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The Nitrogen Cycle: Definition, Facts & Steps

The Nitrogen Cycle: Definition, Facts & Steps
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Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Oh the places we'll go on our nitrogen cycle tour! Nitrogen is an element that cycles through you, me, plants, and other organisms. This lesson will not only follow nitrogen on this cycle, but will also share some nitrogen facts!

Definition of the Nitrogen Cycle

As you bite into your scrumptious turkey, lettuce, and tomato sub sandwich you're probably not thinking even though these nitrogen atoms used to be in cow poop, this sandwich is still going to be delicious!

Sorry if I ruined your lunch, but those nitrogen atoms are quite the sightseers, visiting the air, microorganisms, plants, animals, and the soil though the nitrogen cycle, or the circulation of nitrogen on Earth. Nitrogen is one type of atom, like oxygen, carbon, and hydrogen. Let's follow some nitrogen atoms through the nitrogen cycle to see where they go!

Steps of the Nitrogen Cycle

You are surrounded by nitrogen! In fact, 78% of the air you breathe is nitrogen, but how does the nitrogen from the air get into your turkey sandwich? Good question! Well, here are the stops a nitrogen atom would take on its voyage through the nitrogen cycle:

1. Let's start with the air you are breathing. When nitrogen is in the air it is called atmospheric nitrogen and comes in the form of N2, which means two nitrogen atoms stuck together. Plants can't do a whole lot with atmospheric nitrogen, but microorganisms like nitrogen-fixers, which are special bacteria that can change the nitrogen into a useable form through a process called nitrogen fixation. Let's take a look at how nitrogen fixation takes place.

  • Atmospheric nitrogen makes its way into the soil where nitrogen-fixing bacteria on the roots of some plants change it to ammonium (nitrogen attached to hydrogen atoms, NH4+). There are also some free-living bacteria (not on the roots of plants) that are nitrogen-fixers.
  • Lightning can also change atmospheric nitrogen into nitrogen oxides, another type of nitrogen attached to oxygen atoms. This makes up only a small percentage of nitrogen fixation.

2. Bacteria and archaea (another type of microorganism) in the soil change the ammonium into nitrites (NO2-) and then nitrates (NO3-) through nitrification, which is, in essence, when bacteria change ammonium into nitrates. Nitrates are nitrogen attached to oxygen atoms.

3. Now that the atmospheric nitrogen has been changed into nitrates, let's see what happens next. Assimilation is when plants use the nitrogen for all sorts of things like building leaves or making DNA (deoxyribonucleic acid). Animals and other organisms eat the plants, and the nitrogen gets incorporated into those bodies as well.

4. Eventually plants, animals, and other organisms die and decay, releasing nitrogen back into the soil. Bacteria and fungi (mushrooms, for example) help break down the dead organisms, and through ammonification, nitrogen is turned back into ammonium. The ammonium is turned back into nitrates by bacteria (you basically go back to step 2).

5. Special bacteria can turn nitrates back into atmospheric nitrogen through a process called denitrification, which is how nitrogen in the soil is released into the atmosphere again. And, you are back at step 1!

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