Nitrogen Cycle

The nitrogen cycle describes the stages in which the important gas nitrogen is converted and circulated from the nonliving world to the living world and back again. There are five main steps in the nitrogen cycle, four of which are carried out by bacteria. Since nitrogen cannot be used by the cells of living things until it has been converted by bacteria into a useful form, these nitrogen-fixing bacteria not only play a key role in the nitrogen cycle but are in fact essential to all life on Earth.

All plants and animals need a certain amount of nitrogen in their systems in order to live. Nitrogen is an important component of amino acids, which are the building blocks of life, and nucleic acids, which make up genetic material. Although nitrogen is a free gas (it remains a gas under normal atmospheric conditions) that makes up nearly 80 percent of Earth's atmosphere, only bacteria can use it in this form. This means that for every plant and animal on Earth, the abundant nitrogen in the atmosphere is entirely useless unless it is somehow combined with hydrogen or oxygen. Bacteria that produce compounds of nitrogen, called nitrates, work to form this necessary combination. Most nitrates are produced by bacteria in the soil and are therefore absorbed by plants. Animals obtain the nitrates either by eating plants or other animals that eat plants. When plants and animals die, the nitrates in their systems are returned to the atmosphere and back to the beginning of the nitrogen cycle.

THE FIVE STEPS OF THE NITROGEN CYCLE

The nitrogen cycle has five main steps: fixation, nitrification, assimilation, ammonification, and denitrification. All except the third step (assimilation) are carried out by bacteria.

Nitrogen Fixation. The first step, nitrogen fixation, is probably the most important since it frees what was previously unusable nitrogen. Only a few types of bacteria are able to break the chemical bonds that hold the paired atoms in nitrogen gas together. Nitrogen-fixing bacteria that live in the soil have an enzyme called nitrogenase that they use to break that bond or "fix" and capture nitrogen gas from the atmosphere and convert it to useable ammonia. Blue-green algae, called cyanobacteria, is one example of bacteria that can change atmospheric nitrogen to ammonia.

Nitrification. The second step in the cycle, called nitrification, occurs when the converted nitrogen is combined with oyxgen and hydrogen. This combination results in the production of nitrogen compounds, like nitrates,

that other organisms can use. During the third stage, assimilation, plants take up these nitrates with their roots and use the nitrates to build proteins and nucleic acids. In other words, the plants use the nitrates to grow. Animals, like plants, also need proteins to function and grow, and proteins are composed of amino acids that contain nitrogen atoms.

Ammonification. The fourth stage in the nitrogen cycle is called ammonification. This is the process in which these nitrogen compounds are returned to the soil and reconverted to ammonia by other bacteria. Ammonification occurs in two ways. The first is when a plant or animal dies and their organic matter is acted upon by decomposer bacteria. These bacteria play an important role because the nitrogen trapped in dead animals and plants would be useless and wasted were it not converted. A second way for ammonification to occur is when animals leave their urine and feces on the ground. Almost immediately by the same decomposer bacteria work to release the nitrogen found in these waste products.

Denitrification. The last step in the nitrogen cycle is called denitrification and involves the reduction of ammonia to nitrogen gas. Again, this process is carried out by a certain type of bacteria whose actions eventually release nitrogen gas back into the atmosphere. The complete nitrogen cycle is, therefore, a process in which nitrogen from the atmosphere is captured and converted, passes through a number of organisms (including bacteria), and is finally returned to the atmosphere to enter the cycle again.

There is a more dramatic way that nitrogen is converted to a usable form. The atmosphere of Earth is roughly four parts nitrogen to one part oxygen, and these two gases never normally react with one another. However, during a thunderstorm when lightning strikes, the temperature and the pressure of the air is such that the two gases react chemically and form nitrous oxide gases. In the rainwater that accompanies the storm, nitrous oxide is dissolved and falls to Earth as nitric acid. This enters the nitrogen cycle when it soaks into the soil and forms nitrites and nitrates. However, far more nitrates are formed by bacteria in the soil than by atmospheric lightning flashes.