The Epic Interactive Encyclopedia 1997/The_Epic_Interactive_Encyclopedia_97.iso/n/nitrogen_cycle/infotext

Text File | 1992-09-02 | 1KB | 29 lines

In ecology, the process of nitrogen passing
through the ecosystem. Nitrogen, in the form
of inorganic compounds (such as nitrates) in
the soil, is absorbed by plants and turned
into organic compounds (such as proteins) in
plant tissue. A proportion of this nitrogen
is eaten by herbivores and used for their own
biological processes, with some of this in
turn being passed on to the carnivores, which
feed on the herbivores. The nitrogen is
ultimately returned to the soil as excreta
and when organisms die and are converted back
to inorganic form by bacterial decomposers.
Although about 78% of the atmosphere is
nitrogen, this cannot be used directly by
most organisms. However, certain bacteria and
cyanobacteria are capable of nitrogen
fixation; that is, they can extract nitrogen
directly from the atmosphere and convert it
to compounds such as nitrates that other
organisms can use. Some nitrogen-fixing
bacteria live mutually with leguminous plants
(peas and beans) or other plants (for
example, alder), where they form
characteristic nodules on the roots. The
presence of such plants increases the nitrate
content, and hence the fertility, of the
soil.