Nitrogen Cycle and Oceanic Dead Zones


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The paper gives a description of the nitrogen cycle and its effects on the environment. It examines the role of nitrogen in life, and looks at how living organisms use fixed nitrogen. It describes the different ways through which nitrogen fixation occurs. The article takes note of recent development in the nitrogen cycle. It describes how ammonia-rich fertilizers contribute to the formation of dead zones and shows the effects of the dead zones to marine life. The paper describes how humans have changed the nitrogen cycle by increasing the amount of nitrogen in the atmosphere. Agriculture is one of the major human activities that are responsible for the increased nitrogen content. Other human activities such as burning of vegetation and deforestation are also included. The paper concludes by examining how people can eliminate dead zones or reduce their effects by embracing good farming practices.











Keywords: nitrogen, nitrogen fixing bacteria, dead zones, fixation, ammonia

Nitrogen Cycle and Oceanic Dead Zones

At eighty percent, nitrogen is the most abundant gas in the atmosphere. All living things require nitrogen as it helps in synthesizing organic molecules. Nitrogen is an important element of proteins and DNA. The nitrogen cycle shows the movement of nitrogen between the earth and the atmosphere (Burdass, 2005). It shows how nitrogen is converted to usable organic substances and how it is converted back to nitrogenous elements. Organisms can only used nitrogen that has been combined with hydrogen, a process known as nitrogen fixation. The fixation of nitrogen can occur naturally or it can be human induced. Natural fixation is only responsible for a very small percentage of the usable nitrogen; most of it comes from human activities. Atmospheric fixation is responsible for 5-8% of nitrogen fixation (Burdass, 2005). It occurs when there is lightning, since the lightning makes it possible for nitrogen and oxygen to combine, thus forming different nitrogen oxides.

Another method of ensuring nitrogen fixation is industrial fixation, during the Haber process. The process is mostly used to make fertilizers that contain nitrogen. During this process, nitrogen from the atmosphere is captured and combined with hydrogen under very high temperatures and pressures. The resulting compound is ammonia, which is used in making of the fertilizers. Fertilizers have contributed to increased food production around the world and it has ensured that more people have enough food. Fixation can also happen biologically. The nitrogen-fixing bacteria are responsible for fixing sixty percent of the nitrogen gas. The nitrogen-fixing bacteria can be free living in the soil, or they can be formed through a symbiotic association. Some of these associations can be formed with root nodule legumes such as peas and Rhizobium, or root nodules with non-legumes. Some of the fixed nitrogen includes ammonia, nitrous oxide and nitric oxide. Some of the organisms do not use ammonia since they find it poisonous.

The nitrifying bacteria present in plants converts ammonia ions into nitrite. Since it is poisonous to most organisms, it is then converted to nitrate by a nitrobacter, through a process known as nitrification. Nitrogen reenters the atmosphere through the process of denitrification, whereby nitrate is converted to nitrogen by the help of denitrifying bacteria. The process is usually most effective in wet conditions such as in swampy grounds, where oxygen is very little. When the gases are formed, they are then released back into the atmosphere and this completes the nitrogen cycle. Denitrification removes the nitrogen from the soil and this may cause soil infertility (Burdass, 2005). The ammonification process ensures that the nitrogen organic compounds are converted into ammonia, and this is enabled by decomposers. These organic forms of nitrogen can be in form of dead plants and animals, and they are converted into inorganic nitrogen.

Nitrogen is important in marine life and it affects the cycles of other elements such as phosphorus and carbon. It is present in many different chemical forms, and its conversion to other elements is enabled by marine organisms. Marine organisms use nitrogen for energy and as a way of synthesizing their structural components. Even in marine life, the most important and usable form of nutrient is not readily available. Marine organisms require dissolved nitrogen gas. Some of the research carried out indicates that nitrogen interferes with the biological productivity of marine life and changes in fixed nitrogen cause changes in the carbon dioxide available in the atmosphere. Some people however, disagree with this (Capone, 2008).

