As we all know that nitrogen is present in the atmosphere in the maximum amount i.e. 78% of nitrogen is there in the air. All the living beings use nitrogen in one or the other ways for several purposes and this is the reason that nitrogen cycle exist in the environment. Nitrogen that is used up by living beings again returns to the atmosphere in different forms and ways, so that it can be used up again. Therefore, this nitrogen continuously enters the atmosphere and returns to the cycle.
1. Nitrogen fixation: It is the process in which atmospheric nitrogen is fixed so that it can be used for different purposes by the living organisms. This fixation occurs when atmospheric nitrogen combines with many other elements to give some useful compounds. The work of nitrogen fixation is done by some of the micro- organisms and it can also take place due to lightening. The bacteria that fix the nitrogen have nitrogenase enzymes that helps them covert atmospheric nitrogen to ammonium compound that are generally used by plants. The bacteria that are involved in this process are Azobacter and Clostridium along with blue- green algae. Rhizobium also helps to fix nitrogen. These bacteria live in the roots of leguminous plants maintaining a mutalistic relationship with the plant. Plant benefits the bacteria with supply of food and in return bacteria benefit plant with ammonium compounds by fixing the atmospheric nitrogen. Beside bacteria’s, when the heat is generated due to lightening, nitrogen in the atmosphere combines with oxygen and moisture and comes down as rain. Therefore, this rain that contains nitrogen in the form of nitrates is used by the plants. This way, fixation of nitrogen takes place in the atmosphere.
2. Nitrification: In this process, ammonia is converted into nitrates. It takes place in two steps that involves oxygen and is done by two main types of nitrifying bacteria called nitrosomonas and nitrobacter.
The reaction here is: NH3 + O2+ alkanity+ nitrosomonas= NO2
NO2+O2+alkanity+ nitrobacter= NO3
Nitrification takes place to reduce the toxicity of ammonia.
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3. Nitrogen assimilation: In this process, nitrate is absorbed by the roots of the plants from the soil and when these plants are eaten by the consumers due to this, energy is transferred to them in the form of protein that gets back to the atmosphere and mixes in the soil when they finally dies.
4. Ammonification: In this process some of the ammonifying bacteria like bacillus vulgaris and bacillus mycoides converts protein present in the soil to the ammonia.
5. Denitrification: It is just opposite of nitrification, where the portion of nitrogen that is not used by the plants is converted to nitrogen by some denitrifying bacteria such as pseudomonas denitrificans.
Oxygen is one of the important gases for all the living beings on the earth. It is the second largest gas that is present in the atmosphere after nitrogen. Therefore the cycling of oxygen is very much important for continuation of life in the earth. The main sources of oxygen are the plants. As we all know that plants take in carbon dioxide and give out oxygen. Therefore, this maintains the presence of this gas in the atmosphere. Now, secondly, human beings and animals take in oxygen and give out carbon dioxide, the process known as respiration and again the expelled carbon dioxide is used by plants during the process of photosynthesis. This way this cycle continues in the atmosphere. But plants are not only the source of oxygen, rather when water comes down in the form of rain and strikes rocks and different other hard surface, it increases surface area due to which oxygen present in the air mixes with the water current and passes to ponds, streams and lakes. This is the reason that water animals get dissolved oxygen from the water by breaking of the organic matters.
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This way oxygen enters the atmosphere through photosynthesis and rainfall and exist the environment through respiration and decay of the organism. Oxygen is also present in many other forms in combination with oxides and metals. This way oxygen being an important part of living organism is constantly cycled in the atmosphere.
Both these cycle are very important for the survival of life on the earth, so keeping the environment clean will help to keep the balance of air in the atmosphere.