The Nitrogen Cycle: Building Blocks of Life

Nitrogen is an essential component of amino acids, proteins, and DNA, making it fundamental for all living organisms. However, atmospheric nitrogen (N₂) is unusable to most life forms until it's "fixed" into a different chemical form:

Definition:

"A biological process which transforms the inert oxygen present in the atmosphere to a more usable form for living organisms".

Nitrogen Cycle Explained – Stages of Nitrogen Cycle

Process of the Nitrogen Cycle consists of the following steps – Nitrogen fixation, Nitrification, Assimilation, Ammonification and Denitrification. These processes take place in several stages and are explained below:

Nitrogen Fixation Process

It is the initial step of the nitrogen cycle. Here, Atmospheric nitrogen (N2) which is primarily available in an inert form, is converted into the usable form -ammonia (NH3).

During the process of Nitrogen fixation, the inert form of nitrogen gas is deposited into soils from the atmosphere and surface waters, mainly through precipitation.

The entire process of Nitrogen fixation is completed by symbiotic bacteria, which are known as Diazotrophs. Azotobacter and Rhizobium also have a major role in this process. These bacteria consist of a nitrogenase enzyme, which has the capability to combine gaseous nitrogen with hydrogen to form ammonia.

Nitrogen fixation can occur either by atmospheric fixation- which involves lightening, or industrial fixation by manufacturing ammonia under high temperature and pressure conditions. This can also be fixed through man-made processes, primarily industrial processes that create ammonia and nitrogen-rich fertilisers.

Types of Nitrogen Fixation

Atmospheric fixation: A natural phenomenon where the energy of lightning breaks the nitrogen into nitrogen oxides, which are then used by plants.

Industrial nitrogen fixation: It is a man-made alternative that aids in nitrogen fixation by the use of ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen. Later, it is converted into various fertilizers such as urea.

Biological nitrogen fixation: We already know that nitrogen is not used directly from the air by plants and animals. Bacteria like Rhizobium and blue-green algae transform the unusable form of nitrogen into other compounds that are more readily usable. These nitrogen compounds get fixed in the soil by these microbes.

Nitrification

Nitrification is a microbial process carried out by Nitrosomonas. Reduced nitrogen compounds are sequentially oxidised to nitrite and nitrate in this biological process.

Two groups of autotrophic nitrifying bacteria primarily accomplish the nitrification process. The conversion of ammonia to nitrate is completed fundamentally by soil-living bacteria and other nitrifying bacteria.

Along with other genera, including Nitrosococcus and Nitrosospira, Nitrosomonas is the most commonly distinguished genus associated with this first step of Nitrification.

Step – 1

In the initial stage of Nitrification, ammonia is oxidised to nitrite in the presence of ammonia-oxidising bacteria. The reaction is as follows:

Ammonia (NH3) + Oxygen (O2) → Nitrogen Dioxide (NO2)- + 3 molecules of Hydrogen (3H+) + 2 electrons

Step – 2

In the second stage of Nitrification, nitrite is oxidised to nitrate in the presence of nitrite-oxidising bacteria. The reaction is as follows:

Nitrogen Dioxide (NO2)- + Water (H2O) → Nitrate (NO3)- + 2 molecules of Hydrogen (2H+) + 2 electrons

Nitrobacter is the commonly distinguished genus, which plays an essential role in this second step of Nitrification. Other genera, including Nitrospina, Nitrospira and Nitrococcus, can also oxidise nitrite autotrophically.

Assimilation

Primary producers – plants take in the nitrogen compounds from the soil with the help of their roots, which are available in the form of ammonia, nitrite ions, nitrate ions or ammonium ions and are used in the formation of the plant and animal proteins. This way, it enters the food web when the primary consumers eat the plants.

Ammonification: 

Ammonification is the process by which organic nitrogen compounds from dead organisms, waste products, and decaying matter are converted into ammonia (NH₃) or ammonium ions (NH₄⁺). This conversion is carried out by a variety of soil microorganisms, including bacteria and fungi, which break down proteins and other nitrogen-containing compounds in dead plants and animals.

The two primary forms of nitrogen produced during ammonification are:

Ammonia (NH₃): A gaseous compound that can be toxic at high concentrations but is often converted into more usable forms by the environment.

Ammonium ions (NH₄⁺): A form that plants can directly absorb through their roots.

The Process of Ammonification

When an organism dies, or waste materials are excreted, the nitrogen within the organic matter exists primarily in complex molecules like proteins, nucleic acids, and urea. Specialized decomposer organisms, particularly saprotrophic bacteria and fungi, start to break down these molecules through enzymatic reactions. These microorganisms release enzymes like proteases and ureases that decompose proteins into amino acids and urea into ammonia.

Once ammonia is produced, several things can happen:

Direct absorption by plants: Ammonium ions (NH₄⁺) can be absorbed directly by plant roots, providing them with a ready source of nitrogen for growth.

Further transformation: In the presence of nitrifying bacteria, ammonia can be converted into nitrites (NO₂⁻) and then nitrates (NO₃⁻), which are other plant-usable forms of nitrogen.

Volatilization: If conditions in the soil are too alkaline, ammonia can escape into the atmosphere, reducing the amount of nitrogen available to plants.

Denitrification

Denitrification is the process in which the nitrogen compounds make their way back into the atmosphere by converting nitrate (NO3-)  into gaseous nitrogen (N). This process of the nitrogen cycle is the final stage and occurs in the absence of oxygen. Denitrification is carried out by the denitrifying bacterial species- Clostridium and Pseudomonas, which will process nitrate to gain oxygen and gives out free nitrogen gas as a byproduct.

Nitrogen Cycle in Marine Ecosystem

The process of the nitrogen cycle occurs in the same manner in the marine ecosystem as in the terrestrial ecosystem. The only difference is that it is carried out by marine bacteria.

The nitrogen-containing compounds fall into the ocean as sediments get compressed over long periods and form sedimentary rock. Due to the geological uplift, these sedimentary rocks move to land. Initially, it was not known that these nitrogen-containing sedimentary rocks are an essential source of nitrogen. But, recent researches have proved that the nitrogen from these rocks is released into the plants due to the weathering of rocks.

Importance of Nitrogen Cycle

The importance of the nitrogen cycle are as follows:

1. Helps plants to synthesis chlorophyll from the nitrogen compounds.
2. Helps in converting inert nitrogen gas into a usable form for the plants through the biochemical process.
3. In the process of ammonification, the bacteria help in decomposing the animal and plant matter, which indirectly helps to clean up the environment.
4. Nitrates and nitrites are released into the soil, which helps in enriching the soil with the necessary nutrients required for cultivation.
5. Nitrogen is an integral component of the cell and it forms many crucial compounds and important biomolecules.

Nitrogen is also cycled by human activities such as the combustion of fuels and the use of nitrogen fertilizers. These processes increase the levels of nitrogen-containing compounds in the atmosphere. The fertilizers containing nitrogen are washed away in lakes, rivers and result in eutrophication.