Humans have greatly altered the nitrogen cycle and this is reflected in marine life as well. The fixed organic nitrogen in the ocean is converted back to nitrates through the process of remineralization. This is made possible by ammonification and nitrification. Nitrification occurs by combining ammonium and nitrite oxidation. It is made possible by the presence of certain bacteria, which enhance the oxidation process. Nitrification can only happen when there is oxygen and light available (Capone, 2008). This means that it cannot happen in dead zones, where there is no oxygen, or in deep waters, where it is difficult for light to penetrate.

The nitrogen cycle continues to develop and recently, researchers discovered anaerobic ammonium oxidation (anammox) and anaerobic methane oxidation (ANME). Anammox uses nitrite or nitrate as the electron acceptor. The anammox bacteria are present in wastewater treatment systems, oxygen maximum zones, marine sediments and other ecosystems. Some of the conditions suitable for the growth of anammox include dissolved oxygen concentration, low concentrations of nitrite and a pH that ranges between 6.7 and 8.3. This means that anammox can be found in abundance in wetlands. The NC10 bacteria enable the creation of anaerobic methane bacteria. As the name suggests, it does not require any oxygen to thrive well (Zhu et al., 2010).

Anammox bacteria are negatively affected by higher concentrations of sulphides. Nitrite is also formed during the anammox process. It can be formed when ammonia oxidation process is incomplete or when there is limited substrate in the nitrate reduction during denitrification. The first step of nitrification involves using hydroxylamine to oxidize ammonia. This is made possible by an ammonia-oxidizing bacteria or crenarchea. Half of the nitrogen production in hypoxic marine ecosystems is enabled by anammox bacteria. When sewages, wastewater from industries and fertilizers containing nitrogen leach into wetlands, they decrease the amount of oxygen available. This increases the emission of ammonia and methane (Zhu, et al., 2010)

Although organisms need fixed nitrogen, too much of it can affect the atmosphere negatively. Some of the activities that can cause an increase in nitrogen are burning fossil fuels and grasslands, deforestation and draining marshes. Nitrate levels increase when forests are burned. Research conducted showed that the bacteria responsible for converting ammonia to nitrate, is found in abundance in places where forests are burnt. The same research indicated that charcoal enhanced nitrate production even after the fire had stopped (American Society of Agronomy, 2010). The production of industrial fertilizers is one of the major causes of increased nitrogen in the atmosphere. Without knowing it, people have increased nitrogen by destroying the natural vegetation, and planting crops such as soybeans and peas, which contain nitrogen-fixing bacteria. Nitric acid the source of acid rain and it is formed when nitric oxide dissolves in water.

The abundance of nutrients is called eutrophication. Although some people may consider this beneficial, this is not always the case. In freshwater bodies, eutrophication enhances the growth of aquatic weeds, and this can render the water body useless. Although plants require fixed nitrogen, a lot of it can cause damaging effects in the ecosystem and can cause soil infertility. The extra nitrogen filters the nutrients in the soil and increases the bacterial composers. These bacteria cause the formation of nitrate and release the hydrogen ions, which then cause soil acidity. When the nitrates dissolve, they percolate into water bodies and they dump minerals such as calcium and potassium. This causes the production of phytoplankton, which contribute to the formation of dead zones in the coastal regions affected. When the phytoplankton die, they take up the available oxygen, meaning that no life can survive in that area.

The abundance of nitrogen in the water enhances the growth of algae. When a coastal area becomes a dead zone, the living organisms die and some move away to other areas where the water is not deoxygenated (Tobin & Dushek, 2005). The U.S. Gulf of Mexico is one of the most recognized dead zones, especially during summer because of the flowing water from the snow. As the rivers and streams flow through different regions, they carry large amounts of nutrients from the farms and lawns, most of which use fertilizers containing nitrogen and other compounds. Dead zones change the ecology of a place, as some of the organisms move to other suitable regions. It has contributed to the decreasing number of marine life, since some animals, such as crabs, are not able to move from the hypoxic waters and they die (Hill, 2010).

