Nitrogen Cycle

Overview

Nitrogen is a component of proteins and nucleic acids and so a supply of nitrogen is essential to all living things

Nitrogen is available to living things in a variety of forms, the source used depending on the species:

Heterotrophs (such as animals) can only use organic nitrogen i.e. nitrogen already incorporated into organic molecules

Plants use the inorganic nitrogen-containing ions nitrate (NO₃^-) and ammonium (NH₄⁺)

Some bacteria are able to use atmospheric nitrogen (N₂ gas) as a nitrogen supply

Other bacteria are involved in the interconversion of the inorganic nitrogen ions; ammonium, nitrite (NO₂^-) and nitrate

Some bacteria and fungi decompose dead organisms converting organic nitrogen to inorganic nitrogen in the process

The movement of nitrogen between different groups of organisms and between the different molecular types is known as the nitrogen cycle

The Players and their roles

A number of different groups of organism play important roles in the nitrogen cycle. These include:

Nitrogen-fixing bacteria. 

These fix atmospheric nitrogen gas (N₂) to ammonia (NH₃) in a reduction reaction.

The equation for this reaction is: N₂ + 3H₂ ----> 2NH₃

Some nitrogen-fixing bacteria are free-living in the soil e.g. Azotobacter

Some, e.g. Rhizobium, form a mutualistic (symbiotic) relationship with legumes (peas, beans, clover etc. These are members of the Papilionaceae) in which the bacteria live in nodules on the plant's roots. The bacteria provide the plant with fixed nitrogen, the plant provide the bacteria with carbohydrates. More details of this relationship is available on this site here. The diagram below shows root nodules in a member of the Papilionaceae

Image from http://database.sunsite.ualberta.ca/pls/biod1/ditrl.display.detail?p_mno=135

Decomposers

These are bacteria and fungi that decompose dead animals and plants and, in the process, convert their organic nitrogen (that found in proteins and nucleic acids) into inorganic ammonium (NH₄⁺)

Nitrifying Bacteria

These are bacteria that interconvert inorganic nitrogen molecules:

Nitrosomonas converts ammonium (NH₄⁺) into nitrites (NO₂^-)

Nitrobacter converts nitrites (NO₂^-) into nitrates (NO₃^-)

Collectively these processes are known as nitrification

Nitrification only occurs when the soil conditions are appropriate i.e. not waterlogged, too cold or too acidic. If the soil conditions are not appropriate ammonium accumulates

Denitrifying Bacteria

These are bacteria that convert nitrates (NO₃^-) to nitrites (NO₂^-) and then to nitrogen gas (N₂)

These bacteria thus bring about the conversion of inorganic nitrogen into atmospheric nitrogen; a process known as denitrification

The denitrifying bacteria include:

Pseudomonas

Thiobacillus 

These are anaerobic bacteria so denitrification only occurs under anaerobic conditions (e.g. when the soil is waterlogged

Some physical processes also have a role in the cycle:

Lightning: the high energy available from lightning is sufficient to fix atmospheric nitrogen to nitrates

The Haber-Bosch process : this is an entirely artificial process that is used in the manufacture of ammonium for fertilizer - but since it contributes  to the total fixation of atmospheric nitrogen it is often included

Leaching : the loss of nitrates from the soil as a result of heavy rain fall. Nitrates leach into bodies of water (e.g. lakes) enriching them and making them more fertile. This process is known as eutrophication. 

 

Diagrams of the whole Nitrogen Cycle

There are many different ways of representing the nitrogen cycle - some more visual than others, some more detailed.

I have included a few - find one that works for you either among these or elsewhere - BUT DO KNOW AT LEAST ONE

You must also know the names of all of the species listed on this page as taking part in the cycle

This straightforward diagram illustrating the key points (but not giving details of, for example, nitrogen fixation) is from http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm

A very visual diagram, but lacking in some detail, taken from http://ferl.becta.org.uk/display.cfm?resID=3598

A fairly visual diagram but lacking in detail taken from http://www.soils.agri.umn.edu/academics/classes/soil2125/doc/s9chap2.htm

A detailed diagram shown in an unusual form, this is from http://www.alken-murray.com/Nitrogen.html

A mildly amusing animated version (intended for GCSE students) is here: http://www.bbc.co.uk/schools/gcsebitesize/biology/ecology/nitrogencyclerev1.shtml

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