Nutrient Cycles
What Are the Processes and Features of the Nitrogen Cycle?
The Nitrogen Cycle
Nitrogen is an important component of amino acids and nucleic acids, which are critical to all living things. On land, this nitrogen is made available to living things when plants absorb it as a nutrient from the soil.
Nitrogen makes up about 80% of Earth’s atmosphere, which is the reservoir of nitrogen for living things. However, plants can only use nitrogen in the forms of ammonium (NH4+), nitrate (NO3–), and organic forms such as amino acids. Animals can only use nitrogen in organic forms that they get from food. Atmospheric nitrogen is primarily made available to these organisms through nitrogen-fixing bacteriaglossary term (opens in a new window).
The nitrogen cycleglossary term (opens in a new window) is the series of chemical reactions by which nitrogen is converted among its various chemical forms. The nitrogen cycle can be broken down into five main phases:
- Some species of bacteriaglossary term (opens in a new window) and archaeaglossary term (opens in a new window) convert atmospheric nitrogen into nitrogen compounds into the soil.
- Plants absorb nitrogen compounds from the soil and change it into their own molecules.
- Animals receive nitrogen compounds when they eat plants.
- Both animals and plants return nitrogen compounds to the soil through waste and decomposition.
- Some bacteria decompose nitrogen compounds in the soil into nitrogen gas.
The processes of the nitrogen cycle include fixation, mineralization (ammonification), nitrification, and denitrification. The primary way in which usable nitrogen enters an ecosystem is through nitrogen-fixing bacteria. Many of these live in close association with plants. Some live inside the roots of plants in root nodules. In the fixation process, certain species of bacteria convert N2 gas from the atmosphere into ammonia (NH3). Some bacteria perform nitrification, during which ammonium ions (NH4–) are converted into nitrate. Once nitrogen has been converted into ammonium or nitrate ions, it can be absorbed by plants.
The process of mineralization or ammonification decomposes organic nitrogen into ammonium compounds.
Other bacteria use nitrate instead of oxygen during cellular respiration. These bacteria are performing denitrification, converting nitrate back into N2 gas.
Biological systems work to achieve and maintain balance. However, the ecological function of the microorganisms can both contribute to maintaining the health of organisms and ecosystems, and contribute to disrupting the health of organisms and ecosystems.
For example, since the agricultural boom in the 19th and 20th centuries, there has been an increased use of nitrogen-containing fertilizers. There are many problems associated with over-fertilization. Excess inorganic nitrogen added to soil can be harmful to plants. Also, these fertilizers often lack other important minerals naturally found in soil. Continuously growing crops using such fertilizers without adding these mineral supplements can decrease the concentration of these minerals, making soil less productive.
Runoff rich in nitrogen compounds can also cause problems in aquatic environments. Nitrogen compounds can fertilize algae, causing them to grow at an accelerated rate, producing an algal bloom. The algae can become overcrowded and die. Microorganisms, such as bacteria, use the oxygen in the water as they decompose the dead algae. The microorganisms can use so much oxygen that other living things in the water, such as fish, suffocate.
Teacher Note: Misconception
Students may think that because nitrogen and phosphorus are nutrients, adding more to the environment must always be beneficial to the environment. In excess, nitrogen and phosphorus, as well as atmospheric carbon, can disrupt the flow of nutrient cycles and cause harm to living things.
