Nitrogen gets into the air primarily through the breakdown of organic matter and nitrogen fixation, which releases nitrogen gas back into the atmosphere.
Here's a more detailed explanation:
Nitrogen Cycle and Atmospheric Nitrogen
Nitrogen is a crucial element for life, but in its gaseous form (N2), it's largely unusable by plants and animals. The nitrogen cycle describes how nitrogen moves through the environment. A key part of this cycle involves returning nitrogen to the atmosphere.
Processes that Release Nitrogen into the Air:
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Denitrification: This is the primary process that returns nitrogen gas (N2) to the atmosphere. It's carried out by denitrifying bacteria, which convert nitrates (NO3-) in the soil back into nitrogen gas under anaerobic conditions (i.e., in the absence of oxygen). This typically occurs in waterlogged soils or sediments.
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Ammonification/Decomposition: When plants and animals die, decomposers (bacteria and fungi) break down their organic matter, releasing ammonia (NH3). While some ammonia is used by plants or converted to other forms of nitrogen in the soil, some can be converted back into nitrogen gas through a series of processes, including denitrification.
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Volcanic Activity: Volcanic eruptions can release small amounts of nitrogen gas and other nitrogen-containing compounds into the atmosphere.
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Industrial Processes & Human Activity:
- Combustion: Burning fossil fuels (coal, oil, and natural gas) can release nitrogen oxides (NOx) into the air. While not nitrogen gas, these compounds contribute to the nitrogen cycle and can eventually be converted into nitrogen gas.
- Fertilizer Use: While fertilizers primarily aim to add nitrogen to the soil for plant growth, some of the applied nitrogen can be lost to the atmosphere as nitrogen gas or other nitrogen compounds through processes like denitrification.
Nitrogen Fixation & The Opposing Force
It's important to note that while these processes add nitrogen to the atmosphere, another process called nitrogen fixation removes nitrogen from the atmosphere, converting it into usable forms for plants and other organisms. Nitrogen fixation is carried out by:
- Bacteria: Certain bacteria, including Rhizobium in the root nodules of legumes (e.g., beans, peas), can convert atmospheric nitrogen into ammonia.
- Cyanobacteria (Blue-Green Algae): These microorganisms can also fix nitrogen, both in aquatic and terrestrial environments.
- Industrial Processes: The Haber-Bosch process is an industrial method used to produce ammonia fertilizer from atmospheric nitrogen and hydrogen.
However, the nitrogen released back into the atmosphere through the processes listed earlier generally balances out (or, with human impact, exceeds) the amount of nitrogen being fixed.
