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Monday, January 17, 2011

The Nitrogen Cycle - 17/01/11

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. Nitrogen is required for all organisms as it is the essential component of DNA, RNA, and protein. However, most organisms cannot use atmospheric nitrogen hence, this cycle is important in the sense that it converts the atmospheric nitrogen into usable forms for living organisms.

(click to enlarge)

As seen from the diagram below, the carbon cycle consists of many cycles within it. These cycles consist of five important processes that are driven by microorganisms, namely:
1) Fixation
2) Uptake
3) Decomposition
4) Nitrification
5) Denitrification

Process #1 - Nitrogen Fixation
This is the process whereby N2 is converted to ammonium. Without this, organisms cannot attain nitrogen as they cannot use the atmospheric nitrogen. Ways nitrogen can be fixated in the biosphere include:
- Bacteria
- Lightning
- Rain

Process #2 - Uptake
This is the process where ammonia produced by nitrogen fixing bacteria is quickly incorporated into protein and other organic nitrogen compounds. When we eat, we are using nitrogen that has been fixed initially by nitrogen fixing bacteria.

Process #3 - Decomposition
When organisms die, or when organisms defecate, decomposers (such as bacteria and fungi) consume the organic matter or animal waste. During this decomposition process, nitrogen contained within the dead organism is converted to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO3-) through the process called nitrification.

Process #4 - Nitrification
Nitrification is whereby ammonium produced by decomposition is converted to nitrate.
Process #5 - Denitrification
The processes above remove nitrogen from the atmosphere and pass it through ecosystems. Therefore denitrification is important as it replenishes the atmosphere, by reducing nitrates to nitrogen gas.
This process can be carried out either through bacteria or the sea.


Done by: Yangting :D
Credits: 1,2,Pic


  1. We all learnt that lightning can fix nitrogen into the soil, but i think no one actually knows how. So I did some research and basically, lightning, which has an enormous amount of energy breaks nitrogen molecules in the atmosphere. This enables their atoms to combine with oxygen in the air, forming nitrogen oxides. When it rains, these oxides dissolve in the rain forming nitrates and are then carried into the earth.

  2. Hello Yangting!

    Thanks for your post! :) I understand the nitrogen cycle much more clearly now. However, I was wondering about the significance of the nitrogen cycle, and the implications of change to the nitrogen cycle. So I decided to do some research and this is what I came across.

    Nitrogen is vital for many processes and systems on Earth; more specifically, it is crucial for sustaining life on Earth.

    As we've learnt in biology this year and last year, it is a component in all amino acids, and proteins (as mentioned by Yangting in her post). Furthermore, nitrogenous bases in DNA and RNA couldn't exist without nitrogen, hence the name! From this we can pretty much see that without nitrogen, our DNA would look pretty strange and we probably wouldn't even be alive.

    Plants too, need nitrogen. It is used in chlorophyll molecules (which allow for photosynthesis and further growth).

    Our atmosphere is composed of 78% nitrogen, as we all know. However, this does not mean that all of it can be used by plants. Nitrogen fixation is necessary to convert gaseous nitrogen into forms usable by living organisms and thus it is a crucial part of food production as without plants, we would not have herbivores, and thus there would be no carnivores or omnivores, and without any of these, we wouldn't exist either! The amount of this "fixed" form of nitrogen dictates how much food can be grown on a piece of land as well.

    So what we've learnt so far is that usable nitrogen is pretty important for life on Earth. But what are the human impacts on the nitrogen cycle? How are we playing a part?

    At the moment, extensive cultivation of legumes, the increasing use of the Haber process [as learnt in chemistry: N2(g)+3H2(g) ⇌2NH3(g)] in creating chemical fertilizers and pollution caused by vehicles and industries, there is now more than twice the amount of usable nitrogen in the atmosphere than there was in the past! Of course, these changes to the nitrogen cycle are most significant in countries that are heavily dependent on agriculture, such as some parts of Asia. Nitrous oxide (a greenhouse gas that significantly contributes to global warming) emissions have also increased.

    Ammonia and nitrous oxide are precursors of tropospheric ozone production, which contributes to smog, acid rain, damages plants and increases nitrogen inputs to ecosystems.

    While this may have short-term positive impacts, like more plants, it may also result in nitrogen saturation, which weakens productivity and can damage the health of plants, animals, fish, and humans through eutrophication, for instance (see post on eutrophication).

    So what we can see from this is that more is not necessarily better and that it is important to keep everything on Earth properly balanced.

    Please feel free to correct me if I've made mistakes or to add on to my points!