The Nitrogen Cycle

Nitrogen is a vital molecule that organisms need in manufacturing proteins, which build up the protective structures in their bodies, as well as for DNA, which provides hereditary information for all living things. Although most of the atmosphere is made up of nitrogen gas (78%), only a select group of organisms can harness nitrogen in the gas state. This means that nitrogen enters the ecosystem by plants absorbing it from the soil, often in the form of nitrate ions. Plants will use this inorganic nitrogen to manufacture organic compounds absorbed by animals when they consume these plants. This nitrogen then passes through the food chain. 

The Nitrogen Cycle The Nitrogen Cycle

Create learning materials about The Nitrogen Cycle with our free learning app!

  • Instand access to millions of learning materials
  • Flashcards, notes, mock-exams and more
  • Everything you need to ace your exams
Create a free account
Table of contents

    Let's learn about the nitrogen cycle and its steps which are nitrogen fixation, ammonification, nitrification, assimilation and denitrificati, as well as its importance for all living beings.

    What are Nutrient Cycles

    These are some important terms we must understand when studying nutrient cycles:

    • Reservoirs - where the majority of the nutrient in question is situated; in the nitrogen cycle, this will be in the atmosphere.
    • Sources - the organism or processes which return the nutrients to the reservoir; in this case, this will be the denitrifying bacteria in the soil, which convert nitrate ions back into nitrogen gas in a process called denitrification.
    • Sinks - the sinks are the most prominent site of nutrient consumption, so in this instance, this will be the soil, where nitrate ions are available for plants to absorb.

    Although atmospheric nitrogen is not very accessible, this is where you will find the majority of the Earth's nitrogen, so the atmosphere is a reservoir.

    These are the types of organisms which are involved in nutrient cycles:

    • Producers - photosynthetic organisms harness sunlight's energy to convert inorganic molecules into organic compounds.
    • Consumers - organisms that cannot create their own food must ingest producers or other consumers in the case of secondary and tertiary consumers.
    • Decomposers - these organisms break down dead organic matter and release absorbable, inorganic ions in return.

    This is the process by which nutrient cycles occur:

    1. Producers absorb the nutrients as inorganic molecules that would otherwise be useless to other organisms and subsequently convert them into valuable organic compounds.
    2. Consumers then ingest these producers and absorb these organic compounds.
    3. The nutrients travel through the food chains as secondary and tertiary consumers ingest organisms containing these organic compounds.
    4. Once these producers and consumers die, they are acted on by decomposers which break down the organic compounds and release them as inorganic compounds which producers can absorb.

    Not all nitrogen is taken up by consumers due to a variety of reasons (not consuming the whole organism, being unable to utilise all the nutrients, and nutrient loss through urea and faeces). This does not mean that the nitrogen goes to waste, as any dead or faecal matter will be broken down by saprobionts (decomposers) and enter into the cycle.

    A saprobiont is a decomposing microorganism which externally digests its food (dead, organic matter).

    Nitrogen Cycle Definition

    The nitrogen cycle consists of 5 main processes carried out by different types of microorganisms in the soil:

    • Nitrogen fixation
    • Nitrification
    • Assimilation
    • Ammonification
    • Denitrification

    The nitrogen cycle Stages, nitrogen fixation, ammonification, nitrification, assimilation, denitrification StudySmarterFig. 1 - The nitrogen cycle consists of different processes or stages.

    Here are descriptions of the processes and organisms involved.

    Nitrogen Fixation

    Nitrogen fixation is arguably the most important stage of the nitrogen cycle. This is because it offers a pathway for the abundance of nitrogen present in the atmosphere to become readily available for plants in the soil. Nitrogen will become present in soils through precipitation and is then acted on by saprobiontic bacteria. Atmospheric nitrogen fixation has the potential to occur when lightning strikes, and we can fixate nitrogen industrially under high pressures and temperatures. Still, it is the nitrogen-fixing microorganisms in the soil that we will be focussing on.

    Bacteria known as diazotrophs contain nitrogenase enzymes, so they have the ability to convert nitrogen into ammonia. Examples of nitrogen-fixing bacteria include azotobacter and rhizobium.

    We can split the word diazotroph into "di" = two + "azo" = nitrogen + "troph" meaning "pertaining to food or nourishment". This gives the word a meaning of dinitrogen (N2) utilising.

    There are two types of nitrogen-fixing diazotrophs, namely free-living and mutualistic:

    • Free-living nitrogen-fixing bacteria are abundant in the soil and convert gaseous nitrogen into ammonium by reducing it. This reduction is endothermic, resulting in energy being absorbed by the bacteria, which they can then use to manufacture various nitrogen-containing compounds such as proteins and nucleic acids. After death, these bacteria decaying releases these organic compounds into the soil.
    • Mutualistic nitrogen-fixing bacteria are certain types of microorganisms that live on plants' root nodules. These bacteria will reduce gaseous nitrogen to ammonia and use this ammonia to manufacture nitrogen-containing compounds such as amino acids. The host plant will then take up these useful organic compounds and, in return, give the bacteria useful carbohydrates and sugars, which it can thrive off. This is an example of a symbiotic relationship between two organisms.

