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The Nitrogen Cycle

Nitrogen is an element essential to life. It is found in proteins, DNA and RNA, and in chlorophyll molecules important to photosynthesis. Most living organisms cannot absorb nitrogen in its gaseous form, and rely on a process all fixation to obtain nitrogen.

Nitrogen Fixation:

Nitrogen gas (N2) makes up about 78% of the air in our atmosphere, which in turn holds most of the nitrogen on Earth.

  • N2 (representing nitrogen in the atmosphere), falls to the ground
  • Nitrogen fixation occurs when bacteria convert nitrogen gas (N2) into organic compounds that living organisms can take up.
  • Legumes, unlike most plants, have bacteria living in their roots that can fix nitrogen.
  • These nitrogen-fixing bacteria supply nitrogen to the plant in a form that it can use. In exchange, the bacteria receive carbon-hydrates from the plant that are used to obtain energy.

Ammonification:

Ammonification occurs when bacteria or fungi convert nitrogen gas (N2) or nitrogen compounds into ammonium ions (NH4+).

  • Soil contains bacteria that use enzymes to convert nitrogen gas (N2) into ammonium ions (NH4+). The ammonium ions bind to soil particles.

Nitrification:

Nitrification is a two-step process: soil bacteria first convert ammonium ions (NH4+) into nitrite ions (NO2-), then convert nitrite ions into nitrate ions (NH3).

  • The soil-bound ammonium can be released by nitrifying bacteria in the soil. These bacteria convert ammonium ions (NH4+) into nitrite ions (NO2-). Some plants can take up nitrogen in the form of nitrite ions

Denitrification:

Denitrification occurs when soil bacteria convert nitrate ions (NO3-) into nitrogen gas (N2). Then the nitrogen returns to the atmosphere.

But wait! What happens when living organisms take up nitrogen? When that happens it is in the middle of this cycle. Let’s go back and look.

Assimilation:

When living organisms take up nitrogen, it is called assimilation.

  • Plants can absorb eith nitrate ions (NO2-) or ammonium ions (NH4+) through their roots.
  • When animals and humans eat plants, they assimilate, or take up, nitrogen from the plants. In this way, all the nitrogen in animal and human bodies comes from plants.

Ammonification:

Ammonification occurs when bacteria or fungi convert nitrogen gas (N2) or nitrogen compounds into ammonium ions (NH4+).

  • When plants and animals die, their remains are full of nitrogen. Decomposers, like fungi and some bacteria, convert ammonia (NH3) from animal excretions and dead organic matter into ammonium ions (NH4+).
  • This creates organic nitrogen = N

Nitrification:

The Nitrogen Cycle picks up where it was before - at the Nitrification step. From here it proceeds exactly as it did before.

  • Nitrification
  • Denitrification
  • Cycle Completion
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