Nitrogen Cycle Diagram

Why Is the Nitrogen Cycle Important? | Nitrogen Fixation | Nitrogen Cycle Process

Nitrogen – What is it?

Nitrogen is required for life as a constituent of proteins and nucleic acids. Nitrogen exists in a variety of forms and is constantly cycled between these forms by a variety of microorganisms.

While nitrogen is abundant in the atmosphere as diatomic nitrogen gas (N2), it is exceedingly stable and requires a tremendous deal of energy to convert to other forms.

Historically, the biologically accessible forms of nitrogen, NO3- and NH3, were frequently limited; however, modern anthropogenic operations, such as fertilizer manufacturing, have significantly enhanced nitrogen availability to living organisms.

Nitrogen cycling between its various forms is a complicated process involving a variety of bacteria and environmental factors.

What is Nitrogen Cycle?

The nitrogen cycle is a biogeochemical process that converts nitrogen into a variety of chemical forms as it travels through the atmosphere, terrestrial, and marine environments. Nitrogen conversion can occur in either biological or physical processes.

Fixation, ammonification, nitrification, and denitrification are all critical processes in the nitrogen cycle. The bulk (78 percent) of the Earth’s atmosphere is nitrogen, making it the primary source of nitrogen.

However, atmospheric nitrogen is scarce for biological use, resulting in a nitrogen shortage in a variety of ecosystems.

Ecologists are particularly interested in the nitrogen cycle because nitrogen availability can alter the rate of critical ecosystem processes such as primary production and breakdown.

Human activities such as the combustion of fossil fuels, the use of synthetic nitrogen fertilizers, and the discharge of nitrogen into wastewater have had a tremendous effect on the global nitrogen cycle.

Human manipulation of the global nitrogen cycle has the potential to have a detrimental effect on both the natural environment and human health.

Nitrogen Cycle Process

The Nitrogen Cycle is a process in which nitrogen is converted into a variety of forms and then returned to the atmosphere through the soil and organisms. Numerous processes are involved, including nitrogen fixation, nitrification, denitrification, degradation, and putrefaction.

Nitrogen is essential to life because it is a key component of proteins and nucleic acids. Nitrogen occurs in many forms and is continuously cycled among these forms by a variety of bacteria.

Although nitrogen is abundant in the atmosphere as diatomic nitrogen gas (N2), it is extremely stable, and conversion to other forms requires a great deal of energy.

Historically, the biologically available forms NO3- and NH3 have often been limited; however, current anthropogenic processes, such as fertilizer production, have greatly increased the availability of nitrogen to living organisms.

The cycling of nitrogen among its many forms is a complex process that involves numerous types of bacteria and environmental conditions.

Nitrogen Cycle Steps

The nitrogen cycle is composed of five steps, namely:

  1. Nitrogen fixation (N2 to NH3/ NH4+ or NO3-)
  2. Nitrification (NH3 to NO3-)
  3. Assimilation (Incorporation of NH3 and NO3- into biological tissues)
  4. Ammonification (organic nitrogen compounds to NH3)
  5. Denitrification (NO3- to N2)

 Nitrogen Cycle Steps 1: Nitrogen Fixation

Nitrogen fixation is the conversion of gaseous nitrogen (N2) to ammonia (NH3 or NH4+) by biological fixation or to nitrate (NO3-) via high-energy physical processes.

N2 is an exceptionally stable compound, and it takes a tremendous deal of energy to break the bonds connecting the two N atoms. N2 can be transformed directly into NO3- using extremely high temperatures, pressures, and energies.

Combustion, volcanic activity, lightning discharges, and industrial operations are all examples of such processes. However, a bigger proportion of physiologically accessible nitrogen is created naturally as a result of the biological conversion of N2 to NH3/NH4+.

A tiny group of bacteria and cyanobacteria are capable of breaking the bonds between molecular nitrogen and combining it with hydrogen using the enzyme nitrogenase.

Nitrogenase is an enzyme that is inactive in the absence of oxygen. The exclusion of oxygen can be performed in a variety of ways.

On the roots of certain plants, certain bacteria reside behind layers of oxygen-depleted slime. Rhizobium, the most abundant soil bacteria, lives in oxygen-depleted zones in nodules on the roots of legumes and some other woody plants. Aquatic filamentous cyanobacteria make use of cells that lack oxygen called heterocysts.

Nitrogen Cycle Step 2: Nitrification

Nitrification is a two-step process that converts NH3/NH4+ to NO3-.