    A symbiotic relationship involves two different species becoming dependent on one another for resources to survive.


    Ammonification is the process by which nitrogen is transferred from the living part biome to the non-living part of the biome. The saprobiontic decomposers in the soil (mostly bacteria and fungi in this case) will break down any organic material containing nitrogen and release ammonia in return. This organic material can be from protein, vitamins and nucleic acids found in dead organisms or from the urea and faeces produced by animals. The ammonia will then pick up hydrogen ions by reacting with acidic residues in the soil.

    Clostridium and Streptomyces are examples of ammonifying bacteria.


    Nitrification is the process by which chemoautotrophic bacteria convert ammonium ions into nitrate ions which producers can then absorb. These bacteria rely on a substantial concentration of ammonium ions in the soil as they harness the energy released from the oxidation of ammonium ions for their biochemical processes. Free-living bacteria convert ammonium ions to nitrate ions in the soil; these are aerobic chemoautotrophs, so the soil must be oxygen-rich. Ammonium ions are first oxidised to nitrite ions, which are then subsequently oxidised to nitrate ions, with both reactions releasing energy.

    Chemoautotrophs are organisms that harness chemical energy from chemical compounds, mostly inorganic, such as hydrogen sulfide (H2S), ammonia (NH3) or nitrite (NO2-) to make their own food through a process called chemosynthesis.

    Here are the chemical equations for the reactions:

    Ammonium ions oxidised to nitrite ions:

    2NH4+ + 3O2 → 2NO2+ 4H+ + 2H2O

    Nitrite ions oxidised to nitrate ions:

    2NO2 + O2 → 2NO3


    Producers which live in the soil, or autotrophs, take up the nitrogen by actively transporting the nitrate and ammonium ions through their roots. The ions must be actively transported most of the time because there is always a high concentration of ions in the plant roots, but the soil can sometimes be nutrient deficient. Nitrate ions are transported throughout the plant and converted to ammonia, which is converted to amine groups that can easily be used to manufacture useful nitrogen-containing compounds.

    Reductive amination and transamination are both methods by which an amine group is introduced to a molecule. The difference between these two processes is that reductive amination involves the reduction of a carbonyl group to form an amine group. In contrast, transamination is the addition of an amine group transferred from another molecule.


    Denitrification is the process by which nitrogen returns to the atmosphere completing the cycle. Anaerobic saprobionts called denitrifying bacteria reduce nitrate ions to oxygen and release nitrogen gas in the process. These denitrifying bacteria are abundant in waterlogged soils because the water often pushes out any oxygen available to the microorganisms in the soil. This reduction of nitrate ions can be detrimental to the plant as they require a constant supply for manufacturing nitrogen-containing compounds.

    Importance of the Nitrogen Cycle

    The nitrogen cycle is important for many different reasons:

    • It provides producers with a sustainable source of nitrate ions that they can absorb and use to build up essential nitrogen-containing compounds like proteins, nucleic acids and chlorophyll.
    • Allows important organic compounds which have nitrogen as a major component to pass through the ecosystem. Herbivorous consumers ingest the producers, and carnivorous consumers will ingest their prey.
    • Saprobiontic decomposers break down dead matter and faeces; in doing so, they make nutrients readily available for producers while simultaneously cleaning up the environment.
    • Allows the inert gas nitrogen to become available for terrestrial organisms.

    Why do Plants Require Nitrogen

    Plants require nitrogen for the manufacturing of many different structures and molecules throughout the plant:

    • Chlorophyll - nitrogen is a major component of chlorophyll, so lots of nitrogen is needed to produce the maximum amount of chlorophyll pigment so the plant can photosynthesise as much as possible.
    • DNA - nitrogen is present in the nucleotides, which are the building blocks of DNA. DNA is the genetic material that provides the instructions for all biochemical processes inside a plant and the hereditary information for the plant.
    • ATP - the monomers which make up the energy transfer molecule adenosine triphosphate contain nitrogen. ATP is pivotal in controlling energy transfer in metabolic processes.
    • Amino acids - nitrogen makes up a substantial proportion of amino acid residues, which are the building blocks when manufacturing proteins. These proteins are essential in manufacturing protective structures and useful enzymes in the plant.

