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The Nitrogen Cycle: Processes, Players, and Human Impact

Introduction

Nitrogen is one of the primary nutrients critical for the survival of all living organisms. It is a necessary component of many biomolecules, including proteins, DNA, and chlorophyll. Although nitrogen is very abundant in the atmosphere as dinitrogen gas (N 2 ), it is largely inaccessible in this form to most organisms, making nitrogen a scarce resource and often limiting primary productivity in many ecosystems. Only when nitrogen is converted from dinitrogen gas into ammonia (NH 3 ) does it become available to primary producers, such as plants.

In addition to N 2 and NH 3 , nitrogen exists in many different forms, including both inorganic (e.g., ammonia, nitrate) and organic (e.g., amino and nucleic acids) forms. Thus, nitrogen undergoes many different transformations in the ecosystem, changing from one form to another as organisms use it for growth and, in some cases, energy. The major transformations of nitrogen are nitrogen fixation, nitrification, denitrification, anammox, and ammonification (Figure 1). The transformation of nitrogen into its many oxidation states is key to productivity in the biosphere and is highly dependent on the activities of a diverse assemblage of microorganisms, such as bacteria, archaea, and fungi.

Since the mid-1900s, humans have been exerting an ever-increasing impact on the global nitrogen cycle. Human activities, such as making fertilizers and burning fossil fuels, have significantly altered the amount of fixed nitrogen in the Earth's ecosystems. In fact, some predict that by 2030, the amount of nitrogen fixed by human activities will exceed that fixed by microbial processes (Vitousek 1997). Increases in available nitrogen can alter ecosystems by increasing primary productivity and impacting carbon storage (Galloway et al . 1994). Because of the importance of nitrogen in all ecosystems and the significant impact from human activities, nitrogen and its transformations have received a great deal of attention from ecologists.

Nitrogen Fixation

Nitrogen gas (N 2 ) makes up nearly 80% of the Earth's atmosphere, yet nitrogen is often the nutrient that limits primary production in many ecosystems. Why is this so? Because plants and animals are not able to use nitrogen gas in that form. For nitrogen to be available to make proteins, DNA, and other biologically important compounds, it must first be converted into a different chemical form. The process of converting N 2 into biologically available nitrogen is called nitrogen fixation. N 2 gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms, and it requires a large amount of energy to break this bond. The whole process requires eight electrons and at least sixteen ATP molecules (Figure 2). As a result, only a select group of prokaryotes are able to carry out this energetically demanding process. Although most nitrogen fixation is carried out by prokaryotes, some nitrogen can be fixed abiotically by lightning or certain industrial processes, including the combustion of fossil fuels.

Some of these bacteria are aerobic, others are anaerobic; some are phototrophic, others are chemotrophic (i.e., they use chemicals as their energy source instead of light) (Table 1). Although there is great physiological and phylogenetic diversity among the organisms that carry out nitrogen fixation, they all have a similar enzyme complex called nitrogenase that catalyzes the reduction of N 2 to NH 3 (ammonia), which can be used as a genetic marker to identify the potential for nitrogen fixation. One of the characteristics of nitrogenase is that the enzyme complex is very sensitive to oxygen and is deactivated in its presence. This presents an interesting dilemma for aerobic nitrogen-fixers and particularly for aerobic nitrogen-fixers that are also photosynthetic since they actually produce oxygen. Over time, nitrogen-fixers have evolved different ways to protect their nitrogenase from oxygen. For example, some cyanobacteria have structures called heterocysts that provide a low-oxygen environment for the enzyme and serves as the site where all the nitrogen fixation occurs in these organisms. Other photosynthetic nitrogen-fixers fix nitrogen only at night when their photosystems are dormant and are not producing oxygen.

Genes for nitrogenase are globally distributed and have been found in many aerobic habitats (e.g., oceans, lakes, soils) and also in habitats that may be anaerobic or microaerophilic (e.g., termite guts, sediments, hypersaline lakes, microbial mats, planktonic crustaceans) (Zehr et al . 2003). The broad distribution of nitrogen-fixing genes suggests that nitrogen-fixing organisms display a very broad range of environmental conditions, as might be expected for a process that is critical to the survival of all life on Earth.

Nitrification

Nitrification is the process that converts ammonia to nitrite and then to nitrate and is another important step in the global nitrogen cycle. Most nitrification occurs aerobically and is carried out exclusively by prokaryotes. There are two distinct steps of nitrification that are carried out by distinct types of microorganisms. The first step is the oxidation of ammonia to nitrite, which is carried out by microbes known as ammonia-oxidizers. Aerobic ammonia oxidizers convert ammonia to nitrite via the intermediate hydroxylamine, a process that requires two different enzymes, ammonia monooxygenase and hydroxylamine oxidoreductase (Figure 4). The process generates a very small amount of energy relative to many other types of metabolism; as a result, nitrosofiers are notoriously very slow growers. Additionally, aerobic ammonia oxidizers are also autotrophs, fixing carbon dioxide to produce organic carbon, much like photosynthetic organisms, but using ammonia as the energy source instead of light.

Unlike nitrogen fixation that is carried out by many different kinds of microbes, ammonia oxidation is less broadly distributed among prokaryotes. Until recently, it was thought that all ammonia oxidation was carried out by only a few types of bacteria in the genera Nitrosomonas , Nitrosospira , and Nitrosococcus . However, in 2005 an archaeon was discovered that could also oxidize ammonia (Koenneke et al . 2005). Since their discovery, ammonia-oxidizing Archaea have often been found to outnumber the ammonia-oxidizing Bacteria in many habitats. In the past several years, ammonia-oxidizing Archaea have been found to be abundant in oceans, soils, and salt marshes, suggesting an important role in the nitrogen cycle for these newly-discovered organisms. Currently, only one ammonia-oxidizing archaeon has been grown in pure culture, Nitrosopumilus maritimus , so our understanding of their physiological diversity is limited.

The second step in nitrification is the oxidation of nitrite (NO 2 - ) to nitrate (NO 3 - ) (Figure 5). This step is carried out by a completely separate group of prokaryotes, known as nitrite-oxidizing Bacteria. Some of the genera involved in nitrite oxidation include Nitrospira , Nitrobacter , Nitrococcus , and Nitrospina . Similar to ammonia oxidizers, the energy generated from the oxidation of nitrite to nitrate is very small, and thus growth yields are very low. In fact, ammonia- and nitrite-oxidizers must oxidize many molecules of ammonia or nitrite in order to fix a single molecule of CO 2 . For complete nitrification, both ammonia oxidation and nitrite oxidation must occur.

Ammonia-oxidizers and nitrite-oxidizers are ubiquitous in aerobic environments. They have been extensively studied in natural environments such as soils, estuaries, lakes, and open-ocean environments. However, ammonia- and nitrite-oxidizers also play a very important role in wastewater treatment facilities by removing potentially harmful levels of ammonium that could lead to the pollution of the receiving waters. Much research has focused on how to maintain stable populations of these important microbes in wastewater treatment plants. Additionally, ammonia- and nitrite-oxidizers help to maintain healthy aquaria by facilitating the removal of potentially toxic ammonium excreted in fish urine.

Traditionally, all nitrification was thought to be carried out under aerobic conditions, but recently a new type of ammonia oxidation occurring under anoxic conditions was discovered (Strous et al . 1999). Anammox (anaerobic ammonia oxidation) is carried out by prokaryotes belonging to the Planctomycetes phylum of Bacteria. The first described anammox bacterium was Brocadia anammoxidans . Anammox bacteria oxidize ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen (Figure 6). Anammox bacteria were first discovered in anoxic bioreactors of wasterwater treatment plants but have since been found in a variety of aquatic systems, including low-oxygen zones of the ocean, coastal and estuarine sediments, mangroves, and freshwater lakes. In some areas of the ocean, the anammox process is considered to be responsible for a significant loss of nitrogen (Kuypers et al . 2005). However, Ward et al . (2009) argue that denitrification rather than anammox is responsible for most nitrogen loss in other areas. Whether anammox or denitrification is responsible for most nitrogen loss in the ocean, it is clear that anammox represents an important process in the global nitrogen cycle.

Denitrification

Denitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere. Dinitrogen gas (N 2 ) is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist (Figure 7). Some of these gases, such as nitrous oxide (N 2 O), are considered greenhouse gasses, reacting with ozone and contributing to air pollution.

Unlike nitrification, denitrification is an anaerobic process, occurring mostly in soils and sediments and anoxic zones in lakes and oceans. Similar to nitrogen fixation, denitrification is carried out by a diverse group of prokaryotes, and there is recent evidence that some eukaryotes are also capable of denitrification (Risgaard-Petersen et al . 2006). Some denitrifying bacteria include species in the genera Bacillus , Paracoccus , and Pseudomonas . Denitrifiers are chemoorganotrophs and thus must also be supplied with some form of organic carbon.

Denitrification is important in that it removes fixed nitrogen (i.e., nitrate) from the ecosystem and returns it to the atmosphere in a biologically inert form (N 2 ). This is particularly important in agriculture where the loss of nitrates in fertilizer is detrimental and costly. However, denitrification in wastewater treatment plays a very beneficial role by removing unwanted nitrates from the wastewater effluent, thereby reducing the chances that the water discharged from the treatment plants will cause undesirable consequences (e.g., algal blooms).

