Nitrogen is everywhere! In fact, N2 gas makes up about 78% of Earth’s atmosphere by volume, far surpassing the O2 we often think of as “air”.
But having nitrogen around and being able to make use of it are two different things. Your body, and the bodies of other plants and animals, have no good way to convert into a usable form. We animals and our plant compatriots just don’t have the right enzymes to capture, or fix, atmospheric nitrogen.
Still, your and proteins contain quite a bit of nitrogen. Where does that nitrogen come from? In the natural world, it comes from bacteria!
What is nitrogen Cycle?
Nitrogen cycle, circulation of nitrogen in various forms through nature. Nitrogen, a component of proteins and nucleic acids, is essential to life on Earth. Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms.
Through a series of microbial transformations, however, nitrogen is made available to plants, which in turn ultimately sustain all animal life.
The steps, which are not altogether sequential, fall into the following classifications: nitrogen fixation, nitrogen assimilation, ammonification, nitrification, and denitrification.
Nitrogen fixation, in which nitrogen gas is converted into inorganic nitrogen compounds, is mostly (90 percent) accomplished by certain bacteria and blue-green algae.
A much smaller amount of free nitrogen is fixed by abiotic means (e.g., lightning, ultraviolet radiation, electrical equipment) and by conversion to ammonia through the Haber-Bosch process.
Steps Of The Nitrogen Cycle
#1.Nitrogen Fixation.
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
#1. Atmospheric fixation.
A natural phenomenon where the energy of lightning breaks the nitrogen into nitrogen oxides, which are then used by plants.
#2. 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 fertilisers such as urea.
#3. Nitrogen Fixation by Free-Living Heterotrophs.
Many heterotrophic bacteria live in the soil and fix significant levels of nitrogen without the direct interaction with other organisms. Examples of this type of nitrogen-fixing bacteria include species of Azotobacter, Bacillus, Clostridium, and Klebsiella.
#4. Associative Nitrogen Fixation.
Species of Azospirillum are able to form close associations with several members of the Poaceae (grasses), including agronomically important cereal crops, such as rice, wheat, corn, oats, and barley. These bacteria fix appreciable amounts of nitrogen within the rhizosphere of the host plants.
#5. Symbiotic Nitrogen Fixation.
Many microorganisms fix nitrogen symbiotically by partnering with a host plant. The plant provides sugars from photosynthesis that are utilized by the nitrogen-fixing microorganism for the energy it needs for nitrogen fixation. In exchange for these carbon sources, the microbe provides fixed nitrogen to the host plant for its growth.
One example of this type of nitrogen fixation is the water fern Azolla’s symbiosis with a cyanobacterium Anabaena azollae. Anabaena colonizes cavities formed at the base of Azolla fronds.
#6. Legume Nodule Formation.
The Rhizobium or Bradyrhizobium bacteria colonize the host plant’s root system and cause the roots to form nodules to house the bacteria. The bacteria then begin to fix the nitrogen required by the plant.
Access to the fixed nitrogen allows the plant to produce leaves fortified with nitrogen that can be recycled throughout the plant. This allows the plant to increase photosynthetic capacity, which in turn yields nitrogen-rich seed.
#2.Nitrification.
In this process, the ammonia is converted into nitrate by the presence of bacteria in the soil. Nitrites are formed by the oxidation of ammonia with the help of Nitrosomonas bacteria species. Later, the produced nitrites are converted into nitrates by Nitrobacter. This conversion is very important as ammonia gas is toxic for plants.
The reaction involved in the process of Nitrification is as follows:
#3.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.
#4.Ammonification.
When plants or animals die, the nitrogen present in the organic matter is released back into the soil. The decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into ammonium. This process of decomposition produces ammonia, which is further used for other biological processes.
#5.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.
What role did bacteria play in nitrogen cycle?
- Nitrogen-fixing bacteria, which convert atmospheric nitrogen to nitrates.
