explain in brief about nitrogen cycle 



Asked by nazlimeerah | 29th Dec, 2018, 02:50: PM

Expert Answer:

Nitrogen cycle:

The process of circulation of nitrogen between the atmosphere, soil, plants and animals is called the nitrogen cycle. Atmospheric nitrogen is fixed by biological nitrogen fixers as well as through physical processes such as lightning.

Nitrogen Fixation during Lightning

  • During lightning, nitrogen and oxygen from the atmosphere combine together and form nitrogen oxides. These gases react with rainwater to form dilute nitric acid. The nitric acid then reacts with minerals present in the soil to form nitrates.
  • Plants take in nitrates from the soil through their root system and convert them into proteins. Animals consume these plants and the proteins then enter into their bodies indirectly.
  • When plants and animals eventually die, the nitrogenous compounds present in their bodies are broken down into ammonia. This process is known as ammonification.
  • Ammonia is converted to nitrates by nitrifying bacteria such as Nitrosomonas and Nitrobacter through a process called nitrification.
  • Nitrates may be stored in the form of humus, or they can be leached from the soil and carried to lakes and reservoirs.
  • Nitrates can also be converted to free molecular nitrogen by denitrifying bacteria such as Pseudomonas through a process called denitrification and returned to the atmosphere.
  • Due to constant recycling of nitrogen, the percentage of nitrogen in the atmosphere remains more or less constant. 

Biological Nitrogen Fixation

  • The process of conversion of free nitrogen from the atmosphere into soluble nitrates by microorganisms is known as biological nitrogen fixation. 
  • The microbes which carry out biological nitrogen fixation are commonly called biological nitrogen fixers. Examples: Rhizobium and Azotobacter
  • Some bacteria such as Rhizobium leguminosarum live in the root nodules of leguminous plants such as beans, fenugreek, groundnut, pea and gram and are able to convert free nitrogen from the atmosphere into soluble nitrates.
  • The bacteria not only obtain food but also protection from the root nodules. In return, they enrich the soil with nitrogen and increase its fertility. The host plant and other plants sown later in the same soil use the nitrates produced by the bacteria.
  • Some free-living bacteria like Azotobacter convert inorganic molecular nitrogen into organic compounds such as amino acids and proteins.
  • Some bacteria recycle the nitrates back to the soil through the process of nitrification. During nitrification, the nitrogenous wastes from dead plants and animals are converted into ammonia by the action of bacteria such as Bacillus ramosus and Clostridium spp.
  • Ammonium compounds are formed from ammonia in the subsequent reaction. Finally, these ammonium compounds get converted first into nitrites by Nitrosomonas bacteria, and then into nitrates by Nitrobacter.
  • Nitrification is followed by denitrification, wherein nitrates are converted into nitrogen gas (N2) by the action of denitrifying bacteria such as species of Nitrosomonas, Pseudomonas, Alkaligenes and Bacillus. The nitrogen gas is then released into the atmosphere. 

Answered by Sheetal Kolte | 2nd Jan, 2019, 03:23: PM