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Jetzt kostenlos anmeldenNitrogen 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.
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.
These are some important terms we must understand when studying nutrient cycles:
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:
This is the process by which nutrient cycles occur:
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).
The nitrogen cycle consists of 5 main processes carried out by different types of microorganisms in the soil:
Fig. 1 - The nitrogen cycle consists of different processes or stages.
Here are descriptions of the processes and organisms involved.
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:
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.
The nitrogen cycle is important for many different reasons:
Plants require nitrogen for the manufacturing of many different structures and molecules throughout the plant:
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:
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.
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 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.
The nitrogen cycle is important to humans because nitrogen is essential in building up molecules like DNA and amino acids in all living beings.
Bacteria are needed in the nitrogen cycle to fix gaseous nitrogen and reduce to ammonium and nitrate ions.
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.
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.
Flashcards in The Nitrogen Cycle30
Start learningWhich different types of organisms are involved in the nitrogen cycle?
Decomposers (aerobic and anaerobic), producers, and consumers.
What is a saprobiont?
An organism which digests their food externally and absorbs the products.
Which five processes are involved in the nitrogen cycle?
Nitrogen fixation, ammonification, nitrification, assimilation and denitrification.
What is a producer?
A photosynthetic organism which manufactures its own food using the light energy from the sun.
What is a consumer?
Any organism (all animals are) which consumes another organism.
What is a herbivorous consumer?
A consumer which only eats plants.
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