Botkin Environmental Science Earth as Living Planet 8th txtbk

120 CHAPTER 6 The Biogeochemical Cycles
oxygen will increase. Conversely, if more organic carbon
escapes burial and is oxidized to produce CO 2 , then the
CO 2 concentration in the atmosphere will increase. 13
The Nitrogen Cycle
Nitrogen is essential to life in proteins and DNA. As we
discussed at the beginning of this chapter, free or diatonic
nitrogen (N 2 uncombined with any other element) makes
up approximately 78% of Earth’s atmosphere. However,
no organism can use molecular nitrogen directly. Some
organisms, such as animals, require nitrogen in an organic
compound. Others, including plants, algae, and bacteria,
can take up nitrogen either as the nitrate ion (NO 3 ) or the
ammonium ion (NH 4 ). Because nitrogen is a relatively
unreactive element, few processes convert molecular nitrogen
to one of these compounds. Lightning oxidizes
nitrogen, producing nitric oxide. In nature, essentially all
other conversions of molecular nitrogen to biologically
useful forms are conducted by bacteria.
The nitrogen cycle is one of the most important
and most complex of the global cycles (Figure 6.18). The
process of converting inorganic, molecular nitrogen in the
atmosphere to ammonia or nitrate is called nitrogen fixation.
Once in these forms, nitrogen can be used on land
by plants and in the oceans by algae. Bacteria, plants, and
algae then convert these inorganic nitrogen compounds
into organic ones through chemical reactions, and the nitrogen
becomes available in ecological food chains. When
organisms die, bacteria convert the organic compounds
containing nitrogen back to ammonia, nitrate, or molecular
nitrogen, which enters the atmosphere. The process
of releasing fixed nitrogen back to molecular nitrogen is
called denitrification.
Land plants
3500
Atmosphere
4,000,000,000
Nitrogen fixation
(by bacteria)
Industrial
fixation
80
20
Fixation in
lightning