Nitrogen Cycle: Explanation, Steps & Importance - UPSC Notes

The Nitrogen cycle is the process of conversion of Nitrogen to Nitrate. The amount of nitrogen in the air we breathe is 78% accompanied with 21% of oxygen and the remaining percent is other gases. And this nitrogen cannot be used directly by plants and animals. To use Nitrogen, it is important to convert them into nitrates, nitrates and ammonia compounds. All this conversion is possible only by the Nitrogen cycle.

In this article on the Nitrogen Cycle UPSC, we will learn what is the nitrogen cycle, why is the nitrogen cycle important, how to draw the nitrogen cycle diagram, and many more relevant things which will be very beneficial for the UPSC aspirants.

What is Nitrogen Cycle?

Nitrogen is one of the essential structural components of the DNA, RNA and protein molecules. Because of this Nitrogen is required by all organisms for their proper growth. Though it is known that the majority of Nitrogen inhaled by organisms is in the form of N2, this much atmospheric nitrogen is not available in this form. The reason behind this is that there is a strong bond present between N2 molecules, which makes it inert or unreactive in nature. So, for the conversion of this unreactive or inert molecule into reactive state N2 molecules need to be converted into ammonium (NH4+), nitrate (NO3-), or organic nitrogen. All these forms of Nitrogen can be easily used by organisms.

If Nitrogen is taken in N2 form it can affect the natural ecosystem and also can limit the growth of plants. Nitrogen exists in different forms i.e. both organic and inorganic forms with different oxidation states. So, the movement of different forms of Nitrogen into the atmosphere, biosphere and geosphere is studied under the Nitrogen cycle. This cycle is one of the major biogeochemical cycles. The below nitrogen cycle flow chart gives the steps involved in the cycle:

Fig: Nitrogen cycle diagram

Also, check the NCERT Notes on Endogenic Processes here.

Nitrogen Cycle Steps

Nitrogen cycle consists of five major steps. By going through all these steps Nitrogen moves through the biosphere, atmosphere, geosphere. The steps which are involved in the Nitrogen cycle are: Nitrogen fixation, Nitrogen Uptake, Nitrogen mineralisation, Nitrification and Denitrification.

In all these steps one organism which plays a crucial role is microorganisms, mainly bacteria. As all these steps are mediated by different microorganisms.

Nitrogen Fixation

The process of conversion of the N2 form of Nitrogen into ammonium or NH4+ is called Nitrogen fixation. Through this process only organisms can obtain Nitrogen directly from the atmosphere.

• This nitrogen fixation process is performed by nitrogen-fixing organisms. Certain bacterias like Rhizobium have the ability to fix atmospheric nitrogen and convert them into ammonium form through metabolic processes.
• Nitrogen fixing bacterias always show symbiotic relationships with host plants. This symbiotic relationship is commonly seen in Legume plants like peas, beans, clover. During this relationship process, nitrogen-fixing bacteria inhabit legume root nodules and receive carbohydrates and a favorable environment from their host plant in return they fix Nitrogen for the plants.
• Along with Nitrogen fixing symbiotic associating bacterias there are free-living bacterias also which survive without plant hosts. Examples of such bacterias: Blue green algae or cyanobacteria.
• Along with Nitrogen fixing bacterias, high energy natural phenomena also has a tendency to break N2 strong bonds such a process is called chemical transformation.

Nitrogen Assimilation

The ammonium (NH4+) which is produced by Nitrogen fixing bacteria is taken up by host plants, other organisms, and bacteria itself. These consumed ammonium forms of Nitrogen are incorporated into protein and other Nitrogen compounds like DNA. This ammonium form is taken by other higher organisms like human being through food chain

NH4+ → Organic N

Read the article on Major Minerals and Their Characteristics!

Nitrogen Ammonification

Once Nitrogen is incorporated into organic matter, they get converted into inorganic nitrogen through the mineralisation process. This process is also known as the decaying process. This process occurs when organisms die and undergo decomposition. During this decomposition process Nitrogen is converted into ammonium.

After this step, ammonium form of Nitrogen can be used by plants also and undergoes conversion into Nitrate (NO3-) through a process known as Nitrification.

Organic N → NH4+

Nitrification

Some amount of ammonium which is produced during the decomposition process is converted into Nitrate form through a process of Nitrification. This chemical reaction requires oxygen to be carried out. So, it can be said that this reaction occurs in an oxygen rich environment only.

