Nitrogen (N) is an essential component of DNA, RNA, and proteins, the building blocks of life. All organisms require nitrogen to live and grow. Nitrogen is an incredibly versatile element, existing in both inorganic and organic forms as well as many different oxidation states. The movement of nitrogen between the atmosphere, biosphere, and geosphere in different forms is described by the nitrogen cycle (Figure 1), one of the major biogeochemical cycles.
Figure 1: The nitrogen cycle. Yellow arrows indicate human sources of nitrogen to the environment. Red arrows indicate microbial transformations of nitrogen. Blue arrows indicate physical forces acting on nitrogen. And green arrows indicate natural, non-microbial processes affecting the form and fate of nitrogen. Five main processes cycle nitrogen through the biosphere, atmosphere, and geosphere: 1. Nitrogen fixation,
2. Nitrogen uptake (organismal growth),
3. Nitrogen mineralization (decay),
4. Nitrification, and
Microorganisms, particularly bacteria, play major roles in all of the principal nitrogen transformations.
(A) Nitrogen fixation
N2 NH4+ Nitrogen fixation is the process wherein N2 is converted to ammonium, essential because it is the only way that organisms can attain nitrogen directly from the atmosphere. Certain bacteria, for example Rhizobium, are the only organisms that fix nitrogen through metabolic processes. Nitrogen fixing bacteria often form symbiotic relationships with host plants. This symbiosis is well-known to occur in the legume family of plants (e.g. beans, peas, and clover). In this relationship, nitrogen fixing bacteria inhabit legume root nodules and receive carbohydrates and a favorable environment from their host plant in exchange for some of the nitrogen they fix. There are also nitrogen fixing bacteria that exist without plant hosts, known as free-living nitrogen fixers, for e.g. blue-green...
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