Ammonification

When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.

Ecological Implications of Human Alterations to the Nitrogen Cycle

Many human activities have a significant impact on the nitrogen cycle. Burning fossil fuels, application of nitrogen-based fertilizers, and other activities can dramatically increase the amount of biologically available nitrogen in an ecosystem. And because nitrogen availability often limits the primary productivity of many ecosystems, large changes in the availability of nitrogen can lead to severe alterations of the nitrogen cycle in both aquatic and terrestrial ecosystems. Industrial nitrogen fixation has increased exponentially since the 1940s, and human activity has doubled the amount of global nitrogen fixation (Vitousek et al . 1997).

In terrestrial ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity. With increased nitrogen availability there is often a change in carbon storage, thus impacting more processes than just the nitrogen cycle. In agricultural systems, fertilizers are used extensively to increase plant production, but unused nitrogen, usually in the form of nitrate, can leach out of the soil, enter streams and rivers, and ultimately make its way into our drinking water. The process of making synthetic fertilizers for use in agriculture by causing N 2 to react with H 2 , known as the Haber-Bosch process, has increased significantly over the past several decades. In fact, today, nearly 80% of the nitrogen found in human tissues originated from the Haber-Bosch process (Howarth 2008).

Much of the nitrogen applied to agricultural and urban areas ultimately enters rivers and nearshore coastal systems. In nearshore marine systems, increases in nitrogen can often lead to anoxia (no oxygen) or hypoxia (low oxygen), altered biodiversity, changes in food-web structure, and general habitat degradation. One common consequence of increased nitrogen is an increase in harmful algal blooms (Howarth 2008). Toxic blooms of certain types of dinoflagellates have been associated with high fish and shellfish mortality in some areas. Even without such economically catastrophic effects, the addition of nitrogen can lead to changes in biodiversity and species composition that may lead to changes in overall ecosystem function. Some have even suggested that alterations to the nitrogen cycle may lead to an increased risk of parasitic and infectious diseases among humans and wildlife (Johnson et al . 2010). Additionally, increases in nitrogen in aquatic systems can lead to increased acidification in freshwater ecosystems.

Nitrogen is arguably the most important nutrient in regulating primary productivity and species diversity in both aquatic and terrestrial ecosystems (Vitousek et al . 2002). Microbially-driven processes such as nitrogen fixation, nitrification, and denitrification, constitute the bulk of nitrogen transformations, and play a critical role in the fate of nitrogen in the Earth's ecosystems. However, as human populations continue to increase, the consequences of human activities continue to threaten our resources and have already significantly altered the global nitrogen cycle.

References and Recommended Reading

Galloway, J. N. et al . Year 2020: Consequences of population growth and development on deposition of oxidized nitrogen. Ambio 23 , 120–123 (1994).

Howarth, R. W. Coastal nitrogen pollution: a review of sources and trends globally and regionally. Harmful Algae 8 , 14–20. (2008).

Johnson, P. T. J. et al. Linking environmental nutrient enrichment and disease emergence in humans and wildlife. Ecological Applications 20 , 16–29 (2010).

Koenneke, M. et al. Isolation of an autotrophic ammonia-oxidizing marine archaeon. Nature 437 , 543–546 (2005).

Kuypers, M. M. M. et al. Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation. Proceedings of the National Academy of Sciences of the United States of America 102 , 6478–6483 (2005).

Risgaard-Petersen, N. et al. Evidence for complete denitrification in a benthic foraminifer. Nature 443 , 93–96 (2006).

Strous, M. et al. Missing lithotroph identified as new planctomycete. Nature 400 , 446–449 (1999).

Vitousek, P. M. et al. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7 , 737–750 (1997).

Vitousek, P. M. et al. Towards an ecological understanding of biological nitrogen fixation. Biogeochemistry 57 , 1–45 (2002).

Ward, B. B. et al. Denitrification as the dominant nitrogen loss process in the Arabian Sea. Nature 460 , 78–81 (2009).

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Nitrogen Cycle

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The nitrogen cycle refers to the cycle of nitrogen atoms through the living and non-living systems of Earth. The nitrogen cycle is vital for life on Earth. Through the cycle, atmospheric nitrogen is converted to a form which plants can incorporate into new proteins.

Nitrogen Cycle Explained

Nitrogen was originally formed in the hearts of stars through the process of nuclear fusion. When ancient stars exploded, they flung nitrogen-containing gases across the Universe. When the Earth was formed, nitrogen gas was the main ingredient in its atmosphere.

Today, the Earth’s atmosphere is about 78% nitrogen, about 21% oxygen, and about 1% other gases. This is an ideal balance because too much oxygen can actually be toxic to cells. In addition, oxygen is flammable. Nitrogen, on the other hand, is inert and harmless in its gaseous form. However, nitrogen gas is not accessible to plants and animals for use in their cells.

Here we will discuss how nitrogen plays a vital role in the chemistry of life – and how it gets from the atmosphere, into living things, and back again.

Nitrogen Cycle Steps

The basic steps of the nitrogen cycle are illustrated here:

We’ll discuss each part of the process below.

Nitrogen Fixation

In the process of nitrogen fixation , bacteria turn nitrogen gas from the atmosphere into ammonia.

These nitrogen-fixing bacteria, often called “diazotrophs,” have an enzyme called “nitrogenase” which combines nitrogen atoms with hydrogen atoms. Ammonia is a nitrogen compound that can dissolve in water, and is easier for other organisms’ enzymes to interact with.

Interestingly, the enzyme nitrogenase can only function when oxygen isn’t present. As a result, organisms that use it have had to develop oxygen-free compartments in which to perform their nitrogen fixation!

Common examples of such nitrogen-free compartment sare the Rhizobium nodules found in the roots of nitrogen-fixing legume plants. The hard casing of these nodules keeps oxygen out of the pockets where Rhizobium bacteria do their valuable work of converting nitrogen gas into ammonia.

You can see the oxygen-free Rhizobium nodules, visible as big round lumps, on the roots of this cowpea plant:

Nitrification

In nitrification , a host of soil bacteria participate in turning ammonia into nitrate – the form of nitrogen that can be used by plants and animals. This requires two steps, performed by two different types of bacteria.

First, soil bacteria such as Nitrosomonas or Nitrococcus convert ammonia into nitrogen dioxide. Then another type of soil bacterium, called Nitrobacter , adds a third oxygen atom to create nitrate.

These bacteria don’t convert ammonia for plants and animals out of the goodness of their hearts. Rather, they are “ chemotrophs ” who obtain their energy from volatile chemicals. By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes.

The process can be thought of as a rough (and much less efficient) analog to the cellular respiration performed by animals, which extract energy from carbon-hydrogen bonds and use oxygen as the electron acceptor, yielding carbon dioxide at the end of the process.

Nitrates – the end product of this vital string of bacterial reactions – can be made artificially, and are the main ingredient in many soil fertilizers. You may actually hear such fertilizer referred to as “nitrate fertilizer.” By pumping the soil full of nitrates, such fertilizers allow plants to grow large quickly, without being dependent on the rate at which nitrogen-fixing bacteria do their jobs!

Interestingly, high-energy environments such as lightning strikes and volcanic eruptions can convert nitrogen gas directly into nitrates – but this doesn’t happen nearly enough to keep modern ecosystems healthy on its own!

Assimilation

In nitrogen assimilation , plants finally consume the nitrates made by soil bacteria and use them to make nucleotides, amino acids, and other vital chemicals for life.

Plants take up nitrates through their roots and use them to make amino acids and nucleic acids from scratch. Animals that eat the plants are then able to use these amino acids and nucleic acids in their own cells.

Ammonification

Now we have moved nitrogen from the atmosphere into the cells of plants and animals.

Because there is so much nitrogen in the atmosphere, it may seem that the process could stop there – but the atmosphere’s supply is not infinite, and keeping nitrogen inside plant and animal cells would eventually result in big changes to our soil, our atmosphere, and our ecosystems!

Fortunately, that’s not what happens. In a robust ecosystem like ours, anywhere that energy has been put into creating an organic chemical, there is another form of life that is waiting to extract that energy by breaking those chemical bonds.

A process called “ammonification” is performed by soil bacteria which decompose dead plants and animals. During the process, these decomposers break down amino acids and nucleic acids into nitrates and ammonia and release those compounds back into the soil.

There, the ammonia may be taken up again by plants and nitrifying bacteria. Alternatively, the ammonia may be converted back into atmospheric nitrogen through the process of denitrification.

Denitrification

In the final step of the nitrogen cycle, anaerobic bacteria can turn nitrates back into nitrogen gas.

This process, like the process of turning nitrogen gas into ammonia, must happen in the absence of oxygen. As such it often occurs deep in the soil, or in wet environments where mud and muck keep oxygen at bay.

In some ecosystems, this denitrification is a valuable process to prevent nitrogen compounds in the soil from building up to dangerous levels.

Why is the Nitrogen Cycle Important?

