Nitrogen Fixation
Nitrogen is an essential nutrient for all life forms to synthesise many components of their bodies. Nitrogen is an essential constituent of proteins, nucleic acids, chlorophyll and vitamins. Nitrogen is also found in other biologically important compounds such as alkaloids and urea. Though 78 percent of our atmosphere is nitrogen, plants and animals cannot avail this component from air. Hence, nitrogen has to be fixed for them. Nitrogen fixation The process of conversion of atmospheric nitrogen into nitrogenous compounds and making it available for plants is called as nitrogen fixation. The organisms which fix nitrogen to plants are called as nitrogen fixers. Microorganisms in the soil convert atmospheric nitrogen into ammonium compounds, thereby enabling the survival of plants and animals. Modes of nitrogen fixation Nitrogen fixation by microbes: Symbiosis is the phenomenon by which two organisms maintain relationship with each other to be mutually benefited. In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment. Organisms involved in this type of relationship are called as symbionts. Nitrogen fixation by lightning: Lightning also help in fixing atmospheric nitrogen into the soil. The temperature and pressure during lightning convert atmospheric nitrogen into oxides of nitrogen. These nitrogen compounds combine with rain water to form nitric and nitrous acid which mix with the soil. These acids are utilised by various organisms.   Benefits of nitrogen fixation Nitrogen cycle The sequence in which nitrogen in the atmosphere is passed into the soil and life forms, and then released back into the atmosphere, is called the nitrogen cycle. Percentage of nitrogen in atmosphere is kept constant through nitrogen cycle. a) Nitrogen fixation: The process of conversion of atmospheric nitrogen into nitrogenous compounds and making it available for plants is called as nitrogen fixation. Nitrogen fixation can happen by biological processes or physical processes. Nitrogen is introduced to the soil by fertilisers or animal and plant residues.   b) Ammonification: The process of production of ammonia from organic compounds is called ammonification. Ammonification is the process carried out by a variety of microorganisms that breakdown protein, amino acids, and other nitrogen-containing compounds of dead and waste organic matter to form ammonia. c) Nitrification: It is the process by which ammonia is converted to nitrites (NO2-) and then to nitrates (NO3-).  This process naturally occurs in the environment, where it is carried out by specialised bacteria. Nitrosomonas and nitrobacter are the bacteria which bring about nitrification. d) Denitrification: It is the biological conversion of nitrate to nitrogen gas, nitric oxide or nitrous oxide.  These compounds are gaseous compounds and are not readily available for microbial growth. Therefore, they are typically released into atmosphere. This process is brought about by denitrifying bacteria.

#### Summary

Nitrogen is an essential nutrient for all life forms to synthesise many components of their bodies. Nitrogen is an essential constituent of proteins, nucleic acids, chlorophyll and vitamins. Nitrogen is also found in other biologically important compounds such as alkaloids and urea. Though 78 percent of our atmosphere is nitrogen, plants and animals cannot avail this component from air. Hence, nitrogen has to be fixed for them. Nitrogen fixation The process of conversion of atmospheric nitrogen into nitrogenous compounds and making it available for plants is called as nitrogen fixation. The organisms which fix nitrogen to plants are called as nitrogen fixers. Microorganisms in the soil convert atmospheric nitrogen into ammonium compounds, thereby enabling the survival of plants and animals. Modes of nitrogen fixation Nitrogen fixation by microbes: Symbiosis is the phenomenon by which two organisms maintain relationship with each other to be mutually benefited. In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment. Organisms involved in this type of relationship are called as symbionts. Nitrogen fixation by lightning: Lightning also help in fixing atmospheric nitrogen into the soil. The temperature and pressure during lightning convert atmospheric nitrogen into oxides of nitrogen. These nitrogen compounds combine with rain water to form nitric and nitrous acid which mix with the soil. These acids are utilised by various organisms.   Benefits of nitrogen fixation Nitrogen cycle The sequence in which nitrogen in the atmosphere is passed into the soil and life forms, and then released back into the atmosphere, is called the nitrogen cycle. Percentage of nitrogen in atmosphere is kept constant through nitrogen cycle. a) Nitrogen fixation: The process of conversion of atmospheric nitrogen into nitrogenous compounds and making it available for plants is called as nitrogen fixation. Nitrogen fixation can happen by biological processes or physical processes. Nitrogen is introduced to the soil by fertilisers or animal and plant residues.   b) Ammonification: The process of production of ammonia from organic compounds is called ammonification. Ammonification is the process carried out by a variety of microorganisms that breakdown protein, amino acids, and other nitrogen-containing compounds of dead and waste organic matter to form ammonia. c) Nitrification: It is the process by which ammonia is converted to nitrites (NO2-) and then to nitrates (NO3-).  This process naturally occurs in the environment, where it is carried out by specialised bacteria. Nitrosomonas and nitrobacter are the bacteria which bring about nitrification. d) Denitrification: It is the biological conversion of nitrate to nitrogen gas, nitric oxide or nitrous oxide.  These compounds are gaseous compounds and are not readily available for microbial growth. Therefore, they are typically released into atmosphere. This process is brought about by denitrifying bacteria.

#### Activities

Activity 1 Ext.vt.edu has created a virtual lab to make the activity of 'Nitrogen cycle' understandable. Student has to click on the small human face icon to enter the activity. It gives the student a clear-cut idea about the nitrogen cycle in the form of a slide show. There are also other icons provided which by clicking allow the user to know about different steps and the processes involved in the nitrogen cycle. Click on the 'Farm button to view different phenomena involved in the nitrogen cycle.  Go to Activity Activity 2 Ontrack-media.net has displayed 'Nitrogen cycle' in a single page.  Student has to select the 'Play' button inroder to view the animation. By selecting the 'Step button' user can view the way in which nitrogen flows in the different  parts of the environment.  Student  can roll over the labels inroder to study the information displayed about the nitrogen cycle Go to Activity

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