    Nitrogen Deficiency in Plants

    Nitrate ion availability in the soil can be caused by heavy rains, which leads to waterlogged land. This wet soil leads to their being a dominating anaerobic saprobiont population, meaning that denitrifying bacteria prevail and the nitrate ion concentration of the soil decreases. Dry soils with little to no moisture can also hamper plants' development as they will struggle to uptake water-soluble ions. Plants that have a nitrogen deficiency may have the following characteristics:

    • The plant's leaves will grow much slower; this is especially noticeable in younger leaves.
    • Shoot development is much slower.
    • Leaves may exhibit a yellow discolouration.
    • Reduced flowering and fruit production.

    The Nitrogen Cycle in Marine Ecosystems

    Nitrogen is transferred through marine ecosystems in the same way as terrestrial ecosystems. Aquatic saprobiontic microorganisms break down dead organic matter on the sea bed and release nitrate ions which aquatic plants can absorb. Recent research has found that sedimentary rocks are a major source of nitrogen as they release gaseous nitrogen when they erode.

    Use of Nitrogen Fertilisers

    Nitrogen is one of the three main components of NPK fertilisers, popular with farmers worldwide as nitrogen, phosphorus, and potassium are essential for plant development. Nitrogen fertilisers may be needed when the soil is waterlogged, so it contains mostly denitrifying saprobionts, when the soil is too dry, meaning soluble nitrate ions cannot move around, or even when crops are being grown on infertile soil. Farmers almost always use fertilisers to increase the yield and size of their crops. Despite their apparent benefits, fertiliser runoff can be detrimental to nearby rivers and estuaries.

    Fertiliser runoff is a massive problem for nearby aquatic ecosystems as the leaching of nutrient-rich soils will cause algal populations to surge and result in algal blooms on the water's surface. These blooms will block sunlight from penetrating to the depths of the water and restrict plant life, having consequences for the rest of the ecosystem.

    The Nitrogen Cycle - Key takeaways

    • The nitrogen cycle is a geochemical cycle that describes the conversion of nitrogen into different chemical forms -organic and inorganic- through its circulation process between the terrestrial, marine and atmospheric environments.
    • The main stages of the nitrogen cycle include nitrogen fixation, ammonification, nitrification, assimilation, and denitrification.
    • Nitrogen is a primary bioelement and is the building block of amino acids, proteins and nucleic acids. Thanks to the nitrogen cycle, nitrogen is recycled in ecosystems and can be continuously reused.
    • Human activities such as the overuse of nitrogen fertilizers cause disturbances in the nitrogen cycle. These effects include leaching, eutrophication and reduced species diversity.
    Frequently Asked Questions about The Nitrogen Cycle

    How does the use of fertiliser affect the nitrogen cycle? 

    The overuse of fertiliser affect the nitrogen cycle because it leads to the over concentration of nutrients in soils, which can result in leaching and huge amounts of nitrogen becoming trapped in marine sediment.

    How is the nitrogen cycle important to humans? 

    The nitrogen cycle is important to humans because nitrogen is essential in building up molecules like DNA and amino acids in all living beings.

    Why are bacteria needed in the nitrogen cycle? 

    Bacteria are needed in the nitrogen cycle to fix gaseous nitrogen and reduce to ammonium and nitrate ions.

    Which process in the nitrogen cycle releases nitrogen gas into the atmosphere? 

    The process in the nitrogen cycle that releases nitrogen gas into the atmosphere is denitrification which involves denitrifying bacteria reducing nitrate ions to oxygen, and releasing nitrogen gas in the process.

    How do humans impact the nitrogen cycle? 

    Humans impact the nitrogen cycle through diverse activities. Factories industrially produce nitrogen through processes like the Haber-Bosch and the cyanamide process, so release massive amounts of nitrogen oxides into the atmosphere.

    About StudySmarter

    StudySmarter is a globally recognized educational technology company, offering a holistic learning platform designed for students of all ages and educational levels. Our platform provides learning support for a wide range of subjects, including STEM, Social Sciences, and Languages and also helps students to successfully master various tests and exams worldwide, such as GCSE, A Level, SAT, ACT, Abitur, and more. We offer an extensive library of learning materials, including interactive flashcards, comprehensive textbook solutions, and detailed explanations. The cutting-edge technology and tools we provide help students create their own learning materials. StudySmarter’s content is not only expert-verified but also regularly updated to ensure accuracy and relevance.

    Learn more
    StudySmarter Editorial Team

    Team The Nitrogen Cycle Teachers

    • 11 minutes reading time
    • Checked by StudySmarter Editorial Team
    Save Explanation

    Study anywhere. Anytime.Across all devices.

    Sign-up for free

    Sign up to highlight and take notes. It’s 100% free.

    Join over 22 million students in learning with our StudySmarter App

    The first learning app that truly has everything you need to ace your exams in one place

    • Flashcards & Quizzes
    • AI Study Assistant
    • Study Planner
    • Mock-Exams
    • Smart Note-Taking
    Join over 22 million students in learning with our StudySmarter App