Nitrogen is an essential ingredient for life as we know it. Its unique chemical bonding properties allow it to create structures such as DNA and RNA nucleotides, and the amino acids from which proteins are built. Without nitrogen, these molecules would not be able to exist.

It’s thought that the first nucleotides and amino acids formed naturally under the volatile conditions of early Earth, where energy sources like lightning strikes could cause nitrogen and other atoms to react and form complex structures

This process might have naturally produced self-replicating organic chemicals – but in order to reproduce and evolve, life needed to figure out how to make these nitrogen compounds on demand.

Today, “nitrogen fixers” are organisms that can turn nitrogen gas from the atmosphere into nitrogen compounds that other organisms can use to produce nucleic acids, amino acids, and more. These nitrogen fixers are such a vital part of the ecosystem that agriculture cannot occur without them.

Ancient peoples learned that if they did not alternate growing nitrogen-consuming crops with nitrogen-fixing crops, their farms would become fallow and unable to support growth. Today, most artificial fertilizers contain life-giving nitrogen compounds as their main ingredient to make the soil more fertile.

The Danger of Too Much Nitrogen

While the importance of nitrogen to plant and animal life might make it sound like there’s no such thing as too much, there are actually some dangers that can arise from putting too many nitrates in the soil.

Like anything else, nitrogen compounds can be toxic in high concentrations. Just like too much oxygen is toxic to air-breathers, plants can suffer harmful effects from nitrogen overdose.

Nitrates can also be directly toxic to humans – when consumed in large quantities in food or water, nitrates can increase cancer risks and interfere with blood chemistry , leaving blood unable to properly carry oxygen.

“Blue baby syndrome” is one side effect seen in humans who consume high levels of nitrates in their food or water.

Within Ecosystems

Another acute worry is the danger of throwing ecosystems out of balance. Some organisms can use nitrogen compounds to grow faster than others – and that means that when there’s lots of nitrogen around, these organisms can grow so fast that they cause harm to other organisms.

One concern that has been raised about the use of artificial nitrate fertilizer is that when it gets into rivers, lakes, and even the ocean, it can cause runaway growth of plant life there .

More plant life might sound like a good thing – but not when aquatic plants include algae that can block sun and oxygen from getting to other aquatic organisms, and even produce toxins that make humans and other animals sick!

Nitrate fertilizer in water supplies has been blamed for some blooms of “red tides,” “brown tides,” and Pfiesteria bacteria – all of which produce toxins that can sicken or kill humans and other animals.

The question of how to keep farmlands fertile without using nitrate fertilizers is still being investigated by scientists. It is hoped that someday, sustainable practices using natural or genetically engineered nitrogen-fixing plants may allow farmers to produce high crop yields without adding high concentrations of artificial nitrates to the soil.

Examples of the Nitrogen Cycle

The story of thanksgiving.

The story of the first Thanksgiving goes that the pilgrims feasted with the Indians to celebrate their first harvest in the New World. But why was this harvest a big enough deal to throw a feast over? And why, exactly, was it important that the Indians and the European settlers ate together?

When the European settlers came to the Americas, they had very little idea of how to survive here. Having worked farms in back in England for generations, the pilgrims assumed that farming here would be very much the same. That turned out not to be the case. The pilgrims had a difficult time growing or finding enough food to last them through the winter.

One of the reasons for that was that there was not much nitrogen in the soil where the pilgrims landed. Their crops were not nitrogen-fixing, and they hadn’t brought any large livestock. This was a major problem, as manure had been a common source of fertilizer in the old world. After trying in frustration to grow crops in the American soil, the Europeans were shown how to solve their problems by the American Indians.

By burying dead fish in their crop fields, the pilgrims restored nitrogen from the fish’s proteins and nucleotides to the ground. As a result, their crops flourished – and the first European settlers learned from the American Indians how to survive in the New World.

The Three Sisters

Some tribes of Native Americans traditionally grow three crops together – corn, beans, and squash.

Often referred to as “the three sisters,” this crop combination is ingenious for several reasons. For one, eating these three plants in combination provides humans with proteins containing all the essential amino acids.

For another, it includes a nitrogen-fixing plant – beans.

The beans contain Rhizobium nodules in their roots, which contain bacteria that can convert atmospheric nitrogen into a form that’s usable by soil bacteria and, ultimately, plants.

Just like burying fish in the fields, growing beans alongside corn and squash assures that the soil does not become too depleted to grow new plants. Even a single crop of corn or squash may grow better alongside nitrogen-fixing beans, as their Rhizobium bacteria nurtures the surrounding soil!

Artificial Fertilizer

Humans first began fertilizing their crops using natural nitrogen-containing substances such, such as dead fish and animal manure. These waste products of animal life contained proteins, amino acids, and nucleotides which soil bacteria and plants could use to grow.

Today, humans have discovered industrial processes which can turn ammonia into nitrates just like those produced by soil bacteria. Plants can use these nitrates directly, and human industry can produce them in large quantities.

Unfortunately, the human impact on the nitrogen cycle makes changes to the environment, which can have unintended consequences. Just as artificial nitrates promote the growth of “good” plants like crops, they can also promote the growth of “bad” plants and algae that produce toxins and outcompete other life forms.

This can be especially problematic when artificial fertilizers are carried by rainwater from farmlands and lawns into rivers and lakes. The result can be the growth of toxic algae that can strangle wetlands and even get into human drinking water.

1. Which of the following is NOT a reason why plants and animals need nitrogen?

2. How do scientists think was life on Earth able to begin without the enzyme nitrogenase to convert nitrogen gas into ammonia?

3. What would happen if nitrogen compounds were not broken down by decomposers and denitrifiers at the end of the nitrogen cycle?

4. Why are beans a great complimentary crop to grow with corn or squash?

5. Why does placing a dead fish or manure within a field create nitrogen-rich soils?

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Nitrogen Cycle

The nitrogen cycle is a biogeochemical cycle that converts nitrogen into various forms throughout the ecosystem. Nitrogen is an essential element for life that organisms use in the synthesis of amino acids , proteins , and nucleic acids . Yet, while the atmosphere is rich in nitrogen (about 78%), this nitrogen (N 2 ) is largely inaccessible to cells in its gaseous form. Through the nitrogen cycle, atmospheric nitrogen undergoes various transformations. Living organisms use nitrogen and ultimately return it back to the atmosphere.

Nitrogen Cycle Processes

Several processes play a role in the nitrogen cycle, including both biotic (living) and abiotic (non-living) factors. These processes include nitrogen fixation, assimilation, ammonification, nitrification, and denitrification.

Nitrogen Fixation

Nitrogen fixation is the initial step of the nitrogen cycle, converting inert atmospheric nitrogen (N 2 ) into a bio-available form, ammonia (NH 3 ).

Biological Fixation : Some types of bacteria convert nitrogen gas into ammonia. In symbiotic associations, bacteria like Rhizobium colonize the root nodules of legumes, converting atmospheric nitrogen into ammonia. Similarly, non-symbiotic bacteria like Azotobacter and cyanobacteria, especially those in aquatic systems, perform nitrogen fixation. The enzyme central to this process is nitrogenase, which facilitates the reduction of N 2 .
Physical Fixation : Atmospheric processes, such as lightning, also convert atmospheric nitrogen into nitrogen oxides (NO x ). These oxides subsequently react with water, forming nitrates that can be absorbed by plants.

Assimilation

In assimilation, plants take up ammonia and incorporate nitrogen into amino acids, nucleic acids, and other vital organic molecules. Plants predominantly assimilate nitrogen through their roots in the form of nitrates (NO 3 – ) and ammonium ions (NH 4 + ).

Ammonification

As organisms die and waste products accumulate, decomposers—specifically fungi and certain types of bacteria—break down the organic nitrogen within these materials and convert it back into ammonia. This process ensures that nitrogen trapped within organic matter returns to the soil in a form that plants can reuse.

Nitrification

This aerobic process involves the stepwise oxidation of ammonia to nitrite and then to nitrate.

First, bacteria like Nitrosomonas oxidize ammonia to nitrite (NO 2 – ).
Following this, Nitrobacter takes over, oxidizing the nitrite to nitrate (NO 3 – ). Nitrification is a critical step of the nitrogen cycle because most plants predominantly utilize nitrates for their nitrogen needs.

Denitrification

Denitrification is essentially the reverse of nitrogen fixation. Here, the nitrates in the soil transform back into atmospheric nitrogen. Anaerobic bacteria, such as Pseudomonas and Clostridium , reduce nitrates and nitrites to gaseous nitrogen, releasing it back into the atmosphere. This process prevents the accumulation of excess nitrates in terrestrial systems.

Dissimilatory Nitrate Reduction

Unlike denitrification, this process doesn’t return nitrogen to the atmosphere. Instead, certain bacteria reduce nitrates to nitrites or ammonia for energy, especially under anaerobic conditions. However, the nitrogen remains in the ecosystem.

Anaerobic Ammonia Oxidation (Anammox)

In an anaerobic environment, specialized bacteria like Brocadia oxidize ammonia using nitrite as the electron acceptor, producing nitrogen gas. Anammox is particularly relevant in aquatic systems, contributing significantly to the removal of fixed nitrogen from the oceans.

