The Nitrogen Cycle, Acid Rain and Fossil Fuels

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  • 0:04 Nitrogen Fixation
  • 1:19 Nitrification
  • 2:14 Nitrogen Cycle
  • 3:23 Eutrophication
  • 4:46 Acid Rain
  • 6:15 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
Do you know the difference between nitrite and nitrate? Do you have any clue how all the nitrogen in the atmosphere becomes usable? If you can't answer these questions, then you need to watch this lesson on the nitrogen cycle.

Nitrogen Fixation

The Earth's atmosphere is almost 80% nitrogen, however in its pure gas form it cannot be used by living organisms. So where do producers get nitrogen from if they can't use nitrogen from the air? Because atmospheric nitrogen gas is unusable for all organisms, some bacteria have evolved the ability to add hydrogen atoms to nitrogen and create ammonia, which can be easily incorporated into organic molecules like amino acids. This process of creating ammonia from atmospheric nitrogen is called nitrogen fixation. There are many types of nitrogen fixing bacteria. Some are free-living soil bacteria, others are nitrogen fixing symbiotic bacteria that live in roots of some species of plants, and in the oceans a major source of nitrogen fixation is cyanobacteria which are also sometimes called blue-green algae. All of these bacteria are fixing nitrogen for their own use, but they also release excess ammonia that can be used by other organisms; and of course, they can also be directly consumed by other organisms and pass the fixed nitrogen up the food chain that way too.


The nitrification process
Nitrification Process

Ammonia isn't just used by plants and producers to make organic molecules, some aerobic bacteria can use ammonia as an energy source by oxidizing ammonia to make nitrite, which can then be further oxidized by other bacteria to make nitrate.

This process by which ammonia is converted to nitrite and then to nitrate is called nitrification. Fortunately, plants and other producers can also use nitrite and nitrate as sources of nitrogen, so nitrification doesn't really change the amount of available nitrogen in the environment, it just changes its oxidation state. However, there are still yet other bacteria that use nitrate as a source of oxygen and convert nitrate back into atmospheric nitrogen in a process called denitrification.

Nitrogen Cycle

Let's look at an example of a nitrogen cycle in a typical terrestrial ecosystem. Nitrogen fixing bacteria in the soil and in plant roots convert nitrogen from the atmosphere to ammonia. The ammonia is then used by plants or converted to nitrite and then nitrate by nitrifying bacteria. The nitrite and nitrate can also be used by plants. The plants are then eaten by herbivores, which can then be eaten by carnivores. Animal waste products are high in ammonia and ammonia derivatives like urea, both of which are usable sources of nitrogen for plants. If there is an abundance of nitrate in the soil, denitrifying bacteria can convert some of it back to atmospheric nitrogen. Now if the plants in the ecosystem are crops and are removed after harvest, there will be no animal waste or plant litter to replenish the usable nitrogen in the soil. So, a common practice in agriculture is to replenish the soil with synthetic fertilizers that contain ammonia, nitrite or nitrate.


Photo of a large algae bloom
Large Algae Bloom

Over-fertilization of farmland can result in excess nitrogen compounds entering a nearby stream or lake and causing eutrophication, or the ecological process that occurs when excess nutrients are added to a body of water. Eutrophication can occur naturally, and natural eutrophication occurs over a long period of time, which allows the organisms living in the body of water time to adjust to the change gradually and maintain balance. However, eutrophication caused by human activity, such as over fertilization or the dumping of raw sewage from a large human population in a single body of water, causes eutrophication in a much shorter time frame and often results in an ecological imbalance.

This ecological imbalance can be manifested in large algae blooms that can sometimes cover the entire surface of a pond or lake. These large blooms produce a lot of oxygen when exposed to daylight, but at night, the process is reversed and the algae can suck all of the oxygen out of the water and suffocate aquatic animals, causing massive die-offs. Eventually, the algae population itself crashes, and the large amount of dead and decaying matter in the lake causes another sudden change in the ecological balance.

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