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Nitrogen Cycle

The nitrogen cycle is dominated by the N2 gas in the atmosphere. Nitrous oxide, N2O is the second common form. N20 (the gas commonly known as laughing gas) is a greenhouse gas. Seventy-nine percent of the atmosphere is nitrogen in the form of N2 gas. Because N2 has low reactivity, it offsets the high reactivity of oxygen, O2, the other major constituents of the atmosphere. For example when we light a match, the nitrogen does not burn with the oxygen. It does not react with any other element or common compound under ordinary conditions. This property of nitrogen has been called the "fire insurance" of our atmosphere. If the nitrogen was not "diluting" the flammability of 02, every spark from a match could lead to a large fire!

Due to its different valences (3,4,5,), nitrogen can form a multiplicity of compounds into the same element. For example, it can combine with oxygen to form N2O, NO, NO2, or N2O5! As a group, these oxides are (except for N2O5) denoted by NOx.

NOx compounds form an important category of air pollutants, for example, as a result of the nitrogen and oxygen combining in the extremely hot environment of an automobile engine. Nitrogen oxides and hydrocarbons, in the presence of sunlight, give rise to the photochemical smog and tropospheric ozone problems, described in the Atmospheric System. Natural and anthropogenic nitrogen oxides also contribute to acid rain.

Nitrogen - Essential for Life

Nitrogen is an essential element for life. Amino acids, which are the building blocks of proteins, contain nitrogen as NH2, the "amino" part of the molecule. The four building blocks of DNA [Adenine (A), Cytosine (C), Guanine (G), and Thymine (T)] consist of single or double rings of carbon and nitrogen atoms, with various side chains. Nitric oxide is a neurotransmitter.

Thus all living organisms require large amounts of nitrogen. However, in the form of N2, nitrogen is unusable by all organisms except for a few primitive bacteria that are capable of converting N2 gas to ammonia (NH3). This process of conversion is called nitrogen fixation, and makes the nitrogen available for use by organisms. In the atmosphere, nitrogen is fixed (i.e. N2 is converted to NH3) in three ways: (1) bacteria, (2) by humans through a manufacturing process called Haber process used in fertilizer production, and (3) through a chemical process initiated by lightning.

Certain bacteria are diazotrophs (or more simply, nitrogen-fixers). These bacteria possess an enzyme which converts N2 gas into NH3 or "fixes" the nitrogen.

Nitrogen-fixing bacteria
on soybean roots

Diazotrophs may be symbiotic, living as nodules in roots of plants such as legumes. A type of bacterial called cyanobacteria live on lichens, mosses, and ferns. Some cyanobacteria are free-living and capable of photosynthesis.

Thus nitrogen fixation is an important process for biological functioning. Legumes such as peas, clover, and beans have nitrogen-fixing bacteria in their roots. This enables them to grow in nitrogen-poor soil. Plants take up nitrates through their roots, and convert them into proteins and other compounds. Animals get their nitrogen from plants. Wastes and remains of animals and plants contain organic nitrogen compounds which are then broken down by bacteria and converted into compounds such as ammonia (NH3). Other bacteria (denitrifying bacteria), found especially in waterlogged soils, convert nitrates back into nitrogen gas and make it unavailable again. Plants can not use N2, and the N2 can therefore escape into the atmosphere. Farmers normally try to prevent the soil from becoming waterlogged. This is the problem with over-watering houseplants as well.

Lightning is an electrical discharge through the air, and can cause N2 and O2 molecules to change into the atomic form, combine with water to form weak nitric acid (HNO3), and precipitate atmospheric nitrogen to the earth, adding nitrogen to the soil in a usable form (nitrate, NO3). Inside plants and other organisms, the nitrates are converted into amino-acids and other vital compounds.

Modern agriculture uses artificial fertilizers such as ammonium nitrate (NH4NO3) to capture nitrogen. For example, if you examine the box of "plant food" that is a fertilizer designed for "acid-loving" plants such as azaleas or rhododendrons, you see the numbers 30-10-10, where the 30 stands for N, the first 10 for phosphorus, and the second 10 for potassium. This fertilizer then contains 30% of a nitrogen compound, mostly ammonium nitrate (NH4NO3) with some urea. While this improves yield, it upsets the natural balance of nitrogen in the ecosystem. Too much of nitrogen added to the soil through fertilizers washes out into ponds and rivers and causes overgrowth of algae in large patches. These algae blooms prevent light from entering the water and smother other aquatic life.

The Nitrogen Cycle is shown in Figure N1.

Figure N1: The Nitrogen Cycle.


Combustion and lightning fix nitrogen in the atmosphere. When plant matter (biomass) is burned, the organic fixed nitrogen is converted into nitrogen oxides and released. The clearing of forests by fire and burning of leftover debris from farmland creates large emissions of nitrogen oxide.

The oceans and sediments also contain large amounts of nitrogen as nitrates. Ammonia (NH3) is another form of fixed nitrogen. Ammonia is produced by bacteria after they consume organic matter. This accounts for the ammonia smell from the cat's litter-box resulting from the bacterial emissions. Before chlorofluorocarbons were invented, ammonia was the most common refrigerant. While the figure shows the main global routes of cycling nitrogen, in some locations (for example the Los Angeles basin, Mexico City, and in other industrial cities), nitrogen oxides (NOx) and nitric acid (HNO3) form a significant fraction of the local tropospheric environment.




  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.