Marine Nitrogen Cycle

The marine environment offers unique niches for nitrogen transformation. While many processes mirror their terrestrial counterparts, the deep-sea regions offer unique conditions, such as oxygen minimum zones (OMZs) where processes like anammox and denitrification are prevalent. The ocean floor acts as a nitrogen sink in that organic debris falls and deposits as sediments. Over time, compression converts these particles into sedimentary rock . Geological uplifting eventually returns these rocks to the surface, where weathering releases the nitrogen compounds back into the cycle.

Human Impacts and Consequences

Human activities significantly impact the nitrogen cycle. From agriculture to industry, anthropogenic nitrogen dramatically increases the flux of reactive nitrogen in the environment.

Agriculture : The synthesis and widespread use of nitrogen-based fertilizers increase crop yields, but at the cost of nitrogen runoff that causes eutrophication and acidification in aquatic systems. Inorganic nitrogen in water systems also poses toxicity issues for humans and other animals.
Burning of Fossil Fuels : Burning fossil fuels releases NOx into the atmosphere, which returns to the earth’s surface as acid rain or contribute to smog and greenhouse gas accumulation.
Deforestation and Land Use Changes : Deforestation alters the natural nitrogen balance, leading to soil degradation and other ecological shifts.

The repercussions of these impacts are multi-fold, affecting air and water quality, disrupting natural ecosystems, and posing direct and indirect health risks to humans.

Camargoa, Julio A.; Alonso, Álvaro (2006). “Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment”. Environment International . 32 (6): 831–849. doi: 10.1016/j.envint.2006.05.002
Erisman, J. W.; Galloway, J. N.; et al. (2013). “Consequences of human modification of the global nitrogen cycle”. Philosophical Transactions of the Royal Society B: Biological Sciences . 368 (1621): 20130116. doi: 10.1098/rstb.2013.0116
Fowler, David; Coyle, Mhairi; et al. (2013). “The global nitrogen cycle in the twenty-first century”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences . 368 (1621): 20130164. doi: 10.1098/rstb.2013.0164
Galloway, J. N.; Townsend, A. R.; et al. (2008). “Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions”. Science . 320 (5878): 889–892. doi: 10.1126/science.1136674
Voss, M.; Bange, H. W.; et al. (2013). “The marine nitrogen cycle: recent discoveries, uncertainties and the potential relevance of climate change”. Philosophical Transactions of the Royal Society B: Biological Sciences . 368 (1621): 20130121. doi: 10.1098/rstb.2013.0121

Biology Article

Nitrogen Cycle

Nitrogen cycle is an important part of the ecosystem. In this article, we shall explore its implications on the environment in detail.

Table of Contents

What is Nitrogen Cycle
In Marine Ecosystem

Nitrogen Cycle Definition

“Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen present in the atmosphere to a more usable form for living organisms.”

Furthermore, nitrogen is a key nutrient element for plants. However, the abundant nitrogen in the atmosphere cannot be used directly by plants or animals. Read on to explore how the Nitrogen cycle makes usable nitrogen available to plants and other living organisms.

What is the Nitrogen Cycle?

Nitrogen Cycle is a biogeochemical process through which nitrogen is converted into many forms, consecutively passing from the atmosphere to the soil to organism and back into the atmosphere.

It involves several processes such as nitrogen fixation, nitrification, denitrification, decay and putrefaction.

Nitrogen gas exists in both organic and inorganic forms. Organic nitrogen exists in living organisms, and they get passed through the food chain by the consumption of other living organisms.

Inorganic forms of nitrogen are found in abundance in the atmosphere. This nitrogen is made available to plants by symbiotic bacteria which can convert the inert nitrogen into a usable form – such as nitrites and nitrates.

Nitrogen undergoes various types of transformation to maintain a balance in the ecosystem. Furthermore, this process extends to various biomes, with the marine nitrogen cycle being one of the most complicated biogeochemical cycles.

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 (N 2 ) which is primarily available in an inert form, is converted into the usable form -ammonia (NH 3 ).

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 fertilisers 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.

Also Read: Nitrogen Fixation And Nitrogen Metabolism

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:

2NH 3 + 3O 2 → 2NO 2 – + 2H + + 2H 2 O

2NO 2 – + O 2 → 2NO 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.

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.

Denitrification

Denitrification is the process in which the nitrogen compounds make their way back into the atmosphere by converting nitrate (NO 3 -) 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:

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.

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

Nitrogen is abundant in the atmosphere, but it is unusable to plants or animals unless it is converted into nitrogen compounds.
Nitrogen-fixing bacteria play a crucial role in fixing atmospheric nitrogen into nitrogen compounds that can be used by plants.
The plants absorb the usable nitrogen compounds from the soil through their roots. Then, these nitrogen compounds are used for the production of proteins and other compounds in the plant cell.
Animals assimilate nitrogen by consuming these plants or other animals that contain nitrogen. Humans consume proteins from these plants and animals. The nitrogen then assimilates into our body system.
During the final stages of the nitrogen cycle, bacteria and fungi help decompose organic matter, where the nitrogenous compounds get dissolved into the soil which is again used by the plants.
Some bacteria then convert these nitrogenous compounds in the soil and turn it into nitrogen gas. Eventually, it goes back to the atmosphere.
These sets of processes repeat continuously and thus maintain the percentage of nitrogen in the atmosphere.

Further Reading: Other Biogeochemical Cycles

To explore more about the Nitrogen cycle, or the steps involved, keep visiting BYJU’S Biology website or download the BYJU’S app, for further reference.

Frequently Asked Questions

Why is nitrogen important for life.

Nitrogen constitutes many cellular components and is essential in many biological processes. For instance, the amino acids contain nitrogen and form building blocks that make up various components of the human body such as hair, tissues and muscles.

Why do plants need nitrogen?

Plants need nitrogen as this element is an important component of chlorophyll. Consequently, chlorophyll is vital for the process of photosynthesis, so lack of nitrogen can cause deficiency disorders such as stunted growth and other abnormalities

List the different steps that explain the Nitrogen Cycle process.

Nitrogen Fixation

What is Ammonification?

Ammonification occurs during the decomposition of organic matter, where ammonifying bacteria convert organic nitrogen into inorganic components like ammonia or ammonium ions.

What is Nitrification?

Nitrification is a process that converts ammonia into nitrate by bacteria. Initially, the ammonia is converted to nitrite (NO 2 − ) by the bacteria Nitrosomonas , or Nitrococcus , etc., and then to nitrate (NO 3 – ) by Nitrobacter .

What is Denitrification?

Denitrification is the process of converting the nitrate back into molecular nitrogen by bacterias such as Pseudomonas, Thiobacillus, Bacillus subtilis etc.

What is the function of nitrifying bacteria?

Nitrifying bacteria are a small group of aerobic bacteria, which are mainly involved in the conversion of ammonia into nitrates.

Which part of the plant is involved in nitrogen fixation?

The process of nitrogen fixation is carried out naturally in the soil within nodules in the plant’s root systems.

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nitrogen cycle

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nitrogen cycle - Student Encyclopedia (Ages 11 and up)

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 .

Follow the nitrogen and phosphorus cycles and learn why farmers fertilize fields to keep them productive

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 .

Nitrates and ammonia resulting from nitrogen fixation are assimilated into the specific tissue compounds of algae and higher plants. Animals then ingest these algae and plants, converting them into their own body compounds.

The remains of all living things—and their waste products—are decomposed by microorganisms in the process of ammonification , which yields ammonia (NH 3 ) and ammonium (NH 4 +). (Under anaerobic, or oxygen-free, conditions, foul-smelling putrefactive products may appear, but they too are converted to ammonia in time.) Ammonia can leave the soil or be converted into other nitrogen compounds, depending in part on soil conditions.

Nitrification , a process carried out by nitrifying bacteria , transforms soil ammonia into nitrates (NO 3 −), which plants can incorporate into their own tissues.

Nitrates also are metabolized by denitrifying bacteria , which are especially active in water-logged anaerobic soils. The action of these bacteria tends to deplete soil nitrates, forming free atmospheric nitrogen.

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Nitrogen Cycle

What is the nitrogen cycle.

The series of processes by which nitrogen and its different forms are circulated and interconverted in nature with the help of living organisms is called the nitrogen cycle. It shows the path that nitrogen follows through the biogeochemical cycle using its storage reservoirs, such as the atmosphere, living organisms, and soil.

What are the main steps of the nitrogen cycle?

The entire process of the Nitrogen Cycle, one of the important biogeochemical cycle takes place in five stages:

1) Nitrogen Fixation by Bacteria – Converting inert atmospheric nitrogen (N 2 )into biologically available forms such as ammonia (NH 3 ), nitrates, or nitrites

2) Nitrification by Bacteria – Converting ammonia to nitrite and then to nitrate

3) Assimilation by Plants – Absorbing nitrogen from the soil and incorporating them in the plant and animal bodies

4) Ammonification by Decomposers – Converting the dead organic nitrogen of plants or animals back into ammonia

5) Denitrification by Denitrifiers – Reducingnitrates or nitrites and releasing gaseous nitrogen

What role do bacteria play in the nitrogen cycle?

