e-book Biology of the Nitrogen Cycle

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Articles

  1. Ammonification (decay)
  2. The global nitrogen cycle in the twenty-first century
  3. Biological Nitrogen Fixation | Learn Science at Scitable
  4. Nitrogen fixation

For example, water managers can introduce organisms that eat phytoplankton, and these organisms can help reduce the amounts of phytoplankton, by eating them! The nitrogen cycle is a repeating cycle of processes during which nitrogen moves through both living and non-living things: the atmosphere, soil, water, plants, animals and bacteria. In order to move through the different parts of the cycle, nitrogen must change forms. In the atmosphere, nitrogen exists as a gas N 2 , but in the soils it exists as nitrogen oxide, NO, and nitrogen dioxide, NO 2 , and when used as a fertilizer, can be found in other forms, such as ammonia, NH 3 , which can be processed even further into a different fertilizer, ammonium nitrate, or NH 4 NO 3.

A2 Biology - Reactions in the nitrogen cycle (OCR A Chapter 23.3)

There are five stages in the nitrogen cycle, and we will now discuss each of them in turn: fixation or volatilization, mineralization, nitrification, immobilization, and denitrification. In this image, microbes in the soil turn nitrogen gas N 2 into what is called volatile ammonia NH 3 , so the fixation process is called volatilization. Leaching is where certain forms of nitrogen such as nitrate, or NO 3 becomes dissolved in water and leaks out of the soil, potentially polluting waterways.

In this stage, nitrogen moves from the atmosphere into the soil. To be used by plants, the N 2 must be transformed through a process called nitrogen fixation. Fixation converts nitrogen in the atmosphere into forms that plants can absorb through their root systems. A small amount of nitrogen can be fixed when lightning provides the energy needed for N 2 to react with oxygen, producing nitrogen oxide, NO, and nitrogen dioxide, NO 2.

Ammonification (decay)

These forms of nitrogen then enter soils through rain or snow. Nitrogen can also be fixed through the industrial process that creates fertilizer. This form of fixing occurs under high heat and pressure, during which atmospheric nitrogen and hydrogen are combined to form ammonia NH 3 , which may then be processed further, to produce ammonium nitrate NH 4 NO 3 , a form of nitrogen that can be added to soils and used by plants.

Most nitrogen fixation occurs naturally, in the soil, by bacteria. In Figure 3 above , you can see nitrogen fixation and exchange of form occurring in the soil. Some bacteria attach to plant roots and have a symbiotic beneficial for both the plant and the bacteria relationship with the plant [ 6 ]. The bacteria get energy through photosynthesis and, in return, they fix nitrogen into a form the plant needs.

The fixed nitrogen is then carried to other parts of the plant and is used to form plant tissues, so the plant can grow. Other bacteria live freely in soils or water and can fix nitrogen without this symbiotic relationship. These bacteria can also create forms of nitrogen that can be used by organisms. This stage takes place in the soil. Nitrogen moves from organic materials, such as manure or plant materials to an inorganic form of nitrogen that plants can use. This becomes important in the second stage of the nitrogen cycle. Mineralization happens when microbes act on organic material, such as animal manure or decomposing plant or animal material and begin to convert it to a form of nitrogen that can be used by plants.

All plants under cultivation, except legumes plants with seed pods that split in half, such as lentils, beans, peas or peanuts get the nitrogen they require through the soil. Legumes get nitrogen through fixation that occurs in their root nodules, as described above. The first form of nitrogen produced by the process of mineralization is ammonia, NH 3. The NH 3 in the soil then reacts with water to form ammonium, NH 4. This ammonium is held in the soils and is available for use by plants that do not get nitrogen through the symbiotic nitrogen fixing relationship described above.

The third stage, nitrification, also occurs in soils. Nitrates can be used by plants and animals that consume the plants. Some bacteria in the soil can turn ammonia into nitrites. Although nitrite is not usable by plants and animals directly, other bacteria can change nitrites into nitrates—a form that is usable by plants and animals.

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This reaction provides energy for the bacteria engaged in this process. The bacteria that we are talking about are called nitrosomonas and nitrobacter. Nitrobacter turns nitrites into nitrates; nitrosomonas transform ammonia to nitrites. Both kinds of bacteria can act only in the presence of oxygen, O 2 [ 7 ].

The global nitrogen cycle in the twenty-first century

The process of nitrification is important to plants, as it produces an extra stash of available nitrogen that can be absorbed by the plants through their root systems. The fourth stage of the nitrogen cycle is immobilization, sometimes described as the reverse of mineralization. These two processes together control the amount of nitrogen in soils. Just like plants, microorganisms living in the soil require nitrogen as an energy source. These soil microorganisms pull nitrogen from the soil when the residues of decomposing plants do not contain enough nitrogen.

