The Nitrogen Cycle
The nitrogen cycle represents one of the most important nutrient cycles found in terrestrial ecosystems. The air that we breathe is made up of about 78% free nitrogen gas, but our body cannot use this form of oxygen. In order for nitrogen to be used for growth it must be "fixed" (combined) in the form of ammonium (NH4) or nitrate (NO3) ions.
Nitrogen enters the food chain by
means of nitrogen-fixing bacteria and algae in the soil. This nitrogen, which has
been 'fixed', is now available for plants to absorb. These types of bacteria form a
symbiotic relationship with legumes--these types of plants are very useful because the
nitrogen fixation enriches the soil and acts as a 'natural' fertilizer. The
nitrogen-fixing bacteria form nitrates out of the atmospheric nitrogen which can be taken
up and dissolved in soil water by the roots of plants. Then, the nitrates are
incorporated by the plants to form proteins, which can then be spread through the food
chain. When organisms excrete wastes, nitrogen is released into the environment.
Also, whenever an organism dies, decomposers break down the corpse into nitrogen in
the form of ammonia. This nitrogen can then be used again by nitrifying bacteria to
fix nitrogen for the plants.