Nitrogen cycle

The nitrogen cycle describes the process of converting nitrogen to different chemical forms. These nitrogen forms circulate between the terrestrial environment, marine environment and the atmosphere.

The main processes in the nitrogen cycle include nitrogen fixation, ammonification, nitrification and denitrification (Figure 1). Different organisms can access different forms of nitrogen.

Image Figure 1 The nitrogen cycle. Sources: Wikimedia Commons CC BY-SA 4.0

The steps in the nitrogen cycle

The nitrogen cycle is broken down into key steps:

• Nitrogen fixation
• Nitrification
• Assimilation
• Ammonification
• Denitrification
• Anaerobic ammonia oxidation

Nitrogen fixation

Nitrogen fixation refers to the conversion of nitrogen gas (N₂) to nitrogen-containing compounds. This process occurs naturally via lightning, artificially by the Haber-Bosch process, or specialised nitrogen-fixing microorganisms.

Nitrogen fixation by lightning

In the atmosphere, two nitrogen atoms in the nitrogen molecule are held together by a strong triple covalent bond, i.e., the atoms share three pairs of electrons. Lightning carries enough energy to break the covalent bonds of the atmospheric nitrogen. When the nitrogen molecule splits, it bonds to atmospheric oxygen forming nitrogen oxides (NO_x).

Nitrogen fixation by the Haber-Bosch process

Human activities have become a large source of nitrogen fixation. Burning fossil fuels produces fixed nitrogen, while industrial nitrogen fixation is a byproduct of the Haber-Bosch process to produce ammonia. During the process, nitrogen gas combines with hydrogen gas in a high temperature and a high-pressure environment. The ammonia produced is widely used in synthetic fertilisers.

Nitrogen fixation by microorganisms

Free-living nitrogen-fixing bacteria can transform atmospheric nitrogen into inorganic nitrogen forms that plants can access. Plants will use inorganic nitrogen to make amino acids.

Mutualistic nitrogen-fixing bacteria, such as Rhizobium, live in plant nodes (e.g., peas and beans) and acquire carbohydrates from the plant. In turn, they provide amino acids to the plant. The relationship between these bacteria types and the plants is symbiotic or mutualistic.

Nitrification

Nitrification is a two-step oxidation reaction process converting ammonium ions into nitrate ions. The process involves:

1. Oxidation of ammonium ions (NH₄+) to nitrite ions (NO₂-) by Nitrosomonas and Nitrococcus bacteria.
2. Oxidation of nitrate ions (NO₂-) to nitrate ions (NO₃-) by Nitrobacter bacteria.

Nitrifying bacteria gain energy through these oxidation reactions. Both of the reactions require oxygen to occur; therefore, to increase productivity within agriculture, the soil needs to be kept well aerated.

Assimilation

Assimilation occurs when plants and animals use nitrate ions and ammonia to make amino acids and proteins. Nitrate ions and ammonia are formed by nitrogen fixation and nitrification, as we have just read.

Ammonification

Ammonification involves the conversion of organic nitrogen (nitrogen found in the cells of living organisms) into ammonia. Saprobiotic organisms (decomposers) are the key players in this process as they feed on organic matter, break it down and release ammonia. This ammonia now becomes available for nitrification and assimilation processes as it can be converted into ammonium ions.

Denitrification

Denitrification refers to reducing nitrate ions to nitrogen gas by anaerobic bacteria. This process can only happen in anaerobic conditions (no or very little oxygen available). For example, in wetlands, the soil tends to be waterlogged, which sets up the perfect environment for anaerobic bacteria as little oxygen is found in the soil.

Anaerobic ammonia oxidation

During this process, ammonium ions (NH₄+) and nitrite (NO^-2) are converted into atmospheric nitrogen (anammox reaction). This step is a significant process in oceans and needs to occur in anaerobic conditions (absence of oxygen).

The formula for this is:

Why is the nitrogen cycle important to plants and animals?

Nitrogen is one of the main elements used to build a life. It is a component of many cells and is a building block in amino acids, proteins and nucleic acids. In plants, nitrogen is also used to make chlorophyll, a critical pigment involved in trapping light for photosynthesis.

What is the human impact on the nitrogen cycle?

Disruption of the nitrogen cycle will lead to imbalances in the ecosystems. For example, soil containing excess nitrogen is in danger of having low pH, meaning the soil will be too acidic. Soil with a low pH is harmful to bacteria and microorganisms that cannot survive in these conditions. In the oceans and rivers, the accumulation of nitrogen run-off will lead to eutrophication. Eutrophication is the process whereby nutrient levels become excessive in water bodies. You can learn more about this in our article Fertilisers.

Burning of the fossil fuels

The burning of fossil fuels will release nitrogen and nitric oxides into the atmosphere. These actions affect air quality, leading to the production of acid rain and smog production.

Use of nitrogen-containing fertilisers

Fertilisers increase plant growth and overall crop productivity. However, the excessive use of nitrogen-containing fertilisers creates a harmful imbalance of the soil's nutrient levels. These harmful effects can lead to:

• Reduced species diversity. Nitrogen-rich soils will favour the growth of grasses and other rapidly growing vegetation. Rapidly growing vegetation will outcompete other species and reduce biodiversity.
• Leaching. Nutrients will enter streams, rivers and oceans.
• Eutrophication. This is a consequence of leaching as the excessive accumulation of nutrients harms microorganisms and wildlife in water bodies.

Removal of vegetation

Leaching is the removal of nutrients from the soil. For example, removing plants and trees will remove nutrients with them. In addition to this, the soluble nutrients left, such as nitrate ions, will dissolve in rainwater. These dissolved nutrients will be carried deeper into the soil and eventually, the nutrients will travel too deep into the soil for the plant to reach them.

Reducing the human impact

To reduce the adverse effects of human activity on the nitrogen cycle, individuals and companies need to be mindful of their nitrogen footprint. Changes such as choosing renewable sources can lead to the gradual reduction of the amount of fossil fuels burned. Even choosing to eat less red meat can have a positive impact as cattle farming requires a large amount of feed, which is grown with fertilisers.

Nitrogen monitoring is another approach that the agriculture sector could benefit from. This describes the careful monitoring of nutrient levels in soil, minimising the toxic effects of ammonia build-up and, therefore, the effect of eutrophication from nutrient run-off.