The Nitrogen Cycle Explained

Nitrogen is necessary for all life on Earth. Nitrogen is an important component in plant and animal cells. Organisms need nitrogen to produce proteins, nucleic acids, and amino acids. While nitrogen gas (mostly N₂) makes up 78% of the Earth’s atmosphere, it is not usable in that form by most plants and animals. Most living things can only use nitrogen when combined with carbon, hydrogen, or oxygen. The nitrogen cycle describes the way in which microorganisms transform atmospheric nitrogen and the nitrogen contained in organic matter into usable compounds. The steps of the nitrogen cycle are: nitrogen fixation, assimilation, ammonification, nitrification, dentrification, and anaerobic ammonium oxidation.

Processes in the Nitrogen Cycle

The nitrogen cycle begins with atmospheric nitrogen. To be useful to plant and animal life, N2 must be combined (or “fixed”) with hydrogen to form ammonium (NH₄) ions or with oxygen (NO₃) to form nitrate ions. Microorganisms in the Earth’s soil convert N₂ into ammonia. This process is called “nitrogen fixation” and the microorganisms that accomplish it include free-living bacteria, such as Azotobacter, Purple sulphur bacteria, some Cyanobacteria, as well as bacteria that live symbiotically with legumes and other plants, such as Rhizobium.

Plants can use either ammonium or nitrate, which they absorb and use to create organic molecules, like DNA and amino acids. This process is called “assimilation”. Animals, which cannot absorb nitrates, obtain these nutrients by eating plants or animals that consume plants. Ammonification begins when plants and animals die and decompose, or when animals excrete waste. Decomposing bacteria convert organic nitrogen nutrients back to ammonia and ammonium salts. Nitrification occurs when nitrifying bacteria convert ammonium to nitrate. These bacteria obtain their own energy though the oxidation of ammonium and by obtaining carbon from carbon dioxide in order to produce organic compounds. Certain species of these bacteria make nitrite (NO₂) from ammonium and others make nitrate (NO₃-) from nitrite.

Denitrification occurs when microorganisms convert nitrate (NO₃-) into the gases nitric oxide (NO), nitrous oxide (N₂O), and elemental nitrogen (N₂). The bacteria that perform denitrification live in wet soil in anaerobic conditions. They grow on organic matter and use nitrate, instead of oxygen, as an electron receptor in anaerobic respiration. Anaerobic ammonium oxidation, or the annamox reaction, occurs when the Brocadia anammoxidans bacterium converts nitrite and ammonium into nitrogen gas (N₂). This nitrogen gas returns to the atmosphere. This process is responsible for over half of the oceanic nitrogen.

Human Alteration of the Cycle

Humans can add to or subtract from the amount of nitrogen entering the nitrogen cycle. For example, farmers add nitrogen compounds to the soil in their fields as fertilizer. Human activity now accounts for half of the amount of nitrogen fixation annually. The intensive farming of crops like legumes, such as alfalfa and soybeans, also increases the amount of nitrogen-fixing microorganisms in the soil. Human burning of fossil fuels in automobiles, power plants, and industrial plants add nitrogen oxide to the air, which causes the atmospheric nitrogen molecules to break apart.

Humans have changed the nitrogen cycle by increasing the concentration of nitrous oxide (N₂O) and ammonia (NH₃) in the atmosphere. Humans have also increased the concentration of other nitrogen oxides, such as nitric oxide (NO), in certain regions. Through farming, humans have contributed to the loss of calcium and potassium in the soil, and to the acidification of soil and water in some regions. Increased nitrogen runoff in rivers and streams is a pollutant and dangerous to some animal and plant life. The increased amount of nitrogen entering the cycle has endangered plants that thrive in low-nitrogen environments, which has in turn affected the animals that depend on them. Everything in the environment is linked together and to disrupt one aspect of the chain will have an effect on each link in the environmental chain.

The nitrogen cycle provides essential nutrients for plants and animals. Failure to maintain the proper balance of the cycle affects various ecosystems, endangering plant and animal life and human food resources. Human alteration of the cycle also adds to greenhouse gases, acid rain, and pollution. Technological solutions to restore the nitrogen fixation-denitrification balance are being explored, but more research is still needed. Understanding the nitrogen cycle is essential to a well-rounded understanding of the environment and how each aspect is interconnected and effected by other elements.

Additional Resources

• Nitrogen Cycle: An overview of the nitrogen cycle. Includes ways nitrogen is lost and the impact of the nitrogen cycle on water quality.

• The Nitrogen Cycle and Humans: Includes information and graphics on the nitrogen cycle and how changes in the nitrogen cycle change the environment.

• Nitrogen Cycle: This site goes through each step of the nitrogen cycle and provides graphics.

• Human Alteration on the Nitrogen Cycle: A link to an article published in Issues in Ecology, published by the Ecological Society of America on the effects of human activities on the nitrogen cycle.

• Nitrogen: Site includes information on nitrogen and its properties, a simplified and a detailed explanation of the nitrogen cycle and information on how humans affect the cycle.

• Animated Nitrogen Cycle: This site includes an animated image of the nitrogen cycle with explanations of each part.

• Processes of the Nitrogen Cycle: This site gives information on each of the processes and sources in the nitrogen cycle.

• The Nitrogen Cycle on a Farm: A flash slide show of the nitrogen cycle on a farm.

• Alterations to the Nitrogen Cycle: A resource for engineers detailing human alterations to the nitrogen cycle and potential ways to mitigate them.