Nitrogen fixation – Performed by two different groups of bacteria – a) symbiotic nitrogen fixers like Rhizobium, which keep a close association with the host leguminous plant, and b ) free-living, non-symbiotic bacteria like Azotobacter.

Both these group of bacteria use specific enzymes to complete the biological nitrogen fixation process by the following reaction –

N 2 + 8 H + + 8 e − → 2 NH 3 + H 2

Nitrification – Performed by nitrifying bacteria in two steps –

i) Ammonia-oxidizing bacteria such as Nitrosomonas species perform oxidation of ammonia to nitrite by the following reaction –

2NH 4 + + 3O 2 + 8 e − → 2 NO 2 – + 4H 2 + 2H 2 O

ii) Nitrite-oxidizing bacteria such as Nitrobacter species perform oxidation of nitrite (NO 2 – ) to nitrate (NO 3 – ) by the following reaction –

2 NO 3 – + O 2 → 2 NO 3 –

How are plants involved in the nitrogen cycle?

Plants help in the assimilation of nitrogen when they absorb it from the soil in the form of ammonia, nitrite ions, nitrate ions or ammonium ions to form plant and animal proteins. In leguminous plants such as pea and bean, the symbiotic association with Rhizobium helps to assimilate nitrogen directly in the form of ammonium ions.

What role do decomposers play in the nitrogen cycle?

Detritus feeders or decomposers such as fungi and bacteria present in the soil convert the dead organic matter of plants or animals back into ammonia (NH 3 ) or ammonium ions (NH 4 + ).

What is the role of denitrifiers in the nitrogen cycle?

Denitrifiers such as Clostridium and Pseudomonas helps in the reduction ofnitrates (NO 3 ) or nitrites (NO 2 ), resulting in the escape of gaseous nitrogen which again returns to the cycle.

What is the role of lightening in the nitrogen cycle?

Lightning with thunderstorm serves as an important source of fixing nitrogen in the atmosphere apart from bacteria mediated nitrogen fixation. Here the energy of lightning breaks atmospheric nitrogen into nitrogen oxides which can then be utilized by plants for assimilation.

Why is the nitrogen cycle important in nature?

Allowing plants and animals to use nitrogen by converting atmospheric nitrogen to a more chemically available form such as ammonium (NH 4 + ), nitrate (NO 3 – ), or organic nitrogen
Enriching the soil through the formation of Nitrates and nitrites which are essential for the cultivation
Helping in the synthesis of some biomolecules such as amino acids, nucleic acids, and chlorophyll, the building blocks of life
Decomposing dead plant and animal matter by decomposers which cleans up the environment

How do humans impact the nitrogen cycle?

Human activities release excess nitrogen into the environment, eventually disturbing the balance of nitrogen in its different reservoirs in two possible ways:

Burning of Fossil Fuels
Use of Nitrogen-Containing Fertilizers

How does burning of fossil fuels affect the nitrogen cycle to cause climate change?

Burning fossil fuels like coal, petroleum, and natural gas releases excess nitrogen into the environment that accumulates over time. An increase in the concentration of nitrogen is found to affect the climate of the earth by gradually increasing its temperature, causing greenhouse effect and global warming.

How does the use of fertilizer affect the nitrogen cycle?

When artificial fertilizers containing nitrogen as one of the components are washed away from the agricultural fields, it contaminates the nearby water bodies and also the groundwater making it more difficult for the plants to absorb the nitrogen both for the terrestrial and aquatic plants. Since nitrogen fixation by plants is affected, it affects the nitrogen cycle.

Article was last reviewed on Tuesday, February 25, 2020

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Microbes Environ
v.34(3); 2019 Sep

The Nitrogen Cycle: A Large, Fast, and Mystifying Cycle

1 Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2–15 Natsushima-cho, Yokosuka 237–0061, Japan

If a story was written entitled the “Kingdom of Microbial Ecology”, how would the characters and plot be decided? Many microbiologists will agree with the following: the primary story setting involves a magnificent castle (a habitat) in fertile land with bright sunlight (the energy source), and the king and queen are the carbon and nitrogen cycles, respectively. Although this is an unusual way to begin a research highlight article in Microbes and Environments , the nitrogen cycle is one of the most important and commonly researched topics in the field of environmental microbiology.

Nitrogen is among 6 major essential elements of CHNOPS for life and composes the building blocks and intact molecules for metabolism (amino acids and proteins), heredity (nucleotides and nucleic acids), and other important biological functions. Among the essential elements, circulating volumes of biologically available nitrogen and phosphorus are slightly limited in many biospheres of the planet, and, thus, as a whole, the biological demand for nitrogen and phosphorus and their circulation speeds are markedly greater and faster than those for carbon ( 3 ). Furthermore, although the biological transformation pathways of nitrogen compounds are less diverse than those of carbon compounds when all organic forms are taken in consideration, the complexity of the catabolic and anabolic metabolism of inorganic nitrogen compounds and their synergetic processes are beyond those for carbon and cannot be compared to those of other biologically essential elements ( 9 ). This may, at least in part, explain the great interest in as well as difficulties associated with environmental microbiology research on the nitrogen cycle, which has encouraged and motivated many scientists to study it and resulted in a greater abundance of research articles and reviews on the nitrogen cycle and metabolism than on energy, carbon, and other elemental cycles and metabolism in the last decade (a quick Google scholar search shows 3,100,000 hits for “energy cycle/metabolism”, 2,830,000 hits for “carbon cycle/metabolism”, 2,470,000 hits for “nitrogen cycle/metabolism”, and 670,000 hits for “phosphorus cycle/metabolism”).

There are two large nitrogen pools on Earth, atmospheric molecular nitrogen (N 2 ) and (biologically) reactive nitrogen (NO 3 , NH 4 , and organic nitrogens) ( 3 ). The biological nitrogen cycle mainly consists of internal interactions within the reactive nitrogen pool and in- and out-flow between reactive nitrogens and atmospheric N 2 pools. Inter-connections between the two large nitrogen pools are primarily controlled by only two biological (microbial) processes, nitrogen fixation and denitrification ( 3 ), even though the impact of anthropogenic input into the terrestrial and marine reactive nitrogen pools on the global nitrogen cycle have recently increased due to fertilization and the burning of fossil fuels.

Major catabolic and anabolic microbial nitrogen metabolic pathways are shown in Fig. 1 , the complexity of which has undoubtedly motivated scientific curiosity. In the past several years, many studies that have been published in Microbes and Environments have attempted to elucidate microbial nitrogen cycles at both the local and global scales of microbial habitats and communities.

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Object name is 34_223_1.jpg

Microbial pathways in the nitrogen cycle. Abbreviations: AOB, ammonia-oxidizing bacteria; AOA, ammonia-oxidizing archaea; Anammox, anaerobic ammonium oxidation; DNRA, dissimilatory nitrate reduction to ammonium; NOB, nitrite-oxidizing bacteria; N-AOM, oxygenic nitrite-dependent anaerobic oxidation of methane. Key enzymes in these pathways are shown as follows: AMO, ammonia monooxygenase; HAO, hydroxylamine oxidoreductase; NirK, copper-containing nitrite reductase; NirS, cytochrome cd 1 nitrite reductase; Nrf, cytochrome c nitrite reductase; NirB, cytoplasmic nitrite reductase; NAR, membrane-bound nitrate reductase; NAP, periplasmic nitrate reductase; NXR, oxidoreductase; NOS, nitric oxide synthase; Hmp, flavohemoglobins; NorVW, flavorubredoxin; cNor, nitric oxide reductase using c-type cytochromes as an electron donor; qNor, nitric oxide reductase using quinols as an electron donor; Nos, nitrous oxide reductase; HZS, hydrazine synthase; HDH, hydrazine dehydrogenase; Nif, nitrogenase. Unknown enzymatic entities and abiotic reaction steps are shown as question marks.

Nitrogen fixation is a unique ability possessed by microorganisms called diazotrophs, and involves the conversion of very inert N 2 to reactive nitrogens (including NH 3 ) ( 2 ). The conversion of N 2 to reactive nitrogens (such as NO) always occurs with lightning, which provides reactive nitrogens to the land and ocean ( 16 ). Japanese farmers previously noted a relationship between the annual abundance of lightning and rice crop yields, and referred to this lightning as “Inazuma” (the housewife for rice crop growth). However, this abiotic nitrogen fixation input has been estimated to account for only <1/10 of biological nitrogen fixation based on theoretical calculations and only approximately 10 −5 based on laboratory experiments ( 3 , 16 ). Thus, microbial nitrogen fixation and diazotrophs are key in the global nitrogen cycle and even on various scales in biological communities ever since the Earth became a planet of life approximately 4 Ga.

