Geoscience Reference
In-Depth Information
F
IG. 46
Nitrogen cycle (simplified). Nitrogen gas from the air is fixed in the form of ammonia which is converted
into organic compounds; the clockwise arrows indicate the breakdown of such compounds to ammonia
(shown here as ammonium ions); this is oxidized by bacteria first to nitrite then to nitrate which is assim-
ilated in plants to form organic nitrogen compounds again. Nitrate is reduced to nitrogen and nitrous ox-
ide by other bacteria. Anti-clockwise arrows point to the reduction of nitrate to ammonia and, again, the
assimilation of the latter into organic nitrogen compounds.
Perhaps the best known autotrophic soil bacteria are the ubiquitous nitrifying
organisms. Of these, there are two distinct groups, represented by a few species be-
longing to four different genera. The first type includes those that oxidize ammonia
to nitrite; the second type, namely the
Nitrobacter
species, oxidize nitrite to nitrate
