Sarah Taylor

The Nitrogen Cycle Study Guide

Nitrogen is the most abundant element on the earth but is the least available one. The nitrogen cycle is how naturally the chemical nitrogen is made available in biological and living processes.


Approximately 78% of the earth’s atmosphere is covered by nitrogen gas. However, this complete amount is not available for use by biological processes.

Nitrogen is needed to build the most vital macromolecule of life - proteins and nucleic acids.

It gives structure and support to living beings by being components of proteins and nucleic acids.

Animals get nitrogen through the plant and animal diet they consume. Plants require nitrogen gas for their growth and reproduction. Plants tap into CO2 and light energy to form energy resources. Likewise, plants also tap into the nitrogen gas in the atmosphere to make it part of the life processes.

Nitrogen Cycle

Let us look at the basic components that define the nitrogen cycle.

  • The nitrogen cycle fixes the nitrogen from the atmosphere into the soil for use by plants and animals.
  • They are then demineralized upon their death and released into the soil to release nitrogen gas into the atmosphere.
  • The cycle is a continuous process. Both aerobic and anaerobic processes occur in the soil, plants, and animals and back in the soil to the atmosphere.
  • In short, the nitrogen element is transferred and exchanged continuously between the living and non-living things, which are the atmosphere, soil, bacteria, water, plants, animals, and cyanobacteria.
  • Nitrogen Cycle Source

What is the Nitrogen Cycle?

A simple nitrogen cycle consists of five steps that are of biological importance.

- Nitrogen fixation:

The fixation of atmospheric nitrogen to the soil and the plants. - Nitrogen fixation can be a physical, chemical, or biological process, of which biological process is of great significance. Cyanobacteria and other bacteria fix nitrogen in the soil and the root zone environment. - A group of bacteria called Rhizobium lives in the roots of bacteria in specialized structures called nodules which fix the nitrogen directly to the plants. - Nitrogenase is the enzyme involved in the process of nitrogen fixation. This process does not require oxygen and occurs in an anaerobic environment.

- Nitrification:

The process of conversion of reduced nitrogen compounds (mainly ammonia) to nitrate and nitrite - Nitrification is an oxygen-requiring process occurring in the soil mediated by soil bacteria Nitrosomonas and Nitrobacter.

- Assimilation:

The process of incorporation of nitrates and ammonia into the tissues of the plants and animals as components of protein and nucleic acids

- Ammonification:

The process of conversion of organic nitrogen to ammonia

- Denitrification:

The process of reducing ammonia to nitrogen gas released back into the atmosphere -Denitrification is also an anaerobic process in the soil converting nitrate to nitrogen gas. This is the soil checkpoint for nitrogen loss back to the atmosphere as nitrogen gas.


  • The nitrogen cycle helps make the nitrogen available to the plants, which is the common limiting nutrient in nature and agriculture.
  • It is supplied as fertilizer because what is available in the soil is not enough to meet the needs for plant growth in agricultural lands.
  • Plants lacking nitrogen supply turn yellowish and have smaller flowers and fruits, thereby reducing the total yield.
  • However, boosting the nitrogen cycle is important in these soils and can aid in nitrogen fixation by reducing the need to add fertilizers.
  • When excess organic nitrogen runs off into the water resources, it boosts the growth of organic forms; especially aquatic flora called eutrophication, which makes the water toxic for use and life.


1. What are the 5 stages of the nitrogen cycle?

The five stages of the nitrogen cycle are nitrogen fixation, nitrification, assimilation, ammonification, and denitrification.

2. How do you explain the nitrogen cycle?

The nitrogen cycle is the process of converting the atmospheric nitrogen to be used for plants in the soil by the process of nitrogen fixation and nitrification, which are assimilated into plants and animals, brought back to the soil as organic nitrogenous products through the process of ammonification, and then converted back to nitrogen gas to be released into the atmosphere through the process of denitrification.

3. What are the 7 steps of the nitrogen cycle?

The seven steps of the nitrogen cycle are nitrogen fixation, assimilation, ammonification, nitrification, denitrification, dissimilatory nitrate reduction to ammonia, anaerobic ammonia oxidation, and other processes.

4. What is the function of the nitrogen cycle?

The nitrogen cycle is the biogeochemical cycle. The atmospheric free nitrogen molecules are converted into inorganic and organic forms of nitrogen products fixed into the plants and animals as part of the macromolecular compounds like proteins and nucleic acids.

5. What is the order of the nitrogen cycle?

The order of the nitrogen cycle is nitrogen fixation, assimilation, ammonification, nitrification, and denitrification.

6. What is the first step in the nitrogen cycle?

The first step in the nitrogen cycle is nitrogen fixation carried out in the soil in the absence of oxygen by soil bacteria and cyanobacteria.

7. What are biological nitrogen fixers?

The biological nitrogen fixers are nitrogen-fixing soil bacteria, rhizobium bacteria, blue-green algae, and lightning.

We hope you enjoyed studying this lesson and learned something cool about the Nitrogen Cycle! Join our Discord community to get any questions you may have answered and to engage with other students just like you! We promise, it makes studying much more fun! 😎


  1. Nitrogen Cycle. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/6.7/primary/lesson/nitrogen-cycle-bio/. Accessed on 30 Nov, 2021.
  2. What is the nitrogen cycle?. https://www.sciencefacts.net/nitrogen-cycle.html. Accessed on 30 Nov, 2021.
  3. What Is the Nitrogen Cycle and Why Is It Key to Life?. https://kids.frontiersin.org/articles/10.3389/frym.2019.00041. Accessed on 30 Nov, 2021.