Nitrogen gas is a colourless, odourless and non-toxic gas which makes up about 78 percent of the atmosphere. Nitrogen is extremely important to living material; in fact plants, animals and humans could not live without it. Sprent (1987) argued that nitrogen cycle is the most important process to living organisms after the carbon cycle. But nitrogen exists as dinitrogen (N2) in the atmosphere, which living things, particularly plants, cannot synthesize (citation)
The process of nitrogen cycle transforms nitrogen from one form to another (citation). It is defined by (name) as a process in which the nitrogen in the atmosphere goes through many complex chemical and biological changes in the soil, including be combined into living and non-living material, and return back to the air in a continuing cycle (year). The role of bacteria is central to the majority of these processes (Postgate, 1982), either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. Figure 1 shows how these processes fit together to form the nitrogen cycle.
The nitrogen cycle involves five steps: nitrogen fixation, nitrification, assimilation, ammonification and denitrification (Berg, Martin, Solomon, and Ville, 1993, p. 1162). All these steps involve living organisms in a continuous cycle. Bacteria are involved in all these steps except in assimilation process (citation)
The first step in the nitrogen cycle, nitrogen fixation, involves the conversion of gaseous nitrogen (N2) to ammonia (NH3) or nitrate (NO3-). This process is termed nitrogen fixation because “nitrogen is fixed into a form that living things can use” (Berg et al., 1993, p. 1162). There are many ways in which nitrogen could be fixed; these include by combustion, lightning discharges, volcanic eruption, or industrial wastes because all these processes possess enough energy to break up molecular nitrogen.
But the main process of nitrogen fixation comes from biological
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