The nitrogen cycle consists of several key steps that describe the transformation of nitrogen in various forms. Starting with nitrogen gas (N₂) in the atmosphere, the three main steps can be summarized as follows:
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Nitrogen Fixation: In this step, nitrogen gas from the atmosphere is converted into ammonia (NH₃) or related compounds. This process can occur through biological means, such as the action of certain bacteria (e.g., Rhizobium, which associates with leguminous plants), or through abiotic means, such as lightning or industrial processes (e.g., the Haber-Bosch process).
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Nitrification: This process involves the conversion of ammonia into nitrate (NO₃⁻) via two main steps. First, ammonia is oxidized to nitrite (NO₂⁻) by bacteria such as Nitrosomonas. Then, nitrite is further oxidized to nitrate by bacteria such as Nitrobacter. Nitrate is the form of nitrogen that is most readily taken up by plants.
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Denitrification: In this final step, nitrate is reduced back to nitrogen gas (N₂) or, in some cases, nitrous oxide (N₂O) by denitrifying bacteria under anaerobic conditions. This process releases nitrogen back into the atmosphere, completing the nitrogen cycle.
These steps illustrate the flow of nitrogen through different forms and processes in the ecosystem.
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