Nishihara and colleagues investigated nitrogen fixation in chemolithotrophic microbial communities in a hot spring stream in Japan ( 17 – 19 ). Chemolithotrophic nitrogen fixation at high temperatures (up to 92°C) has attracted scientists researching the early evolution of life and the nitrogen cycle, and deep-sea hyperthermophilic methanogens and their nitrogen fixation processes have been extensively examined ( 12 , 20 ). The types of thermophilic diazotrophs supporting nitrogen sources for chemolithotrophic microbial communities in terrestrial geothermal environments lacking a significant reactive nitrogen input were previously unknown. A series of studies by Nishihara and colleagues revealed that nitrogen fixation occurred in thermophilic microbial mats along the hot spring stream at temperatures up to 75°C, and was not associated with the functions of thermophilic methanogenic and sulfate-reducing diazotrophs, previously known as thermophilic diazotrophs, based on activity measurements and molecular analyses ( 17 , 19 ). Nishihara and colleagues also succeeded in isolating new thermophilic chemolithoautotrophs that potentially function as primary diazotrophs in microbial mat communities and showed that they were hydrogenotrophic and/or thiotrophic diazotrophs of the genus Hydrogenobacter ( 18 ). Genetic analyses of nitrogen fixation genes and phenotypes have also been performed on ecologically important diazotrophic microbes, such as a thermophilic cyanobacterium ( 26 ) and plant-associated actinobacterium ( 8 ). Using a metatranscriptomic approach, Masuda et al. ( 11 ) found that nitrogen fixation in paddy fields was primarily driven by deltaproteobacterial populations, such as Anaeromyxobacter and Geobacter , but not by other rhizospheric Proteobacteria and Cyanobacteria that were considered to be the major diazotrophs in paddy soils. Masuda et al. ( 11 ) identified true “Inazuma”, the housewife for rice crop growth, and this study was selected for the Most Valuable Paper (MVP) award of 2017 in Microbes and Environments . It is important to note that the diversity, abundance, and function of plant-associated diazotrophs are significantly controlled by the abundance and input of other reactive nitrogens ( 14 ).

In catabolic and anabolic metabolic pathways, the significance of intermediate nitrogen metabolites, such as nitrite (NO 2 ), nitric oxide (NO) and nitrous oxide (N 2 O), has been recognized in many scientific and social contexts. Nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), assimilatory ammonication, and anaerobic ammonium oxidation (Anammox) each through the transformation of these intermediate nitrogen metabolites and intermediates in natural microbial communities may be multi-directionally transformed by the complex nitrogen metabolism of various populations in response to intra- and extracellular physicochemical conditions and reaction dynamics through interspecies interactions ( Fig. 1 ). Nakagawa et al. ( 15 ) reported the diversity and abundance of a nitrous oxide reductase gene ( nosZ ) in coastal eelgrass sediments and suggested that sulfur-oxidizing Gammaproteobacteria and Bacteroidetes populations contribute to N 2 O removal in eelgrass sediment microbiomes. Siqueira et al. ( 23 ) confirmed the previously proposed hypothesis that the different denitrification pathways and functions of similar Bradyrhizobium species in the soybean rhizosphere ( e.g ., with and without nosZ ) respond to the physicochemical conditions of habitats ( e.g ., in situ O 2 concentrations). Although nitrous oxide reduction (with and without nosZ ) is not key for denitrification, the abundance of nitrate reduction (the expression of napA ) may be more important. Jang et al. ( 5 ) also isolated the novel denitrifying bacterium Bradyrhizobium nitroreducens from rice paddy soil that hosted and co-expressed two types of nitrite reductase genes ( nirK and nirS ), and the majority of denitrifiers are known to possess and use each of the two types of nitrite reductases. These are also excellent examples of the complexity of the nitrogen cycle in natural microbial communities.

Besides denitrification, DNRA is considered to play an important role in energy metabolism with the nitrates and nitrites of microbial communities occurring at relatively electron-donor-enriched and/or electron-acceptor-limiting habitats based on the thermodynamic estimation of energy efficiency. Chutivisut et al. ( 1 ) showed that the denitrifying microbial community of activated sludge from a municipal wastewater treatment plant enriched DNRA populations when it was incubated under the condition of a high donor/acceptor ratio. Furthermore, Microbes and Environments recently published a number of studies on ecologically important nitrogen catabolism by Anammox and Anammox microbial communities ( 13 , 22 ).

In many natural and anthropogenic habitats, nitrifying microbial communities are reliant on the close cooperation of two distinct groups, namely, that between ammonia- and nitrite-oxidizing metabolism (except for Comammox) ( 25 ) and also that between archaeal and bacterial populations. Ammonia-oxidizing archaea (AOA) play a significant role in natural environments and include phylogenetically and physiologically diverse members. Using a metagenomic analysis and the long-term enrichment of a thermophilic nitrifying microbial community obtained from a subsurface geothermal water stream, Kato et al. ( 6 ) demonstrated that previously uncultivated Nitrocaldus and Aigarchaeota members were enriched in the thermophilic ammonium-fed culture and may be involved in not only nitrification, but also denitrification in a long-term culture and indigenous habitats. Isoda et al. ( 4 ) and Nunoura et al. ( 21 ) reported the nitrifying archaeal phylotype diversity and potential niche separation of diverse Thaumarchaeota in Finnish and Japanese forest soils as well as in the world’s deepest Challenger Deep of the Mariana Trench. These AOA oxidize ammonia to hydroxylamine by ammonia monooxygenase (Amo); however, the enzymatic entity that catalyzes the oxidation of hydroxylamine to nitrite, corresponding to hydroxylamine dehydrogenase (Hao) in the case of ammonia-oxidizing bacteria (AOB), remains unclear ( 24 ). AOA are known to produce nitric oxide during their chemolithoautotrophic growth ( 10 ). Using the recombinant copper-containing nitrite reductase (NirK) of Nitrososhaera viennensis and enzymological characterization, Kobayashi et al. ( 7 ) confirmed that NirK is involved in the production of nitric oxide via hydroxylamine oxidation and nitrite reduction during chemolithoautotrophic growth. Under the experimental conditions described, no evidence of nitrite production from hydroxylamine by the recombinant NirK was obtained ( 7 ); however, this study provided key insights into the as-yet-unknown metabolic pathways and enzymatic entities of ammonia oxidation and nitric oxide production by AOA. This concise, but impacted study was selected for the MVP award of 2018 in Microbes and Environments .

The MVPs in Microbes and Environments in the last two years have been awarded to studies on the nitrogen cycle. Therefore, the large, fast, and complex nitrogen cycle has attracted the attention of many environmental microbiologists worldwide, and Microbes and Environments has significantly contributed to lifting the veil of secrecy surrounding “the Queen of the Kingdom of Microbial Ecology”.

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nitrogen cycle

Quick reference.

An important chemical cycle, consisting of a complex series of processes that circulate nitrogen between the atmosphere and animals, plants, and soil. Nitrogen in the atmosphere may be converted into nitrogen oxides by lightning discharges; these are taken up by the soil and form nitrates that are then absorbed by plants and eventually by animals. In addition, nitrogen-fixing bacteria associated with the roots of certain plants convert the element into a form that may be absorbed. Other bacteria break down nitrogen compounds in dead animals and plants, or their wastes, and return it to the soil or, in some cases, to the atmosphere. See also carbon cycle.

From: nitrogen cycle in A Dictionary of Weather »

Subjects: Science and technology — Environmental Science

What are the steps involved in the nitrogen cycle?

Nitrogen cycle consists of four main steps namely:

Nitrogen Fixation

Ammonification/ Decay

Nitrification

De-nitrification

It is important to note that microorganisms play an important role in each of these steps.

Browse more Topics under Microorganisms

Disease-causing microorganisms
Food Preservation
Microorganisms and its uses
Heterotrophs
Inoculation

You can download the Nitrogen Cycle Cheat Sheet PDF by clicking on the Download button below

What is the mechanism of each of these steps?

This is the first step of the nitrogen cycle. This step is characterized by the conversion of atmospheric N2 into ammonia (NH3). Bacteria like Azotobacter and Rhizobium have a major role in this process. They are harbored in the roots of the leguminous plants and help convert inert nitrogen to ammonia. Nitrogen fixation can occur in any of the following ways: atmospheric fixation (involves lightening), industrial fixation(manufacturing ammonia under high temperature and pressure condition)

Assimilation

Once the nitrogen has been fixed in the soil , plants can absorb nitrogen through their roots. This process of absorption is known as assimilation .

Ammonification

This is another process by which ammonia can be generated. Organic remains of plants and animals are broken down in the soil by some bacteria to release ammonia into the soil. These dead and waste matter is used by these microorganisms as food and they release ammonia into the soil.

This occurs in two-steps. The first step is in which NH3/NH$+ is converted to NO3- (nitrates). The bacteria Nitrosomonas and Nitrococcus present in the soil convert NH3 to NO2-, and another bacterium, Nitrobacter converts NO2- to NO3-. These bacteria gain energy through these conversions.

Denitrification

Is the reverse of nitrification that occurs in the deep layers of soil where the bacteria convert NO3- is converted into N2 and other gaseous compounds like NO2 . This occurs because in deep layers of soil, oxygen is not available and the soil bacteria use these nitrogen compounds instead of oxygen.

What are some Essential Mineral Elements?

What is the importance of the nitrogen cycle?