NH4+ → NO3-

Nitrate form decreases the fertility of the soil along with enriching downstream surface and groundwater.

Denitrification

The process in which oxidised forms of Nitrogen such as Nitrate ( NO3-) and Nitrate ( NO2-) is converted into dinitrogen (N2) and ( NO2) nitrous oxide gas. This process is carried out by denitrifying bacteria, which help in conversion of Nitrate to dinitrogen.

Steps involved in conversion of Nitrate to dinitrogen.

NO3- → NO2- → NO → N2O → N2.

Nitric oxide and nitrous oxide are types of gases that have environmental impacts. Nitric oxide (NO) helps in the formation of smog, whereas nitrous oxide (N2O) is an important greenhouse gas, thereby contributing to global climate change. After conversion of nitrates or nitrite into dinitrogen form, it is unlikely to reconvert them into biologically available form.

Read the article on Weathering!

Human Influences on the Nitrogen Cycle

There are several human activities that alter the Nitrogen cycle process. Some major alteration which are caused are listed below:

• Use of Nitrogen fertilizers to crop causes an increase in rate of denitrification and leaching of nitrates into groundwater. By this process Nitrogen enters into the groundwater which further flows into the steam, rivers lake and estuaries. This extra amount of Nitrogen causes an eutrophication process.
• Deposition of nitrogen in the atmosphere is also increased by combustion of fossil fuel and burning of forest.
• Livestock ranching increases a large amount of ammonia into the environment. This ammonia is formed from livestock wastes. Further this Nitrogen enters into the soil system and then the hydrologic system through leaching, groundwater flow, and runoff.

Read the article on Carbon Cycle!

Significance of Nitrogen Cycle in the Ocean

The nitrogen cycle in the ocean is a complex process that involves the transformation and cycling of nitrogen compounds within the marine ecosystem. It plays a crucial role in sustaining marine life, primary productivity, and overall ecosystem functioning.

The nitrogen cycle begins with nitrogen fixation, where atmospheric nitrogen (N2) is converted into ammonia (NH3) or ammonium (NH4+) through biological or industrial processes. This conversion is primarily carried out by nitrogen-fixing bacteria, which live in symbiotic relationships with certain marine plants.

Ammonia generated can be directly utilized by marine organisms, or it can be further transformed through nitrification. Nitrification is the process by which ammonia is oxidized into nitrite (NO2-) by nitrifying bacteria. It is then oxidized into nitrate (NO3-) by different groups of bacteria. Nitrate is a crucial form of nitrogen that serves as a primary nutrient for marine plants, algae, and phytoplankton.

Assimilation is another key process in the nitrogen cycle, where marine organisms incorporate nitrate and ammonia from the surrounding water into their tissues.

The excess nitrogen in the ocean ecosystem is removed through denitrification. Denitrification is the process by which certain bacteria convert nitrate back into atmospheric nitrogen gas, completing the nitrogen cycle. Denitrifying bacteria use nitrate as an electron acceptor in the absence of oxygen.

Activities, such as excessive fertilizer use in agriculture, industrial pollution, and urban runoff, can severely alter the nitrogen cycle in the ocean. These activities can lead to an excess input of nitrogen compounds into the marine environment. This can cause eutrophication, harmful algal blooms, and disruption of the natural balance of the ecosystem.

Read the article on Heating and Cooling of the Atmosphere!

Key Takeaways on Nitrogen Cycle for UPSC Aspirants Introduction: The nitrogen cycle is a biogeochemical cycle that involves the conversion of nitrogen among various chemical forms. It is crucial for all living organisms as nitrogen is a key component of amino acids, proteins, and nucleic acids. Nitrogen Fixation: Atmospheric nitrogen (N2) is converted into ammonia (NH3) by nitrogen-fixing bacteria in the soil, often associated with leguminous plants. Industrial processes like the Haber-Bosch method also fix nitrogen into usable forms. Nitrification: Ammonia is converted into nitrate ions (NO3-) through a two-step process involving nitrifying bacteria. First, bacteria convert ammonia to nitrite (NO2-); then other bacteria convert nitrite to nitrate. Assimilation: Plants absorb nitrates from the soil and incorporate them into plant proteins and nucleic acids. When animals consume plants, they assimilate these nitrogen compounds into their bodies.

Download the Nitrogen Cycle Notes PDF for Key Takeaways!

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