- Bacteria of decay, which convert decaying nitrogen waste to ammonia.
- Nitrifying bacteria, which convert ammonia to nitrates/nitrites.
- Denitrifying bacteria, which convert nitrates to nitrogen gas.
- Fungi, like bacteria, help to convert dead plants and animals and their wastes into ammonia in the soil.
- Plants absorb nitrates from the soil to make proteins.
- Animals consume plants and use it to form animal protein.
- Humans contribute to the cycle by adding nitrogen rich fertilisers to the soil and by using manure
Nitrogen Cycle in Marine Ecosystem
So far, we’ve focused on the natural nitrogen cycle as it occurs in terrestrial ecosystems. However, generally similar steps occur in the marine nitrogen cycle. There, the ammonification, nitrification, and denitrification processes are performed by marine bacteria and archaea.
Some nitrogen-containing compounds fall to the ocean floor as sediment. Over long periods of time, the sediments get compressed and form sedimentary rock. Eventually, geological uplift may move the sedimentary rock to land.
In the past, scientists did not think that this nitrogen-rich sedimentary rock was an important nitrogen source for terrestrial ecosystems. However, a new study suggests that it may actually be quite important—the nitrogen is released gradually to plants as the rock wears away, or weathers.
Importance of Nitrogen Cycle
The nitrogen cycle refers to the movement of nitrogen within and between the atmosphere, biosphere, hydrosphere and geosphere.
The nitrogen cycle matters because nitrogen is an essential nutrient for sustaining life on Earth. Nitrogen is a core component of amino acids, which are the building blocks of proteins, and of nucleic acids, which are the building blocks of genetic material (RNA and DNA).
When other resources such as light and water are abundant, ecosystem productivity and biomass is often limited by the amount of available nitrogen. This is the primary reason why nitrogen is an essential part of fertilizers used to enhance soil quality for agricultural activities.
The importance of the nitrogen cycle are as follows:
- Helps plants to synthesise chlorophyll from the nitrogen compounds.
- Helps in converting inert nitrogen gas into a usable form for the plants through the biochemical process.
- In the process of ammonification, the bacteria help in decomposing the animal and plant matter, which indirectly helps to clean up the environment.
- Nitrates and nitrites are released into the soil, which helps in enriching the soil with the necessary nutrients required for cultivation.
- Nitrogen is an integral component of the cell and it forms many crucial compounds and important biomolecules.
How does Human activity affects nitrogen cycle?
We humans may not be able to fix nitrogen biologically, but we certainly do industrially! About 450 million metric tons of fixed nitrogen are made each year using a chemical method called the Haber-Bosch process, in which N2 is reacted with hydrogen H2 at high temperatures.
Most of this fixed nitrogen goes to make fertilizers we use on our lawns, gardens, and agricultural fields.
In general, human activity releases nitrogen into the environment by two main means: combustion of fossil fuels and use of nitrogen-containing fertilizers in agriculture. Both processes increase levels of nitrogen-containing compounds in the atmosphere.
High levels of atmospheric nitrogen other than N2 are associated with harmful effects, like the production of acid rain as nitric acid, HNO3– and contributions to the greenhouse effect as nitrous oxide, N2O.
Also, when artificial fertilizers containing nitrogen and phosphorus are used in agriculture, the excess fertilizer may be washed into lakes, streams, and rivers by surface runoff. A major effect from fertilizer runoff is saltwater and freshwater eutrophication.
In this process, nutrient runoff causes overgrowth, or a “bloom,” of algae or other microorganisms. Without the nutrient runoff, they were limited in their growth by availability of nitrogen or phosphorus.
Eutrophication can reduce oxygen availability in the water during the nighttime because the algae and microorganisms that feed on them use up large quantities of oxygen in cellular respiration.
This can cause the death of other organisms living in the affected ecosystems, such as fish and shrimp, and result in low-oxygen, species-depleted areas called dead zones.