07 Plant Growth and Mineral Nutrition - part 17 - Nitrogen cycle Nitrogen cycle:

07 Plant Growth and Mineral Nutrition - part 17 - Nitrogen cycle

Nitrogen cycle: 

• It is series of natural processes by which Nitrogen enters successively from air to organisms through soil and back to environment. 
• Plants use photosynthetic product, the sugars to make proteins. To do this, they need nitrogen. 
• Unfortunately, it is very innert (nonreactive).
• Plants need nitrogen in a reactive form usually as nitrate ions.
• Nitrogen is a limiting nutrient in the agricultural ecosystems. 
• It exists as nitrogen atoms with a strong triple covalent bond (N≡N). 
• A regular supply of nitrogen to the plants is maintained through biological and physical process. 

Nitrogen fixation :

• Atmosphere is the source of nitrogen. 
• Can not be used directly. 
• Combines with C, H, N and O to form compounds before being used.
• Conversion of free nitrogen (N₂) of the atmosphere into nitrogenous salts to make it available for the plants, is called nitrogen fixation. 

Types of Nitrogen fixation :

1. Physical fixation 
2. Biological fixation.

1. Physical Nitrogen fixation:

• Occurs in several steps and starts with combination of atmospheric nitrogen with oxygen under the influence of electric discharge and thunder storm produce nitric oxide. 
  

• The nitric oxide is then oxidized to nitrogen peroxide in the presence of oxygen. 
  

• During rains, the nitrogen peroxide combines with rain water to form nitrous acid and nitric acid which come to ground along with rains. 
  

• On ground, the alkali radicals of soil react with nitric acid to produce nitrites and nitrates.(absorbable form) 
  

•  Industrial nitrogen fixation. It occurs by Haber Bosch nitrate process at high temperature and pressure. 
  
• Ammonia is then converted to urea as it is less toxic.

Always Remember  :

• Today nearly 80% of nitrogen found in human tissues originate from the Haber Bosch process.

Biological Nitrogen fixation : 

• It is carried out by prokaryotes called as `Nitrogen fixers' or Diazotrophs'. 
• It accounts nearly 70% of natural nitrogen fixation. Nitrogen fixers are either symbiotic or free living. 
• The cyanobacteria fix significant amount of nitrogen in specialized cells called heterocysts. 
• Nitrogen fixation is high energy requiring process and nitrogen fixers use 16 molecules of ATP to fix each molecule of nitrogen to form ammonia.
  
   Ammonia is then converted into amino acids. 

Nitrification : 

• Most of the soil bacteria participate in converting ammonia into nitrate, the form of nitrogen which can be used by plants and animals. 
• This involves two steps performed by two different types of bacteria.
• First a soil bacteria convert ammonia into nitrogen-di-oxide (nitrite) eg. Nitrosomonas, Nitrosococcus, etc. 
  
• Then another type of soil bacterium called Nitrobacter adds a third oxygen atoms to create nitrate.
  
• These bacteria are chemoautotrophs. 
• By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes.

Symbiotic N2 fixation : 

• The best known nitrogen fixing symbiotic bacterium is Rhizobium. 
• This soil living/ dwelling bacterium forms root nodules in plants belonging to family Fabaceaee.g. beans, gram, groundnut etc. 

Ammonification :

• After the death of plants and animals, various fungi, actinomycetes and some ammonifying bacteria decompose the tissues and convert organic nitrogen into amino acid and then to ammonia and back into the ecosystem. 
• Ammonia (NH₄₊) is now available for uptake by plants and other micro-organisms for growth.
  

Nitrogen assimilation : 

• In soil, nitrogen is present as nitrates, nitrites and ammonia (NH₄₊). 
• It is obsorbed by the green plants and converted to nitrogenous organic compounds like amino acids, DNA, etc. This is known as nitrogen assimilation. 
• From plants, nitrogen as biomolecules like amino acids, enters food chain and moves to animals and then to decomposers through the death of animals. 
• Nitrates are first converted to ammonia but it ishighly toxic and immediately used for conversion into amino acids, which are then transported to other parts of the plants for synthesis of proteins. 
  

Amino Acid synthesis : 

• Amino acids are building blocks of proteins. 
• The amino acids are synthesized through : 

1. Reductive amination : Ammonia reacts with alpha ketoglutaric acid to form glutamic acid (glutamate).
   
2. Transamination : Amino group of one amino acid (-CHNH2) is transfered to keto position (-CO) of other carboxylic acid.
   
3. Amides : 

• Ammonia may be absorbed by amino acid to produce amides. The process is called amidation. 
• The amides are the amino acids having two amino groups. 
• Extra amino group is attached to acidic group (-COOH) in presence of ATP. 
  
• Amides like asparagine and glutamine are formed from glutamic acid and aspartic acidrespectively by addition of another amino group to each. 
• Amides are transported to other parts of plants via xylem vessels. 

Denitrification : 

• It is the process in which anaerobic bacteriacan convert soil nitrates back into nitrogen gas. 
• Denitrifying bacteria removes fixed nitrogen i.e. nitrates from the ecosystem and return it to the atmosphere in inert form. 
• Denitrifying bacteria includes - 

1. Bacillus spp., 
2. Paracoccus spp. and 
3. Pseudomonas denitrificans. 

• They transform nitrates to nitrous and nitric oxides and ultimately to gaseous nitrogen.
  

Sedimentation : 

• Nitrates of the soil are washed away to the sea or leached deep into the earth along with percolating water.

Do you know ? 

