The nitrogen gas (N₂) from the atmosphere can enter the soil primarily through a process known as nitrogen fixation. In the soil, nitrogen gas can be converted into ammonia (NH₃) or related compounds, which can then be utilized by plants and various microorganisms. This process occurs through several mechanisms:
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Biological Nitrogen Fixation: Certain bacteria, such as Rhizobium, which live in symbiotic relationships with leguminous plants (like peas and beans), can fix atmospheric nitrogen into a form that plants can use. Free-living nitrogen-fixing bacteria, like Azotobacter and Clostridium, are also found in the soil and contribute to this process.
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Chemical Nitrogen Fixation: Lightning strikes can also fix nitrogen by combining atmospheric nitrogen with oxygen, forming nitrogen oxides that dissolve in rainwater and are then deposited into the soil.
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Soil Microbial Activity: Various soil microorganisms can further convert fixed nitrogen into forms such as ammonium (NH₄⁺) and nitrate (NO₃⁻), which are readily absorbed by plant roots.
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Nitrification: Some soil bacteria convert ammonium into nitrites (NO₂⁻) and then into nitrates (NO₃⁻) through a process called nitrification. Nitrates are the form of nitrogen that is most easily taken up by plants.
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Organic Matter Breakdown: When plants and animals die, their organic matter is broken down by decomposers (fungi and bacteria), returning nitrogen to the soil in the form of ammonium.
This entire process is a key component of the nitrogen cycle, which is essential for life on Earth, as nitrogen is a critical component of amino acids, proteins, and nucleic acids.