Nitrogen cycle
The nitrogen cycle is a repeating circulation of the element nitrogen in various chemical forms throughout living and non-living things on Earth. By changing forms nitrogen is able to is able to move from the atmosphere, as a gas, to a form that is usable by plant life. The nitrogen cycle can be divided into several processes including: nitrogen fixation, assimilation, ammonification, nitrification, and denitrification. Other processes have been considered in this cycle as scientific research continues.[1]
The nitrogen cycle allows for the continued maintenance of healthy productive ecosystems. The alteration of nitrogen levels can greatly affect plant production and biomass in our environment. The nitrogen cycle allows us to understand how to better grow crops in agriculture to maintain a food supply for the human population but also limit fertilizer pollution in soils that can lead to eutrophication.
Nitrogen
Nitrogen is a critical nutrient in the survival and success of all organisms [2]. Around 78% of the Earth’s atmosphere is made up of nitrogen. This nitrogen in the atmosphere occurs as dinitrogen gas (N2) and is unable to be used directly by living organisms such as plants which can limit nitrogen availability ecosystems [3]. The nitrogen cycle is a key component in many ecosystem processes such as decomposition and primary production. Nitrogen availability can alter the rate of these processes. Nitrogen has several forms including dinitrogen gas (N2), nitrogen oxide (NO), nitrogen dioxide (NO2), ammonia (NH3), ammonium (NH4 +), and ammonium nitrate (NH4NO3).
Processes
Through a series of processes nitrogen can be converted by microbial activities through fixation, assimilation, ammonification, nitrification, and denitrification.[4] These processes make -up the nitrogen cycle and play an important role for all living organisms on Earth.
Nitrogen fixation:
Nitrogen fixation is the process by which nitrogen gas (N2), is transformed into a form, ammonium (NH4-), of nitrogen that can be used by plants. Through this process nitrogen is moved from the atmosphere into the soil on Earth where plant can absorb it through their root system. A small percentage of fixation can occur via abiotic means such as lightening. A majority of nitrogen fixation occurs naturally in
soils by bacteria that have a symbiotic relationship with the plants [5]. In exchange for energy from photosynthesis the bacteria will fix nitrogen into a usable form for the plant by using the enzyme nitrogenase. Nitrogen fixation by bacteria can also produce forms of nitrogen that can be utilized by various organisms. this fixation process requires a great deal of energy and therefore uses a lot of ATP.
Assimilation:
Assimilation of inorganic nitrogen (nitrates and ammonium) is the process by which organic nitrogen compounds form from inorganic nitrogen compounds in an ecosystem. Plants use these ions to make proteins and nucleic acids [6]. Assimilation of nitrate and ammonium is sometimes necessary for plant, fungi and bacteria organism that are unable to fix nitrogen gas from the atmosphere. Nitrogen assimilation requires ATP and reduced ferredoxin from photosynthesizing cells in plants [7].
Ammonification/ mineralization:
Soil nitrogen can be derived from dead organic materials. Ammonification or mineralization is the process where amino acid and organic compounds are decomposed to produce ammonia and ammonium ions. This ammonia is then readily available for uptake by plants and microorganisms that require it for growth [8].
Nitrification:
Nitrification is a two-part oxidation process of ammonia into nitrates and nitrites moderated by many microbial communities in the ecosystem [9]. This process provides extra available nitrogen for plants to take in via their roots. Through the process of nitrification, ammonia (NH3), produced by ammonification, found in soils is transformed into nitrites (NO2-) and nitrates (NO3-). Nitrates are able to be used by plants and plant consuming animals and are formed by ammonia-oxidizing bacteria. Nitrites are not readily available to plants and animal but can be converted to nitrates bacteria. These nitrite-oxidizing bacteria receive energy in exchange for this process [10].
Denitrification:
Denitrification follows the process of nitrification and is where nitrates are returned to the atmosphere as nitrogen gas by denitrifying bacteria in soils [6]. Denitrification generally occurs in anoxic environments with exhausted oxygen levels. This process can lead to a loss in soil nitrogen content which needs to be replaced.
Anthropogenic Changes:
Anthropogenic activities have greatly altered the nitrogen cycle through, fossil fuel combustion, extensive cultivation of legumes and the construction of fertilizers using the Haber-Bosch process. The human use of nitrogen fixation has increased food production but has led to an increase in nitrogen being emitted into the atmosphere [12]. This build up of excess nitrogen can drain from soils into water sources underground or enter water systems via runoff. Nitrogen build up leads to eutrophication, extreme nitrogen levels, leading to issues such as algae blooms due to nitrogen enrichment in the water. This process can decrease oxygen level and have a more last effect on an aquatic system.
References
1. Stein, L. Y., and M. G. Klotz. 2016. The nitrogen cycle. Current Biology 26:R94–R98.
2. LeBauer, D. S., and K. K. Treseder. 2008. Nitrogen Limitation of Net Primary Productivity in Terrestrial Ecosystems Is Globally Distributed. Ecology 89:371–379.
3. Ollivier, J., S. Töwe, A. Bannert, B. Hai, E.-M. Kastl, A. Meyer, M. X. Su, K. Kleineidam, and M. Schloter. 2011. Nitrogen turnover in soil and global change: Key players of soil nitrogen cycle. FEMS Microbiology Ecology 78:3–16.
4. Meng, L., W. Li, S. Zhang, C. Wu, and L. Lv. 2017. Feasibility of co-composting of sewage sludge, spent mushroom substrate and wheat straw. Bioresource Technology 226:39–45.
5. Peoples, M. B., D. F. Herridge, and J. K. Ladha. 1995. Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production? Plant and Soil 174:3–28.
6. Mader, Sylvia S., and Michael Windelspecht. Essentials of Biology. 11th ed., McGraw-Hill Education, 2017.
7. Raven, Peter H., Ray Franklin Evert, and Susan E. Eichhorn. Biology of Plants. New York: W.H. Freeman and Co, 2005.
8. Fath, B. D. 2018. Encyclopedia of Ecology. Elsevier, San Diego, NETHERLANDS, THE.
9. Sims, J. T., and D. C. Wolf. 1994. Poultry Waste Management: Agricultural and Environmental Issues. Pages 1–83 in D. L. Sparks, editor. Advances in Agronomy. Academic Press.
10. Cáceres, R., K. Malińska, and O. Marfà. 2018. Nitrification within composting: A review. Waste Management 72:119–137.
11. Gu, B., Y. Ge, Y. Ren, B. Xu, W. Luo, H. Jiang, B. Gu, and J. Chang. 2012. Atmospheric Reactive Nitrogen in China: Sources, Recent Trends, and Damage Costs. Environmental Science & Technology 46:9420–9427.
12. Nitrogen_Cycle.jpg: Environmental Protection Agency[1]
![[1]](https://upload.wikimedia.org/wikipedia/commons/d/de/Nitrogen_Cycle.jpg){kind=link}
13. https://en.wikipedia.org/wiki/File:Nitrogen_fixation_Fabaceae_en.svg [2]