Immobilization, therefore, ties up nitrogen in microorganisms. However, immobilization is important because it helps control and balance the amount of nitrogen in the soils by tying it up, or immobilizing the nitrogen, in microorganisms.

In the fifth stage of the nitrogen cycle, nitrogen returns to the air as nitrates are converted to atmospheric nitrogen N 2 by bacteria through the process we call denitrification. This results in an overall loss of nitrogen from soils, as the gaseous form of nitrogen moves into the atmosphere, back where we began our story. The cycling of nitrogen through the ecosystem is crucial for maintaining productive and healthy ecosystems with neither too much nor too little nitrogen.

Plant production and biomass living material are limited by the availability of nitrogen. Understanding how the plant-soil nitrogen cycle works can help us make better decisions about what crops to grow and where to grow them, so we have an adequate supply of food. Knowledge of the nitrogen cycle can also help us reduce pollution caused by adding too much fertilizer to soils.

Biological Nitrogen Fixation | Learn Science at Scitable

As you have seen, not enough nitrogen in the soils leaves plants hungry, while too much of a good thing can be bad: excess nitrogen can poison plants and even livestock! Pollution of our water sources by surplus nitrogen and other nutrients is a huge problem, as marine life is being suffocated from decomposition of dead algae blooms. Farmers and communities need to work to improve the uptake of added nutrients by crops and treat animal manure waste properly. The gases that are formed escape into the atmosphere completing the nitrogen cycle.

This can be a harmful process as fixed nitrogen is removed from the soil making it less fertile. A wide range of soil fungi and bacteria, called the decomposers, carry out the ammonification process. The decomposers consume the organic matter, and the nitrogen contained in the dead organism is converted to ammonium ions.

Nitrogen fixation

The ammonium is then converted to nitrates by the nitrifying bacteria. To show the role of microbes in the nitrogen cycle, how microbes can be grown from root nodules and an example of symbiosis. Reflame the loop. Tape the lid on the plate, invert it and incubate for 3—4 days. Dispose of contaminated materials appropriately.


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Examine the MYEA plate without removing the lid, noting the appearance of any colonies growing on the agar. Reproduced from our education resource, Practical Microbiology for Secondary Schools. This publication and its companion Basic Practical Microbiology which contains details of how to carry out a risk assessment are available free to School Members.


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Please contact education microbiologysociety. Homepage Knowledge search Schoolzone: The nitrogen cycle. Schoolzone: The nitrogen cycle 08 May Various processes are responsible for recycling the chemicals necessary for life on Earth. The nitrogen cycle The nitrogen cycle is the movement of nitrogen between the earth and the atmosphere.

Nitrogen fixation Nitrogen gas is composed of two atoms of nitrogen linked by a very strong triple bond. Atmospheric fixation. Lightning allows nitrogen and oxygen to combine to produce various oxides of nitrogen. These are carried by the rain into the soil where they can be used by plants. Industrial fixation. The Haber—Bosch process is used to make nitrogen-containing fertilizers.

This is a very energy-inefficient process. Biological fixation. Biological fixation The reduction of nitrogen gas to ammonia is energy-intensive. Table 1. Reactions of the nitrogen cycle. Nitrogen gas is fixed into forms other organisms can use.

Nitrification Nitrifying bacteria, e. Ammonia or ammonium ions are oxidized first to nitrites and then to nitrates, which is the form most usable by plant. Denitrification Denitrifying bacteria Anaerobic Nitrates are reduced to nitrogen gas, returning nitrogen to the air and completing the cycle. Types of nitrogen-fixing bacteria Some nitrogen-fixing bacteria are free-living in the soil, fixing nitrogen independently of other organisms, e.

Adapting to their environment Nitrogen-fixing bacteria contain an enzyme complex called nitrogenase which catalyses the conversion of nitrogen gas to ammonia. Nitrification This is the oxidation of ammonium compounds to nitrites and then to nitrates by the nitrifying bacteria. Nitrification is a two-step process. Nitrite is toxic to plants and animals in high concentrations. Nitrification occurs in well-drained and aerated soils at neutral pH.

Practical — isolating microbes from root nodules Learning objectives To show the role of microbes in the nitrogen cycle, how microbes can be grown from root nodules and an example of symbiosis. Materials Plant with root nodules, e. Heat to dissolve. Add 0. Dispense and sterilize by autoclaving. Clover obtain seeds from school science suppliers or plants from a field or lawn is recommended as the nodules are relatively soft.

Potato dextrose agar supplemented with 0.