1. Soil nitrogen is replenished by excretion of animals, (as ammonia, urea and uric acid) ammonification and nitrification. 
2. Plastids contain nitrite reductase which reduces it to ammonia.

• It is series of natural processes by which Nitrogen enters successively from air to organisms through soil and back to environment. 
• Plants use photosynthetic product, the sugars to make proteins. To do this, they need nitrogen. 
• Unfortunately, it is very innert (nonreactive).
• Plants need nitrogen in a reactive form usually as nitrate ions.
• Nitrogen is a limiting nutrient in the agricultural ecosystems. 
• It exists as nitrogen atoms with a strong triple covalent bond (N≡N). 
• A regular supply of nitrogen to the plants is maintained through biological and physical process. 

Nitrogen fixation :

• Atmosphere is the source of nitrogen. 
• Can not be used directly. 
• Combines with C, H, N and O to form compounds before being used.
• Conversion of free nitrogen (N₂) of the atmosphere into nitrogenous salts to make it available for the plants, is called nitrogen fixation. 

Types of Nitrogen fixation :

1. Physical fixation 
2. Biological fixation.

1. Physical Nitrogen fixation:

• Occurs in several steps and starts with combination of atmospheric nitrogen with oxygen under the influence of electric discharge and thunder storm produce nitric oxide. 
  

• The nitric oxide is then oxidized to nitrogen peroxide in the presence of oxygen. 
  

• During rains, the nitrogen peroxide combines with rain water to form nitrous acid and nitric acid which come to ground along with rains. 
  

• On ground, the alkali radicals of soil react with nitric acid to produce nitrites and nitrates.(absorbable form) 
  

•  Industrial nitrogen fixation. It occurs by Haber Bosch nitrate process at high temperature and pressure. 
  
• Ammonia is then converted to urea as it is less toxic.

Always Remember  :

• Today nearly 80% of nitrogen found in human tissues originate from the Haber Bosch process.

Biological Nitrogen fixation : 

• It is carried out by prokaryotes called as `Nitrogen fixers' or Diazotrophs'. 
• It accounts nearly 70% of natural nitrogen fixation. Nitrogen fixers are either symbiotic or free living. 
• The cyanobacteria fix significant amount of nitrogen in specialized cells called heterocysts. 
• Nitrogen fixation is high energy requiring process and nitrogen fixers use 16 molecules of ATP to fix each molecule of nitrogen to form ammonia.
  
   Ammonia is then converted into amino acids. 

Nitrification : 

• Most of the soil bacteria participate in converting ammonia into nitrate, the form of nitrogen which can be used by plants and animals. 
• This involves two steps performed by two different types of bacteria.
• First a soil bacteria convert ammonia into nitrogen-di-oxide (nitrite) eg. Nitrosomonas, Nitrosococcus, etc. 
  
• Then another type of soil bacterium called Nitrobacter adds a third oxygen atoms to create nitrate.
  
• These bacteria are chemoautotrophs. 
• By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes.

Symbiotic N2 fixation : 

• The best known nitrogen fixing symbiotic bacterium is Rhizobium. 
• This soil living/ dwelling bacterium forms root nodules in plants belonging to family Fabaceaee.g. beans, gram, groundnut etc. 

Ammonification :

• After the death of plants and animals, various fungi, actinomycetes and some ammonifying bacteria decompose the tissues and convert organic nitrogen into amino acid and then to ammonia and back into the ecosystem. 
• Ammonia (NH₄₊) is now available for uptake by plants and other micro-organisms for growth.
  

Nitrogen assimilation : 

• In soil, nitrogen is present as nitrates, nitrites and ammonia (NH₄₊). 
• It is obsorbed by the green plants and converted to nitrogenous organic compounds like amino acids, DNA, etc. This is known as nitrogen assimilation. 
• From plants, nitrogen as biomolecules like amino acids, enters food chain and moves to animals and then to decomposers through the death of animals. 
• Nitrates are first converted to ammonia but it ishighly toxic and immediately used for conversion into amino acids, which are then transported to other parts of the plants for synthesis of proteins. 
  

Amino Acid synthesis : 

• Amino acids are building blocks of proteins. 
• The amino acids are synthesized through : 

1. Reductive amination : Ammonia reacts with alpha ketoglutaric acid to form glutamic acid (glutamate).
   
2. Transamination : Amino group of one amino acid (-CHNH2) is transfered to keto position (-CO) of other carboxylic acid.
   
3. Amides : 

• Ammonia may be absorbed by amino acid to produce amides. The process is called amidation. 
• The amides are the amino acids having two amino groups. 
• Extra amino group is attached to acidic group (-COOH) in presence of ATP. 
  
• Amides like asparagine and glutamine are formed from glutamic acid and aspartic acidrespectively by addition of another amino group to each. 
• Amides are transported to other parts of plants via xylem vessels. 

Denitrification : 

• It is the process in which anaerobic bacteriacan convert soil nitrates back into nitrogen gas. 
• Denitrifying bacteria removes fixed nitrogen i.e. nitrates from the ecosystem and return it to the atmosphere in inert form. 
• Denitrifying bacteria includes - 

1. Bacillus spp., 
2. Paracoccus spp. and 
3. Pseudomonas denitrificans. 

• They transform nitrates to nitrous and nitric oxides and ultimately to gaseous nitrogen.
  

Sedimentation : 

• Nitrates of the soil are washed away to the sea or leached deep into the earth along with percolating water.

Do you know ? 

1. Soil nitrogen is replenished by excretion of animals, (as ammonia, urea and uric acid) ammonification and nitrification. 
2. Plastids contain nitrite reductase which reduces it to ammonia.