The Significance of the Nitrogen Cycle in Ecosystems

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The Significance of the Nitrogen Cycle in Ecosystems

Nitrogen makes up about 80 per cent of the Earth's atmosphere as a

gas. However the gaseous molecule is very stable and has to be

transformed before it can be used by most organisms as it is only

available to them when it is ammonium or nitrate. It can only be

removed from the atmosphere in two ways: by lightning and by nitrogen

fixation. Only a few species can convert nitrogen by nitrogen fixation

to biologically useful forms. Due to this, biologically useful

nitrogen is often in short supply and can be the limiting factor in an

ecosystem.

There are five main steps in the Nitrogen cycle:

Biological Nitrogen Fixation

This is the conversion of gaseous nitrogen to ammonia using an enzyme

called nitrogenase that only works in the absence of oxygen and

requires large amounts of energy. The equation for nitrogen fixation

is:

N + 8e + 8H + 16ATP ® 2NH + H 16ADP + 16Pi

Often the nitrogen fixers live in close association with a specific

eukaryotic organism. Although their biomass is small their role in the

biosphere is just as important as that of the photosynthetic

autotrophs fixing ninety million tons of gaseous nitrogen per year

(Jones, 1997). Various photosynthetic bacteria, including

Cyanobacteria are the main nitrogen fixers. On land most nitrogen

fixation comes from the bacteria associated with the formation of root

nodules in certain plants with free-living soil bacteria making only a

small contribution.

Some bacteria belonging to the genus Rhizobium live in close

association with the roots of leguminous seed plants such as peas,

soybeans and alfalfa. The...

... middle of paper ...

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8th Septmber 2003

http://scholar.hw.ac.uk/heriotwatt/scholar2003/courses/advanced/Biology/unit2/activity18.asp?outline=no

Jones, A (1997) "Environmental Biology", Routledge, USA and Canada

p81- 84

Bibliography

============

Durbridge, C and Sockett, L "Nitrogen Cycle Lesson Notes" Visited: 4th

September 2003

http://www.nottingham.ac.uk/~pdzres/NITROGEN%20CYCLE%20LESSON%20NOTES.doc

Jones, A(2001) "Environmental Biology, Student Monograph", Learning

and

Teaching Scotland, Scotland, p11-12

Jones, T (2002) "The Nitrogen Cycle" Petsforum Visited: 4th September

2003

http://www.petsforum.com/personal/trevor-jones/nitrogencycle.html

Kimball, JW (2003) "The Nitrogen Cycle" Visited: 4th September 2003

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html

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