Cycling of nitrogen in Atmosphere - Nitrogen Cycle

• The nitrogen cycle is a biogeochemical cycle in nature by which nitrogen moves through both biotic and abiotic components  in the atmosphere.

• In the atmosphere, nitrogen is present in the form of gas. but in the soils it remains present in the form of oxide  like  nitrogen monoxide NO, nitrogen dioxide NO2.

• In some cases it is present in soil in the form of  ammonia NH3. To understand the complete cycle of nitrogen, some steps are being described - 

First Step

• Nitrogen moves from the atmosphere into the soil. Earth’s atmosphere is a big reservoir of nitrogen gas .

• But this nitrogen can not be used by the plants directly because plants do not have any system to absorb nitrogen  in gaseous form.

• To overcome this situation, plants have a process called nitrogen fixation by which  nitrogen in the atmosphere becomes available to  the plants  through their root systems.

• A small amount of nitrogen that can react with oxygen whenever lightning takes place and produces nitrogen oxide, NO, and nitrogen dioxide, NO2.

• These forms of nitrogen may  enter into soils through rain or snow.

• Nitrogen is  also  fixed through the industrial process for the formation of  fertilizer. 

• In nature, most of the nitrogen is fixed by bacteria. Some bacteria are present in the roots of plants as  a symbiotic relationship.

• They derive nutrition  through photosynthesis and in return, they fix nitrogen from the atmosphere and make it available to the  plant .

• There are  some bacteria that live freely in soils or water and can fix nitrogen without any  symbiotic relationship..

SECOND STEP

• Plants uses nitrogen in the form of inorganic form and derives from soil in the form of manure  and stores in organic form as the plant’s nutrients.

• When plants die or parts of plants are detached from the body than they are decomposed by decomposers. This is called  Mineralization. 

• It occurs  when microbes  decompose the  organic material like animal manure or dead parts of plant or animal material and convert it in  form of nitrogen. This nitrogen is again used by plants.

👌👌Remember -  All plants  except legumes like  beans, peas or peanuts  get the nitrogen  through the soil.  But Legumes plants get nitrogen through biological  fixation that occurs in their root nodules,

• Nitrogen is produced by the process of mineralization in the form of  ammonia..

• The ammonia reacts with water in soil  to form ammonium. This ammonium is present  in the soils and  used by the plants.

THIRD STEP

• The third stage, nitrification, also occurs in soils.

• During nitrification the third step, the ammonia in the soils is converted in the form of nitrites, (NO2−)  and nitrates (NO3−.) .

• Nitrates can be used by plants and animals. Some bacteria in the soil can convert ammonia into nitrites.

👌👌Remember - Nitrite is not used  by plants and animals directly and various bacteria  like. Nitrosomonas and nitrobacter convert  nitrites into nitrate and make available to the plants and animals. This reaction provides energy for the bacteria engaged in this process.

• Nitrobacter converts nitrites into nitrates whereas  Nitrosomonas transform ammonia to nitrites.

• Both kinds of bacteria can function only  in the presence of oxygen hence called aerobic bacteria

FOURTH STEP

• Like the  plants, microorganisms that are present in soil also need  nitrogen as an energy source.

• These  microorganisms derive the nitrogen from the soil when they do not get enough nitrogen after decomposition.

• When microorganisms start to withdraw the  ammonium  and nitrate then these forms of nitrogen are no longer available to the plants and may cause nitrogen deficiency. This complete process is termed as Immobilization, 

FIFTH STEP

• Now  nitrogen returns to the air and  nitrates are converted to atmospheric nitrogen  by bacteria like thiobacillus Micrococcus Serratia, Pseudomonas, and Achromobacter  through the process we call it denitrification.