Unfortunately, most of the dead zones occur in regions that provide edible fish to the surrounding communities and to the nation. In most cases, dead zones appear occasionally and when they occur, they cause major losses to these communities since they take away their source of food and their income. The fixed nitrogen gases such as ammonia are active greenhouse gases and they contribute to global warming. It is believed that nitrous oxide is a more potent greenhouse gas than carbon dioxide (Brown, 2011). It is produced during nitrification and denitrification and it is therefore available in abundance. Extra nitrogen fixation also contributes to the reduction of the protective ozone layer, and this causes other harmful effects. Some of the reactive nitrogen contributes to the production of aerosols, which are harmful to human health. They cause some cancers, respiratory and cardiac diseases. The nitrites, which percolate into drinking water cause blue baby syndrome (National Academy of Engineering, n. d.).

Most of the extra nitrogen in the atmosphere is caused by human activities. Through the process of denitrification, it is possible to reduce this extra nitrogen. Since a large part of the extra nitrogen comes from the manufacture and use of nitrogen-rich fertilizers, people can instead choose to reduce the amount of nitrogen in the fertilizers. People could also reduce the greenhouse gases such as nitrous oxide, which can be replaced by nitrogen molecules (National Academy of Engineering, n. d.). Reducing the use of fertilizers is a hard task especially in areas where people depend on farming. However, it is possible to reduce the runoff by creating buffer zones to absorb the runoff (Hill, 2010).

Although some advocate the use of manure over synthetic fertilizers, it is important to note that they too, cause problems. Animal waste is a major contributor of methane, which is a greenhouse gas and it contains nitrogen. One can control the nitrous oxide in the soil by adding nitrification inhibitors to the fertilizers and this ensures that ammonia does not mineralization fast. One way of doing this is by controlling the temperature. When the temperatures are low, ammonia is not released quickly. One of the areas where this is most applicable is in the formation of compost. When there is increased oxygen in the compost, less ammonia is produced. Reducing the temperature to a certain degree will also enhance decomposition (Brown, 2011). Good farming practices such as this will ensure that nitrous oxide is less in the atmosphere.

When ammonia is present in the soil it is taken up by the plants and converts to amino acids, which then convert into proteins. When an animal eats the plants, they take in the nitrogen and convert it into a more usable form. When they die and decompose the nitrogen is taken back to the atmosphere through denitrification, and the nitrogen cycle continues. One cannot deny the fact that the production of synthetic fertilizers have contributed to increased food production around the world. Despite this, excess nitrogen has caused many ecological changes, and has caused a lot of harm. One of the major negative effects has been the creation of dead zones, which has contributed to the reduction of some of the marine life. It is possible to reverse the effects of the extra nitrogen, although it is a challenging task. By adopting good agricultural practices such as good composting, and reducing the use of artificial fertilizers, one can ensure that the level of nitrous oxide, a potent by product of nitrogen, is reduced in the atmosphere.




American Society of Agronomy (2010). Forest fires help power the nitrogen cycle. ScienceDaily. Retrieved from

Brown, S. (2011). Climate change connections. Biocycle 52 (2) 52-54.

Burdass, D. (2005). The nitrogen cycle. Retrieved from

Capone, G. D. (2008). Nitrogen in the marine environment. Burlington, MA: Academic Press.

Hill, K. M. (2010). Understanding environmental pollution. United Kingdom: Cambridge University Press.

National Academy of Engineering. (n. d.). Manage the nitrogen cycle. Retrieved from

Tobin, J. A., & Dushek, J. (2005). Asking about life. New York, NY: Cengage Learning.

Zhu, G., Jetten, S. M., Kuschk, P., Ettwig, F. K., & Yin, C. (2010). Potential roles of anaerobic ammonium and methane oxidation in the nitrogen cycle of wetland ecosystems. Appl Microbiol Biotechnol 86, 1043-1055.


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