In the atmosphere, air consists of 78 percent of nitrogen gas. Every cellular component is made up of nitrogen as one of the major components. All living organisms including plants, animals, humans and even microorganisms use nitrogen for their life processes. Despite all these, the concentration of nitrogen in the atmosphere is still 78 percent, although it is continuously being consumed. How is this possible? Why the air we breathe always has the same proportion of nitrogen gas? Nitrogen cycle explains the reason behind this consistency. Let’s learn more about the nitrogen cycle.
Nitrogen is a vital element. It is a part of most of the cellular components like proteins, nucleic acids (DNA and RNA), vitamins, etc. It is also a major constituent of chlorophyll in plants. Plants obtain their nitrogen from the soil while animals obtain it from plants. A cycle of processes helps in maintaining the concentration of nitrogen in the atmosphere and meet the requirement of every organism. This cycle of nitrogen is called the nitrogen cycle.
The nitrogen cycle is the recycling and reusing of nitrogen in different forms to meet the demand for various environmental activities. This happens through various processes with the help of certain microorganisms. In the atmosphere, nitrogen is available in a diatomic form i.e. N2. Plants and certain organisms can’t use nitrogen in this form. Therefore N2 is converted into nitrogen oxides like NO, NO2, NO3. Various processes like lightning; industrial combustion, forest fire etc. contribute to these oxides.
PROCESS OF NITROGEN CYCLE
Ammonification: Ammonia is obtained from dead and decaying plants and animals by decomposition. This process is called ammonification.
Nitrification: In this step, ammonia obtained is first converted to nitrite (NO2) by bacteria like Nitrosomonas, Nitrococcus, etc., and then to nitrate (NO3) by Nitrobacterium. Bacteria involved in nitrification are called chemoautotrophs. Here is the reaction involved in the process of Nitrification.
2 NH3 + 3O2 ―→ 2NO2– + 2H+ + 2H2O
2NO2– + O2 ―→ 2NO3–
Once the nitrate is utilized by plants, the excess nitrate in the soil is reduced back to nitrogen by Pseudomonas and Thiobacillus bacteria. This process is known as denitrification.
The concentration of usable form of nitrogen in the atmosphere is less. But certain bacteria called N2- fixers help to fix this problem. Nitrogen fixation is the process in which diatomic nitrogen is converted into ammonia by bacteria like Rhizobium, Azotobacter, etc. The conversion is carried out by an enzyme called nitrogenase. Nitrogenase is an oxygen-sensitive enzyme which requires a strict anaerobic condition. A compound called leghaemoglobin acts as an oxygen scavenger and fulfills the demand of the enzyme.
The process of nitrogen fixation is initiated with the nodule formation. Rhizobium like bacteria divides and forms colonies around the root hairs and eventually invades them. There they produce nitrogen-fixing cells. The nitrogenase enzyme in the root nodule catalyzes the formation of ammonia. The whole process is carried out at the expense of ATP which is produced during plant respiration.
STAGES OF NITROGEN CYCLE
Microorganisms: Even though nitrogen has 78 percent share in the atmosphere, it is not in usable form for plants and animals. Here comes the role of microbes. Bacteria like, Rhizobium and blue-green algae convert this non-absorbable form of nitrogen to other compounds of nitrogen that are usable. These nitrogen compounds get fixed in soil by the microbes and the process is called nitrogen fixation. The natural phenomenon of lightning also helps in nitrogen fixation.
Plants: Plants absorb the usable nitrogen compounds from the soil. Their root system helps them in taking up nitrogen from the soil. Later, these nitrogen compounds are utilized for the synthesis of proteins and other nitrogen-containing compounds of cells.
Animals: We know that animals are dependent on plants for their food. While we feed on plants, these nitrogen compounds in plants get passed onto animals.
Back to Atmosphere: During the last stage of the nitrogen cycle, microbes like bacteria and fungi act upon dead plants and animals. This is the process of decomposition. During decomposition, nitrogenous compounds get dissolved into the soil which is again used by the plants. While some other bacteria convert these nitrogenous compounds in the soil into nitrogen gas. Eventually, it goes back to the atmosphere.
These set of processes repeat continuously and thus maintain the percentage of nitrogen in the atmosphere.
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