Back To CourseCLEP Biology: Study Guide & Test Prep
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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.
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.
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.
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.
Another part of the nitrogen cycle is a result of the combustion of organic material. When fossil fuels, like petroleum, coal and natural gas, and other organic materials, like grass, trees and brush, are burned, they release large amounts of carbon dioxide, sulfur dioxide and nitrogen oxides into the atmosphere. But it should be noted that the burning of some of these materials, especially coal, releases much greater quantities of these pollutants into the air than burning natural gas does. This is the major reason why coal is considered to be the dirtiest fossil fuel, and conversely, why natural gas is considered the cleanest. Petroleum lies somewhere in between.
While nitrogen oxides are considered nutrients in the soil, in the atmosphere it's a different story. In the atmosphere, sulfur dioxide and nitrogen oxides react with water to create sulfuric and nitric acids. This results in acid rain, or rain that has been made more acidic by the addition of sulfur dioxide and nitrogen oxides in the atmosphere. In lakes and soils that have very little buffering capacity, acid rain can significantly lower the pH of the water and soil. In some cases, this has led to the death of sensitive aquatic animals and even deforestation in some areas.
Nitrogen fixation is the process of creating ammonia from atmospheric nitrogen. Plants and most other producers cannot utilize atmospheric nitrogen so they must rely on nitrogen fixing bacteria to turn nitrogen gas into ammonia that they can use. There are other bacteria, both in the soil and in aquatic ecosystems that use ammonia as an energy source. These bacteria convert ammonia to nitrite and then to nitrate in a process called nitrification. Nitrification does not change the amount of nitrogen that is available to plants because they can utilize ammonia, nitrite and nitrate as well as many other ammonia derivatives that are abundant in animal waste.
Eutrophication is the ecological process that occurs when excess nutrients are added to a body of water. When excess fertilizers leach into local waterways or when untreated sewage finds its way into a body of water, rapid eutrophication can take place and upset the ecological balance of the aquatic ecosystem. The excess nutrients in the water often cause large blooms of algae that 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.
When fossil fuels and other organic materials are burned, they release carbon dioxide, sulfur dioxide and nitrogen oxides into the atmosphere. In the atmosphere, sulfur dioxide and nitrogen oxides react with water to create sulfuric and nitric acids. This results in acid rain, or rain that has been made more acidic by the addition of sulfur dioxide and nitrogen oxides in the atmosphere. In lakes and soils that have very little buffering capacity, acid rain can significantly lower the pH of the water and soil. In some cases, this can lead to the death of sensitive aquatic animals and deforestation in some areas.
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Back To CourseCLEP Biology: Study Guide & Test Prep
23 chapters | 211 lessons