The earth is a closed system; matter can never be removed or created. Instead, matter moves across the earth in cycles called biogeochemical cycles. These basic cycles are essential for supporting life on Earth, but when they are disrupted, it can cause matter to get “stuck” or move too quickly between stages.
Nitrogen is a macro-nutrient that organisms need in order to form amino acids and nucleic acids (DNA). Nitrogen is also a limiting nutrient, since it is usually less available than other nutrients, but organisms can only grow if they have nitrogen.
The nitrogen cycle has a lot of chemical transformations, which can be difficult to memorize and think about intuitively. Although technically the exam can ask you about the nitrogen cycle, it tends to focus on later chapters and global trends, so don’t focus too much on memorizing all the chemical processes (this isn’t AP Chem!).
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Even though there is a lot of nitrogen in the atmosphere, it is not usable to organisms in that form. The process of nitrogen fixation turns nitrogen gas (N2) into a nitrate (NO3) so that plants can use it.
Nitrogen fixation can happen in two different ways: biotic and abiotic. In the biotic pathway, nitrogen-fixing bacteria such as cyanobacteria convert N2 into ammonia (NH3) which then quickly bonds with hydrogen ions to become ammonium (NH4). Then, during nitrification, specialized bacteria convert NH4 into nitrite (NO2) and then into nitrate (NO3).
Nitrogen fixation can also occur in the atmosphere when N2 is exposed to lightning, fires, or fossil fuel combustion, which converts it directly to NO3. The nitrate then enters the soil through precipitation.
Once nitrogen is in usable form, plants can assimilate it, or incorporate it into their tissues. Consumers who eat the plants will also synthesize some of the nitrogen into their tissues. Some nitrogen also leaches into the ocean, either through runoff or precipitation. This is where aquatic organisms can obtain the nitrogen they need.
When organisms die, decomposers break down their tissues and convert the organic nitrogen (the nitrogen in their tissues) back into inorganic ammonium. This process is called mineralization or ammonification. After ammonification, the nitrification process can begin again.
The final step of the nitrogen cycle is denitrification, which returns nitrogen to the atmosphere. During denitrification, specialized bacteria convert nitrate into nitrous oxide (N2O) and then back into nitrogen gas (N2).
Like we discussed before, nitrogen is a limiting nutrient, which means that it is necessary for organisms, but it is also available in smaller amounts than other essential nutrients. Therefore, plants usually can’t grow due to a lack of nitrogen.
Consequently, humans love to put nitrogen in fertilizers. In fact, humans now fix more nitrogen than the total amount of nitrogen fixed by nature. Nitrogen makes plants grow really fast, but it can also have some negative consequences. Because ecosystems have adapted to a certain level of nitrogen over time, excess nitrogen disrupts balance in the ecosystem. Larger plants which were previously contained by the limited availability of nitrogen start to overtake the smaller plants that don’t need as much nitrogen. This leads to a decrease in the number of species in an ecosystem.
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