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
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
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
Some bacteria belonging to the genus Rhizobium live in close
association with the roots of leguminous seed plants such as peas,
soybeans and alfalfa. These bacteria are normally free-living in the
soil but when they are close to the root of a legume they infect it.
They invade the root hairs and cause the production of an infection
thread in the root hair cell (Heriot-Watt University, 2002). This
enables the bacteria to penetrate deeper into the root tissue and
infect the cells there. The infected root cells become enlarged and
undergo rapid division to form a mass of cells called root nodules.
The plant then supplies the bacteria with sugars which they use to
provide energy for nitrogen fixation. Also the plant makes a compound
called leghaemoglobin which traps any oxygen present. This is
important because nitrogen fixation is an anaerobic process and the
presence of oxygen deactivates the nitrogenase.
The other method of nitrogen fixation is through lightning. The
enormous energy of lightning breaks nitrogen molecules and enables
their atoms to combine with oxygen in the air to form nitrogen oxides.
These dissolve in rain, forming nitrates, that are carried to earth.
Nitrification is the conversion of ammonia to nitrate by soil
bacteria. It is a two stage...