Nitrosomonas and Nitrococcus soil bacteria convert NH3 to NO2, and another soil bacterium, Nitrobacter, oxidizes NO2- to NO3-. Two bacteria obtain energy via these transformations, which both require oxygen to take place.

Nitrogen Cycle Step 3: Assimilation

Assimilation is the process by which plants and animals absorb NO3- and ammonia produced during nitrogen fixation and nitrification.

Through their roots, plants absorb these types of nitrogen and incorporate them into their proteins and nucleic acids. Animals can then utilize the nitrogen found in plant tissues.

Nitrogen Cycle Step 4: Ammonification

Assimilation generates a significant amount of organic nitrogen in the form of proteins, amino acids, and nucleic acids. Ammonification is the process by which organic nitrogen is converted to ammonia.

This process excretes ammonia into the environment, where it is available for nitrification or absorption.

Nitrogen Cycle Step 4: Denitrification

Denitrification occurs when anaerobic bacteria reduce NO3- to gaseous N2. This process happens only in areas with low to no oxygen levels, such as deep in the earth near the water table.

As a result, places such as wetlands are advantageous for denitrification processes that reduce excess nitrogen levels.

Nitrogen Cycle Diagram

The nitrogen cycle diagram illustrates how several steps of the nitrogen cycle affect denitrifying bacteria’ ability to process nitrates and nitrites in groundwater.

Nitrogen Cycle Diagram


The nitrogen cycle diagram above outlines the main steps by which nitrogen is converted between its components.

Common Forms of Nitrogen

  • The most common forms of inorganic nitrogen in the environment are;
  • Diatomic nitrogen gas (N2)
  • Nitrate (NO3-)
  • Nitrite (NO2-)
  • Ammonia (NH3)
  • Ammonium (NH4+).

The presence of nitrogen in aquatic environments as unionized ammonia (NH3) or ammonium (NH4+) is pH and temperature-dependent.

When the pH is less than 8.75, NH4+ is the dominant cation. Increases in pH indicate an increase in the concentration of the hydroxyl ion (OH-) in the water, implying that the above process will shift to the left in order to achieve equilibrium.

Above a pH of 9.75, NH3 is the predominant cation (Hem, 1985). Aquatic life is more harmful to NH3. If biological absorption of NH3 is insufficient, NH3 can accumulate and have a negative effect on aquatic life.

NH4+ ions are highly adsorbed in soils by clay particles and organic materials, which have a net negative surface charge. NH4+ is transformed to NH3 gas and lost to the environment in alkaline soils.

Nitrification occurs when NH4+ in the soil is converted to NO3- under warm growing conditions. NO3- is a highly soluble compound that can rapidly be leached from soils in moist situations.

How Do Humans Impact the Nitrogen Cycle?

Numerous human activities have a tremendous effect on the nitrogen cycle. The combustion of fossil fuels, the application of nitrogen-based fertilizers, and other activities can significantly enhance an ecosystem’s biologically accessible nitrogen.

Furthermore, because nitrogen availability is frequently a constraint on primary productivity in many ecosystems, considerable changes in nitrogen availability can result in significant changes to the nitrogen cycle in both aquatic and terrestrial ecosystems.

Since the 1940s, industrial nitrogen fixation has expanded dramatically, and human activity has doubled global nitrogen fixation.

Nitrogen addition can result in nutrient imbalances in trees, changes in forest health, and decreases in biodiversity in terrestrial ecosystems.

Increased nitrogen availability frequently results in a shift in carbon storage, affecting more than simply the nitrogen cycle.

Fertilizers are widely employed in agricultural systems to boost plant output, but excess nitrogen, typically in the form of nitrate, can leak from the soil, infiltrate streams and rivers, and eventually make its way into our drinking water.

The Haber-Bosch method, which involves inducing N2 to react with H2 to create synthetic fertilizers for agricultural use, has increased dramatically over the last several decades.

Indeed, nearly 80% of the nitrogen found in human tissues today is a result of the Haber-Bosch process.

The majority of nitrogen supplied to agricultural and urban areas eventually finds its way into rivers and nearshore marine systems.

Increased nitrogen levels in nearshore marine environments frequently result in anoxia (lack of oxygen) or hypoxia (insufficient oxygen), reduced biodiversity, changes in food web structure, and general habitat deterioration.

Increased nitrogen levels frequently result in an increase in toxic algal blooms.

In some regions, toxic blooms of specific species of dinoflagellates have been linked to a high rate of fish and shellfish mortality.

