic cylinders into the soil for a depth of 10-15cm. One plot at each site was untreated and used as a control. The other four received 5.12 liters of water with a range of pH of 3.0-6.0 by using a sprinkling that can stimulate 5cm of rain. Samples of litter and soil were collected for acetylene reduction analysis both immediately before and 30minutes after the acid application.

There was no detectable N2 fixation in most of the soil samples. The rate of ethylene production before acid application in moles, C2H4 g-1 soil h-1, was 1.6 x 10-11 to 7.7 x 10-11 for Quinault, zero for Centralia, and zero to 1.4 x 10-11 for Cedar River. Fixation in the untreated litter was somewhat higher: 1.3 x 10-10 for Quinault, zero to 3.9 x 10-1° for Centralia, and zero to 1.9 x 10-1° for Cedar River.

The pouring of acid had little effect on the small amount of fixation that takes place in the soil. Both soil and litter from Centralia and Cedar River were very dry as it was in the summertime in Washington. Most of the water or acid applied was absorbed by the litter layer leaving the soil to dry.

The effect of acid rain in the litter is more significant and is shown in figure 3? At Quinault, litter was moister and the initial rate of fixation is greater compared to that of Centralia and Cedar River. However, there was a drop in fixation after the application of acid of pH 4 or lower.

Rates of N2 fixation were low in litter and slower in soil. As pH inhibits fixation, moisture appears to be the major limiting factor at least in the summertime. Large quantities of acid were found to be not sufficient in saturating the soil with water or to change the pH of the soil and the litter.


An increase in urbanization and industrialization of air quality standards has potentially increased the precipitation of acidity. The low pH of the acid rain will affect the N2 fixation of the soil in various ways. However, the severity of this issue depended on other factors too.

The low rate of N2 fixation in soil and litter is probably due to natural acid conditions. Azotobacter and the blue-green algae are excluded by the acidic pH of the soil. Other than that, moisture of the soil also gives out an impact on the results. Buffering by the canopy of the forests, litter, and soil may have protected the soil and litter microorganisms from the effects of acid rain especially when the occurrence is infrequent.

Acid rain has affected the fluctuant number of Lobaria oregana sp., a species of bacteria in nitrogen fixation. William Denison (1974) has found that L. pulmonaria sp., a species closely related to Lobaria oregana sp., was severely damaged after being exposed to 50µg m-3 of SO2 for five weeks. On the other hand, Red alder contributes to a large amount of fixed N2. Because this species competes with Douglas fir, it is frequently suppressed deliberately.

In conclusion, a decrease in natural N2 fixation has been one of the results of acid rain which was caused by air pollution. Applying artificially fixed N2 to forests will be a decent alternative in the future. However, the best way to conserve natural N2 fixation is to manage the air pollution and the forests as to artificially fixing N2 will cause high energy and money.

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