As we all know by now, the nitrogen cycle helps bring in the inert nitrogen from the air into the biochemical process in plants and then to animals.
Plants need nitrogen to synthesize chlorophyll and so the nitrogen cycle is absolutely essential for them.
During the process of ammonification, the bacteria help degrade decomposing animal and plant matter. This helps in naturally cleaning up the environment.
Due to the nitrogen cycle, nitrates and nitrites are released into the soil which helps in enriching the soil with nutrients needed for cultivation.
As plants use nitrogen for their biochemical processes, animals obtain the nitrogen and nitrogen compounds from plants. Nitrogen is needed as is an integral part of the cell composition. It is due to the nitrogen cycle that animals are also able to utilize the nitrogen present in the air.

Solved Example for You

Q: Nitrates are converted into Nitrogen by

Ammonifying Bacteria
Denitrifying bacteria
Nitrogen-fixing bacteria
All of the above

Solution: The correct answer is “b”. Denitrification is the process where after nutrients are converted back into ammonia, anaerobic bacteria will convert them back into nitrogen gas.

FAQ’s for You

Q1. Explain nitrogen cycle in nature and define all the terms involved in it.

Terms involved in nitrogen cycle are 1. Nitrogen fixation : Plants cannot use free nitrogen present in the air. This nitrogen molecule is converted into nitrates and nitrites which can be taken up and used to make the required molceule. This is called nitrogen fixation which can be done by the bacterias that live in the root nodules of leguminors plants. By physical process during lightning, the high temperatures and pressures is created in the air which convert nitrogen into oxides of nitrogen and dissolve in water and come down along with rain. This is also called nitrification. 2. Ammonification. The nitrogen compounds formed are taken by plants to form proteins which are further converted into ammonia. 3. Denitrification. The nitrates and nitrites of nitrogen are acted upon by other group of microbes e.g. Pseudomonas bacteria,which convert these compounds into free nitrogen gas. Nitrogen cycle: 1. Free nitrogen from atmosphere is converted into nitrates by bacterias or by lightning. 2. Nitrates mix with soil, is absorbed by the plants to make proteins. 3. The proteins in plants and animals are converted into amino acids and ammonia. 4. Ammonia is converted into nitrates nitrates and then these nitrates and nitrites present in soil is acted up on by other group of bacterias called denitrifying bacteria. The process is called denitrification, nitrates are converted into free nitrogen and is released back to the atmosphere.

Q2. In nitrogen cycle, atmospheric nitrogen is fixed by bacteria and converted into ammonia. Ammonia is further converted into other forms of nitrogen. At the end of the cycle it returns to the atmosphere by the process of A. Ammonification B. Nitrification C. Dentrification D. Assimilation

Answer: The nitrogen cycle is the recycling phase of the nitrogen which includes nitrogen fixation, ammonification, nitrification, and denitrification. Denitrification is the process through which the nitrates and nitrites are converted back to atmospheric nitrogen. This process is performed by the anaerobic bacteria. Thus, the correct answer is option C.

Q3. In nitrogen cycle which bacteria are responsible for nitrification?

Answer: In nitrogen cycle Nitrosomonas & Nitrobacter bacteria are responsible for nitrification) During nitrification, the nitrogenous wastes from dead plants and animals are converted into ammonia by the action of bacteria such as Bacillus ramosus, Clostridium spp. etc. The microbes which carry out biological nitrogen fixation are commonly called biological nitrogen fixers. for eg: Rhizobium and Azotobacter. So, the correct answer is ‘Nitrosomonas & Nitrobacter’.

Q4. Explain the nitrogen cycle?

Answer: A continuous series of natural processes by which nitrogen passes successively from air to soil to organisms and back to air or soil involving principally nitrogen fixation, nitrification, decay, and denitrification.

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Nitrogen Cycle

What is the definition of nitrogen cycle.

The nitrogen cycle can be defined as one of the biogeochemical cycles that converts the unusable inert nitrogen existing in the atmosphere into a more usable form of nitrogen for living organisms. Before further discussing the nitrogen cycle, we must know some facts about nitrogen. Nitrogen is an essential constituent of all organisms. Nitrogen atoms can be detected in all proteins and nucleic acids . Nitrogen generally exists as nitrogen gas (N 2 ) in the environment . Nitrogen fixation is the process by which bacteria transform gaseous nitrogen into ammonia, a type of nitrogen that plants may use. Plants provide usable nitrogen molecules to animals when they eat them. In both nature and agriculture, nitrogen is a typical limiting nutrient. A limiting nutrient is one that is in limited quantity and hence restricts development. It is thus clear that nitrogen is an essential constituent of nature. The article focuses on the details of the nitrogen cycle.

What is Nitrogen Cycle

The nitrogen cycle is a biogeochemical process in which nitrogen, in various forms, is circulated from the atmosphere to the living organisms and later back to the atmosphere. Living organisms require nitrogen for the synthesis of nucleic acid and proteins. The atmosphere contains almost 78% of nitrogen present in an inert form (N 2 ). This nitrogen cannot be used by living organisms unless it is converted to ammonia, nitrates, and other usable compounds of nitrogen.

The nitrogen cycle is a cyclic process where the nitrogen travels from inorganic form in the atmosphere and to the organic way in the living organisms. The nitrogen cycle contains several steps, such as nitrogen fixation, assimilation, ammonification, nitrification, and denitrification. This cycle is essential in maintaining a proper ecological balance and is present in both marine and terrestrial ecosystems.

Nitrogen Cycle Steps

There are several steps of the nitrogen cycle as mentioned above, the complete process can be classified into nitrogen fixation, assimilation, ammonification, nitrification, and denitrification steps. Each of the steps is described below in the article.

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Nitrogen Fixation

The first step involves the fixation (conversion) of atmospheric inert nitrogen into a usable form of nitrogen. Here, the N 2 form of nitrogen is converted into NH 3 (Ammonia). This process is carried out by symbiotic bacteria present in the soil called diazotrophs. These are bacteria, primarily known for their nitrogen fixation in nature, e.g., rhizobium.

There are three ways nitrogen fixation can take place.

Atmospheric Nitrogen Fixation: The inert nitrogen present in the atmosphere is converted to nitrous oxide with the help of lightning due to the high -temperature present during lightning. The nitrogen is broken down into nitrogen atoms which react with oxygen to form nitrous oxide, nitrogen peroxide, and nitric oxide. These compounds later dissolve in the rain to form dilute nitric acid. When the dilute nitric acid reaches the Earth's surface, it reacts with the alkalies present to form nitrates that plants can easily absorb.

Biological Nitrogen Fixation: There exist nitrogen-fixing bacteria and blue-green algae that convert nitrogen present in the atmosphere into nitrates. There are two types of nitrogen-fixing bacteria:

Free-Living Bacteria: For example, Azotobacter , and Clostridium.

Symbiotic Bacteria: For example, Rhizobium that is present in root nodules of individual leguminous plants like Nostoc and Anabaena.

Industrial Nitrogen Fixation: It is a human-made alternative where the atmospheric nitrogen is converted into ammonia by Haber's process and later into nitrates in various fertilizers .

Ammonification

The dead remains of plants and animals are buried in the soil. They decay and create ammonia, carbon dioxide, and water, with the help of fungi like actinomyces. This process of formation of ammonia is called ammonification. Already ammonia exists in the soil with the help of nitrogen-fixing bacteria. Ammonification increases the concentration of ammonia in the ground.

Nitrification

The process in which the ammonia is converted into nitrites and later into nitrates is called Nitrification. This process takes place in two steps:

Conversion of Ammonia into Nitrites: This takes place by the action of Nitrosomonas bacteria. They oxidize the ammonia present in the soil and convert them to nitrites. The chemical equation to represent the reaction is mentioned below.

2NH 4 + + 3O 2 → 2NO 2 – + 4H + + 2H 2 O

Conversion of Nitrites to Nitrates: This takes place by the action of Nitrobacter species, which convert the nitrates in the soil into nitrates. The chemical equation of the reaction is mentioned below.

2NO 2 – + O 2 → 2NO 3 –

Assimilation

In this process, the formed nitrates in the soil get absorbed by the plants through their root system. The plants contain nitrates that are consumed by the consumers and then later process through the food chain and enter the food web. Assimilation is the absorption of nitrates and other nitrogen compounds. The nitrogen compounds are essential for the formation of crucial biomolecules.

Denitrification

The plants do not absorb some nitrates. They are converted into atmospheric nitrogen with the help of pseudomonas and clostridium. This process is the last step where the nitrogen compounds present in the soil make their way back to the atmospheric nitrogen.

The Nitrogen Cycle in the Marine Ecosystem

The marine ecosystem also has a similar manner to the nitrogen cycle. The nitrogen from the atmosphere gets absorbed in the water, and nitrogen-containing compounds sediment as rocks on the ocean floor. Many species cannot break the strong bond between the nitrogen. But few bacteria can oxidize the nitrogen molecule and convert it into ammonia. The phytoplankton plants can absorb ammonia. Some bacteria can consume ammonia and release nitrites. The nitrites are then converted to nitrates that can later be used by another microorganism in the marine ecosystem. This process of converting ammonia into nitrates is called nitrification. Larger organisms like the whale, fish, etc. get their supply of nitrogen by consuming phytoplankton. When the fish die eventually, they sediment to the ocean floor. They are decomposed by the bacteria present and release ammonia which is again converted to nitrates by nitrification, and the cycle continues.

Importance of the Nitrogen Cycle

There are various important uses of the nitrogen cycle, some of which are summarised below.

