What Is The Nitrogen Cycle?

by Ella Barnes

The importance of the nitrogen cycle cannot be understated. Nitrogen is a key component of amino acids, from which plant and animal cells and tissue are made [1]. It constitutes around 79% of the earth’s atmosphere [2]. However, plants and animals cannot absorb it in its atmospheric form [2]. Therefore it must undergo various processes in order to become useful to the life forms that depend on it. This process of transformation is called the nitrogen cycle.

Steps of the nitrogen cycle

Firstly, atmospheric nitrogen is absorbed into soil where it undergoes ‘nitrogen fixation’. Nitrogen fixation is the conversion of nitrogen gas into usable forms such as nitrates [3]. This stage of the nitrogen cycle is carried out by nitrogen fixing bacteria that exist in the soil [3]. Some of these bacteria are attached to plant roots that directly absorb the nitrates [2]. Other nitrogen fixing bacteria live freely in the soil [2]. 

‘Nitrogen fixation’ also refers to any industrial process which fixes nitrogen, such as the use of nitrogen-based fertilisers. It refers to any process by which nitrogen is transformed from an inert gas into a more useful compound [3]. 

Plants absorb nitrates through their roots and use them to grow [2]. Animals and humans absorb these nitrates when they eat plants and other life forms. When dead plants, animals, and faeces decompose they return nitrogen to the soil in the form of ammonia [4]. 

Ammonia is then transformed into nitrates by nitrifying bacteria in the soil and reabsorbed by plant roots [4]. Ammonia is also what is found in nitrogen-based fertilisers, so it may enter the soil in this way as well [3].

The final stage of the nitrogen cycle is denitrification whereby bacteria convert nitrates back to gaseous nitrogen and return it to the atmosphere [4].

Human impact on the nitrogen cycle

Human intervention has drastically impacted the nitrogen cycle, particularly since the invention of the Haber-Bosch process in the 1940s [4]. The Haber-Bosch process converts hydrogen and nitrogen into ammonia, which can be used as a fertiliser to increase nitrogen levels in the soil thus encouraging crop growth [5]. 80% of the nitrogen found in humans today can be attributed to the Haber-Bosch process [6]. 50% of the world’s food production depends on ammonia fertilisers[5].

Fossil fuel burning has also contributed to an increase of nitrogen in the atmosphere. Fossil fuel combustion produces nitric oxide which ends up in the air and causes, for example, smog and acid rain [7]. 

Impact of increased nitrogen levels

The human-caused increase of nitrogen levels in the environment has widespread impacts, as an excess of nitrogen is highly damaging.

Let’s first look at the effect of unbalancing the nitrogen cycle through agricultural fertilisers. When there is too much nitrogen in the soil, excess nitrogen in the form of nitrites remains in the soil and leaches out into streams and rivers, resulting in a process called eutrophication [4].

Eutrophication is caused by an overgrowth of algae due to high levels of nitrogen. When the algae dies the microbes in the water use up so much oxygen decomposing it that there is not enough left to support other life forms [2]. This creates something called a ‘dead zone’ or ‘hypoxic zone’.

There is a dead zone off the gulf of Mexico that covers 6,334 square miles or 4 million acres of ocean. The oxygen levels there are so low that very little life can survive [8]. The largest dead zone ever recorded is off the Gulf of Oman and covers 63,000 square miles [12].

The other key impact is nitrogen pollution. Burning fossil fuels releases nitrous oxide. A high concentration of NO₂ causes respiratory problems in humans, such as the development of asthma and an increased risk of respiratory infections [9]. 

A 2018 Greenpeace analysis of global NO2 air pollution identified hotspots across 6 continents, with concentrations around cities and power plants [13].

Nitrous oxide also causes acid rain, which is harmful for ecosystems as many species cannot survive when the pH level, a measure of acidity, of their environment is altered dramatically [10].

Finally, nitrous oxide is a dangerous greenhouse gas. It is 300 times more effective at trapping heat in the atmosphere than carbon dioxide [11].

In conclusion, the nitrogen cycle is fundamental to the survival of all life forms, as it allows for nitrogen to be absorbed and used in the production cells and tissues. Human intervention in the nitrogen cycle through the use of fertilisers and fossil fuels has led to an excess of nitrogen, unbalancing the cycle with damaging effects, such as ocean dead zones and air pollution. 

References: 

[1] G Hanrahan, G Chan, 2005, Nitrogen, Encyclopaedia of Analytical Science, California State University, LA, https://doi.org/10.1016/B0-12-369397-7/00401-5
[2] Miriam R. Aczel, 2019,  What is the Nitrogen Cycle and Why Is It Key to Life?, https://kids.frontiersin.org/articles/10.3389/frym.2019.00041#figure-3 , accessed on 26th November 2021
[3] Encyclopaedia Britannica, https://www.britannica.com/science/nitrogen-fixation , accessed on 26th November 2021
[4] Anne Bernard, 2010, The Nitrogen Cycle: Processes, Players, and Human Impact, https://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632/ , accessed on 26th November 2021
[5] Leigh Krietsch Boerner, 2019, Industrial ammonia production emits more CO2 than any other chemical-making reaction. Chemists want to change that, https://cen.acs.org/environment/green-chemistry/Industrial-ammonia-production-emits-CO2/97/i24 , accessed on 26th November 2021
[6] Robert W.Howarth, 2008, Coastal nitrogen pollution: A review of sources and trends globally and regionally, Harmful Algae, Volume 8 Issue 1, Department of Ecology and Evolutionary Biology, E309 Corson Hall, Cornell University, Ithaca, NY 14853, USA, https://doi.org/10.1016/j.hal.2008.08.015
[7] How Humans Have Disrupted The Nitrogen Cycle, 2009, Science Daily, https://www.sciencedaily.com/releases/2009/06/090604144322.htm , accessed on 26th November 2021
[8] Northern Gulf of Mexico Hypoxic Zone, United States Environmental Protection Agency, https://www.epa.gov/ms-htf/northern-gulf-mexico-hypoxic-zone , accessed on 26th November 2021
[9] Basic Information about NO₂, United States Environmental Protection Agency, https://www.epa.gov/no2-pollution/basic-information-about-no2#Effects , accessed on 26th November 2021
[10] Effects of Acid Rain, United States Environmental Protection Agency, https://www.epa.gov/acidrain/effects-acid-rain , accessed on 26th November 2021
[11] Ee Ling Ng, Deli Chen, Robert Edis, 2016, Nitrogen pollution: the forgotten element of climate change, https://theconversation.com/nitrogen-pollution-the-forgotten-element-of-climate-change-69348 , accessed on 26th November 2021
[12] Jenny Howard, 2019, Dead zones explained, https://www.nationalgeographic.com/environment/article/dead-zones#:~:text=The%20largest%20dead%20zone%20in,6%2C000%20square%20miles%20in%20size, accessed 12 December 2021
[13] Lauri Myllyvirta, Emma Howard, 2018, Mapped: Nitrogen dioxide pollution around the world,https://unearthed.greenpeace.org/2018/10/29/nitrogen-dioxide-no2-pollution-world-map/, accessed 12th December 2021