Nitrogen gas is a major component of air. Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids, proteins, and nucleic acids. The main component of the nitrogen cycle starts with the element nitrogen in the air. Most organisms cannot use nitrogen in this form. Plants take nitrogen in ‘fixed’ form such as ammonia (NH3), nitrate ion (NO3–). Animals secure their share of nitrogen from plants or animals who have fed on plants.
The four processes of the nitrogen cycle transform nitrogen from one form to another. Microbes play major role in these processes.
1. Nitrogen Fixation
Nitrogen in the air becomes a part of biological matter mostly through the actions of bacteria and algae in a process known as nitrogen fixation. Legume plants form nodules on the roots where nitrogen fixing bacteria take nitrogen from the air and convert it into ammonia (NH3). The ammonia is further converted by other bacteria first into nitrite ions, NO2–, and then into nitrate ions, NO3–.
Another method of nitrogen fixation takes place in atmosphere. The lightning breaks nitrogen molecules into atoms which combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, which are carried to the earth.
2. Decay
The proteins made by plants enter and pass through food webs. At each level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay which break down the molecules in excretions and dead organisms into ammonia.
3. Nitrification
Soil-living and nitrifying bacteria convert ammonia to nitrate. Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2−) then bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3−). In this way, nitrogen is made available to the roots of plants. Archael microbes present in soil and ocean convert ammonia to nitrites. Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification (conversion of organic nitrogen to nitrites and nitrates). These reach the soil when they shed their leaves.
4. Denitrification
Denitrification is the reduction of nitrates back into nitrogen gas (N2). Bacteria live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
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