Chlorophyll is an essential pigment for the process of photosynthesis. The nitrogen cycle helps the plants to manufacture chlorophyll from the compound of nitrogen.

It is essential for the survival of plants as plants need nitrates to survive and grow.

During the process of formation of ammonia, the dead and decayed organic matter is decomposed by bacteria. This process helps the environment to be cleaned up from organic matter and also provides essential nutrients required by the soil.

Nitrogen compounds enrich the soil and make it fertile and suitable for growing plants.

Nitrogen is a necessary element in the cells and tissues of living organisms. It forms proteins and nucleic acid, which form the essential elements of life. Without nitrogen compounds, life could not exist.

Combustion of fuels and fertilizers also contains nitrogen that increases the percentage of nitrogen in the atmosphere.

Eutrophication is the accumulation of nitrogen in water bodies when the nitrogen from the fertilizers in the soil is washed away.

Important Points on the Nitrogen Cycle

Some of the interesting facts about the nitrogen cycle are summarised below.

Nitrogen is present in the atmosphere in abundance, but cannot be used by the plants and other organisms directly from the atmosphere.

Nitrogen is fixed in three ways which are atmospheric, industrial, and biological means. The atmospheric nitrogen is converted into ammonia.

Nitrogen-fixing bacteria like Azotobacter and Rhizobium play a vital role in the formation of nitrogen compounds.

Dead and rotten plants are decomposed by fungi like actinomyces and ammonia, carbon dioxide, and water are released. This process is called ammonification.

Nitrosomonas convert ammonia into nitrites and later to nitrates by Nitrobacter bacteria by the process of nitrification.

Plants absorb nitrates, and nitrogen is used to form important cell organelles and biomolecules. The process of absorption of nitrogen compounds from the plants is called assimilation.

The nitrates present in the soil are converted into free nitrogen by pseudomonas bacteria. This process is called denitrification.

The cycle repeats, and the nitrogen percentage in the atmosphere remains stable.

The nitrogen cycle also exists in the marine ecosystem where the phytoplankton plants and other bacteria convert the nitrogen into nitrogen compounds.

This cycle is a critical biogeochemical cycle in nature that is necessary for life processes.

To conclude the article, it can be said that nitrogen is an essential part of the environment and the nitrogen cycle plays an important role in the sustenance of the ecosystem. In this article, we have learnt about the nitrogen cycle and its importance.

FAQs on Nitrogen Cycle

1. What are nitrifying bacteria?

Nitrifying bacteria are the microbes responsible for the conversion of ammonium into nitrates in the nitrification step of the nitrogen cycle. These are generally chemolithotrophs. A common example of nitrifying bacteria includes Nitrosomonas, which converts ammonium into nitrite. Another example is Nitrobacter which converts nitrite into nitrate.

2. How does nitrogen affect the formation of a dead zone?

Dead zones are the area in an aquatic ecosystem where there is a sudden decline in the population of flora and fauna as a result of eutrophication. Eutrophication can diminish oxygen availability as algae and bacteria that feed on them consume enormous amounts of oxygen in cellular respiration, thus diminishing oxygen availability in the water. Other animals living in the impacted habitats, such as fish and shrimp, may die as a result, resulting in low-oxygen, species-depleted regions known as dead zones.

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Essay on the Nitrogen Cycle

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Here is an essay on the nitrogen cycle.

Nitrogen is one of the important elements in biological compounds, mainly of nucleic acid and protein and, therefore, it is essential for life. Atmosphere contains about 78% nitrogen, but free nitrogen cannot be utilised by most of the organisms, except a few blue green algae and some bacteria.

The fixation of nitrogen takes place by physical, chemical and biological means. Small amounts of N 2 are converted to ammonia by electrical discharges in atmosphere i.e., by physical force, and settle down on earth by rain. About 30 million metric tonnes of N 2 are produced by industry however about 200 million metric tonnes of nitrogen are fixed every year by biological organisms.

The biological N 2 fixation takes place by a few organisms like:

1. Symbiotic Bacteria:

Rhizobium, Brady- rhizobium, Frankia.

2. Free-Living Bacteria:

Azotobacter, Azomonas, Dersia etc.

3. Blue Green Algae:

Nostoc, Anabaena, etc., fix nitrogen inside heterocyst where oxygenic photosynthesis does not occur.

In nitrogen cycle, free N 2 gas of atmosphere is converted into ammonia or oxidised to nitrate at different stages. Blue green algae and N 2 fixing bacteria play a significant role in converting the atmospheric gaseous nitrogen into organic nitrogenous compounds and, finally, to nitrate, which is soluble in water.

Nitrates are absorbed and utilised by the plants for the synthesis of amino acids vis-a-vis proteins. The plants are consumed by herbivores. When the plants and animals die, they are decomposed by, bacteria, thus the N 2 becomes released in the atmosphere.

The N 2 cycle (Fig. 2.34) thus consists of the following steps:

1. Nitrogen fixation,

2. Ammonification,

3. Nitrification,

4. Denitrification, and finally, and

5. Release of gases in the atmosphere.

Biological Nitrogen Fixation:

The major share of nitrogen fixation is occupied by biological N 2 fixation. The biological N 2 fixation of atmospheric nitrogen depends on the nitrogenase enzyme system, composed of nitrogenase and nitrogenase reductase. Nitrogenase is very much sensitive to O 2 and it becomes irreversibly inactivated on exposure even at low concentration of O 2 . In leguminous plants, N 2 fixation takes place in root nodules, where the enzyme is protected by red pigment leghemoglobin.

The plants are eaten by herbivores and fixed nitrogen goes into their body and after their death, it becomes decomposed and mixed with the soil.

The nitrogen may be fixed in the soil by the following means:

(a) Physical fixation takes place by lightning and thereby N 2 comes down in soil through rain.

(b) There is huge industrial production of N 2 fertiliser, these are mixed with the soil during cultivation.

(d) Major share of N 2 is contributed by the biological fixers.

Ammonification:

In this process nitrogen in organic matter of dead plants and animals is converted to ammonia and amino acids.

Urea is applied in the field as additional nitrogen source. The microbial decomposition of urea causes the release of ammonia which is returned to atmosphere or may go to neutral aqueous environments as ammonium ions.

Nitrification:

This is the process of conversion of ammonia to nitrate. It takes place in two steps. Ammonia is first oxidised to nitrous acid by Nitrosomonas, Nitrospira and Nitrococcus. The molecular O 2 acts as electron acceptor in this step. In the second step, nitrous acid becomes oxidised and converted into nitric acid, mostly by Nitrobacter. Proper aeration in the soil is essential for the availability of oxygen.

Denitrification:

During this process, nitrate is converted to molecular nitrogen by the different bacteria like Bacillus cereus, Pseudomonas aeruginosa etc. and thus, N 2 goes back into the atmosphere. This is called dissimilatory nitrate reduction. This takes place during depletion of oxygen in the medium.

In assimilatory nitrate reduction process nitrite is converted into ammonia.

The nitrate added to the soil is reduced to NH 3 by plants and fermentative bacteria rather than to nitrogen by denitrifying bacteria.

Biological Nitrogen Cycle: (With Diagram) | Ecosystem
4 Main Phases of Nitrogen Cycle (With Diagram) | Ecosystem | Biology

Essay , Botany , Biogeochemical Cycles , Nitrogen Cycle

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The Nitrogen Cycle ( Edexcel GCSE Biology )

Revision note.

Biology Lead

The Nitrogen Cycle

The nitrogen cycle shows how nitrogen is recycled in ecosystems
Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA)
Instead, they rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds , which can be taken up by plants
The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment

The role of bacteria in the nitrogen cycle

There are four key processes in the nitrogen cycle that are carried out by different types of bacteria
Nitrogen fixing bacteria can be free-living in the soil or can live within the root nodules of some plants
Lightning can also split the bond between the two N atoms, turning them into nitrous oxides like N 2 O and NO 2 that dissolve in rainwater and leach into the soil
These are decomposers, e.g. fungi and bacteria
This ammonia forms ammonium ions in the soil
Initially, nitrifying bacteria convert ammonium ions into nitrites
Different nitrifying bacteria then convert these nitrites into nitrates
Denitrifying bacteria use nitrates in the soil during respiration
This process produces nitrogen gas , which returns to the atmosphere
This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)

The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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The nitrogen cycle is a biogeochemical cycle that converts nitrogen into various forms throughout the ecosystem. Nitrogen is an essential element for life that organisms use in the synthesis of amino acids, proteins, and nucleic acids. Yet, while the atmosphere is rich in nitrogen (about 78%), this nitrogen (N 2) is largely inaccessible to ...
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The entire process of the Nitrogen Cycle, one of the important biogeochemical cycle takes place in five stages: 1) Nitrogen Fixation by Bacteria - Converting inert atmospheric nitrogen (N 2)into biologically available forms such as ammonia (NH 3), nitrates, or nitrites. 2) Nitrification by Bacteria - Converting ammonia to nitrite and then ...
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Introduction

Nitrogen Fixation

Nitrification

Denitrification

Ammonification

Ecological Implications of Human Alterations to the Nitrogen Cycle

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