Even without such economically disastrous consequences, nitrogen addition can result in changes in biodiversity and species composition, which can affect the overall function of an ecosystem.

Certain researchers have even claimed that altering the nitrogen cycle may raise the danger of parasitic and infectious diseases in humans and wildlife.

Additionally, higher nitrogen levels in aquatic systems can contribute to the acidification of freshwater environments.

The atmospheric concentration of nitrous oxide (N2O) has increased due to agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources.

N2O is a carcinogen in the stratosphere, where it degrades and functions as a catalyst for the depletion of atmospheric ozone.

Nitrous oxide is also a greenhouse gas and, after carbon dioxide and methane, is the third-largest contributor to global warming.

While it is not as common in the atmosphere as carbon dioxide, it is roughly 300 times more potent at warming the earth on an equivalent mass basis.

Nitrogen Chemicals and Their Possible Environmental Consequences

Agriculture, through fertilizers and the development of nitrogen-fixing crops, may be responsible for almost half of the nitrogen fixation on Earth. Increased nitrogen inputs (into the soil) have resulted in a massive increase in food production to feed an increasing population – dubbed the ‘green revolution.

However, excess nitrogen can seep from soils into streams. Eutrophication is facilitated by nitrogen enrichment.

Another issue can arise during the nitrification and denitrification processes. Nitrous oxide (N2O) can be generated if the chemical reaction is not finished. This is significant because N2O is a powerful greenhouse gas that contributes to global warming.

Nitrogen molecules in the environment in balance sustain plant life and do not pose a harm to animals. Only when the cycle is out of balance do problems arise.

Why Is the Nitrogen Cycle Important?

The nitrogen cycle is critical to the functioning of numerous cells and activities, including amino acids, proteins, and even our DNA.

It is also required for plants to produce chlorophyll, which is utilized in photosynthesis to provide food. Nitrogen is converted from one chemical form to another as part of numerous biological activities.

The nitrogen cycle is critical to life on Earth. Bacteria use nitrogen from the air to create nutrients in the soil. These nutrients contribute to a plant’s normal growth. Animals and humans consume nitrogen found inside plants.

Why Is the Nitrogen Important?

Nitrogen is critical for plant development (structure), food processing (metabolism), and chlorophyll synthesis. Without sufficient nitrogen in the plant, it cannot grow taller or generate sufficient food (usually yellow).

However, an excess of nitrogen is just as harmful. It can result in the plant being scorched to the point where the leaves shrink and the plant is unable to produce blooms (or grains).

Nitrogen can also be a significant pollutant in groundwater and the oceans. This is why it is critical to understand the nitrogen cycle, despite its complexity.

The majority of nitrogen on Earth is in the form of a gas. Because plants are incapable of extracting nitrogen from the air, bacteria must do it for them.

Certain legume plants (soybeans, peanuts, peas, and clover) have a unique connection with the Rhizobia bacteria. These bacteria form little nodules on the root.

When these nodules are sliced open, they should be pink or crimson. This means that future crops will use less fertilizer, saving both money and the environment.

Nitrogen is a critical component of a variety of cells and processes, including amino acids, proteins, and even our DNA. It is also required for plants to produce chlorophyll, which is utilized in photosynthesis to provide food.

Nitrogen is converted from one chemical form to another as part of numerous biological activities. The nitrogen cycle is the series of transformations that nitrogen through as it flows between the atmosphere, the land, and living beings.

How Is Nitrogen Cycle Important to Humans?

Nitrogen is a critical component of the nucleic acids DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid), which are the most significant biological molecules and are required for the existence of all living beings.

DNA contains genetic information, which translates to the instructions for developing a life form. When plants lack sufficient nitrogen, they are unable to synthesize amino acids (substances that contain nitrogen and hydrogen and make up many of living cells, muscles and tissue).

Without amino acids, plants are unable to synthesize the specific proteins required for cell growth. Without an adequate supply of nitrogen, plant growth is significantly impacted.

When plants receive an excessive amount of nitrogen, they generate an excess of biomass, or organic matter, such as stalks and leaves, but not nearly enough root structure.

In extreme circumstances, plants that absorb an excessive amount of nitrogen from the soil might damage farm animals that consume them.

The Risk of Excess Nitrogen

While the critical nature of nitrogen to plant and animal life may make it appear as though there is no such thing as too much, there are some risks associated with adding too many nitrates to the soil.

As with anything else, large amounts of nitrogen compounds can be harmful. Just as an excess of oxygen is toxic to air-breathing organisms, an excess of nitrogen can be detrimental to plants.

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