Geoscience Reference
In-Depth Information
Chapter 3
Inorganic and Organometallic
Compounds
3.1 Nitrogen
Nitrogen is a key building block in all life forms; it is an essential element in many
fundamental cell components, such as proteins, DNA, RNA, and vitamins, as well
as in hormones and enzymes. Nitrogen is an extremely versatile element, existing
in both inorganic and organic forms as well as in many oxidation states. Nitrogen
gas constitutes almost 80 % of the atmosphere and is therefore present almost
everywhere. However, nitrogen gas is not available for use by most organisms. For
plants and animals to be able to use nitrogen, N
2
gas must first be converted to a
more chemically available form, such as ammonium (NH
4
+
), nitrate (NO
3
-
), or
organic nitrogen (e.g., urea). Furthermore, higher organisms (e.g., animals) cannot
use these simple forms of nitrogen and require even more complex forms, such as
amino acids and nucleic acids.
To sustain the food needs of the world's growing population, it is essential to
maintain modern agriculture, which in turn is dependent on a constant supply of
nitrogen fertilizers. Currently, the main technology for the ''fixation'' of atmo-
spheric nitrogen is the Haber-Bosch method, which was developed in Germany
between 1908 and 1910 and is now the primary process in the production of over
99 % of nitrogen fertilizer materials (Alan
2004
). Nitrogen fertilizers have been
produced and applied in very large amounts to field crops around the world over
several decades; in the years 1980, 1990, and 2000, for example, the world ''fixed''
N production by the Haber-Bosch process, in amounts that increased from 59,290,
to 76,320, to 85,130 KT, respectively.
Fixed nitrogen in the subsurface results from many processes and is applied in
many forms and materials. Fixation by rhizobium-legume combinations or free-
living organisms adds N directly to crop or soil organic forms. Animal manure,
sewage sludge, crop residues, and roots add organic nitrogen materials, which are
then mineralized to give ammonium, which is further transformed to nitrate.
Because plants often cannot utilize all the nitrogen applied to agricultural fields,
some is left in the soil, which subsequently leaches into the groundwater (Pratt and
