What is the Nitrogen Cycle?

The Nitrogen Cycle is a repeating process during which nitrogen moves through both living and non-living things: the atmosphere, soil, water, plants, animals, and bacteria. In order to move through the cycle, nitrogen must change forms. Although nitrogen gas makes up most of Earth's atmosphere, plants cannot use this gas to make organic compounds for themselves and other organisms.1 In soil, Nitrogen exists as nitrogen oxide (NO) and nitrogen dioxide (NO2). Nitrogen can also be found in other forms when used in fertilizer, such as ammonia (NH3), which can be further processed into a different fertilizer ammonium nitrate (NH4NO3).2

Steps of the Nitrogen Cycle

The first stage of the nitrogen cycle is known as nitrogen fixation. In this stage, nitrogen moves from the atmosphere to the soil. Although the nitrogen moves to the soil, it is still unusable to the plants within the soil.4 This step is essential in the process of making nitrogen usable for plants. It is when special bacteria convert the nitrogen gas (N2) to ammonia (NH3) which the plants can use.5 In nature, most nitrogen is harvested from the atmosphere by microorganisms to form ammonia, nitrites, and nitrates that can be used by plants.6

The second stage of the nitrogen cycle is called nitrification.7 This stage takes place in the soil. Nitrogen moves from organic materials, such as manure or plant materials to an inorganic form of nitrogen that plants can use. Eventually, the plant’s nutrients are used up and the plant dies and decomposes. All of the plant matter goes back into the nitrogen cycle in the end. This is the process by which ammonia (NH3) or ammonium (NH4+) is converted to nitrate (No3-).8 The first step that happens is the soil bacteria Nitrosomanas and Nitrococcus convert NH3 to NO2-.9 After another soil bacterium oxidizes NO2- AND NO3-. The bacteria gains energy through these conversions and both require oxygen.10

Step 5 of the nitrogen cycle in Denitrification.11 Denitrification is the reduction of nitrates back into nitrogen gas (N2).12 This process is performed by bacterial species such as Pseudomonas and Paracoccus, under anaerobic conditions.13
They use nitrate as an electron acceptor in the place of oxygen during respiration14.15

The nitrogen cycle was discovered by Jules Reiset,16 three decades after it was discovered that plants need nitrogen to live. Reiset recognized that decaying organic matter releases nitrogen in 1856. Seeing that plants require nitrogen and decomposing ones release it, he discovered the Nitrogen Cycle.17


The nitrogen cycle is the way that in nature, nitrogen is converted into different forms which are used by living organisms. Nitrogen is an essential part of proteins, DNA, and RNA. Approximately 78% of the atmosphere is nitrogen, but is not usable by plants and animals. In terrestrial ecosystems the incorporation of nitrogen can create disproportion such as: nutrient imbalance in trees, changes in forest health, and decline in biodiversity. Though there are different forms of nitrogen, nitrogen is an odorless, colorless, and tasteless element.19

Understanding the Nitrogen Cycle

Student-made video on the Nitrogen Cycle