Geoscience Reference
In-Depth Information
Let us point out that
nitrification
is, unlike what we have discussed
above, the set of reactions that results in the manufacture of nitrates in
presence of oxygen by strictly aerobic autotrophic bacteria. The process
starts with organic nitrogen or ammonium ions in the soil (Fig. 12.8).
Organic nitrogen, ammonium and nitrite act as electron donors and
also as sources of energy of which bacteria utilize a few per cent, the
rest being dissipated as heat. In these conditions, they accomplish their
biosyntheses by utilizing the ambient carbon dioxide. They make much
more nitrate than they need.
Nitrification
12.3.4 Synthesis
Many population types are associated in soils. On the one hand, they do
not have the same functions (some organisms denitrify in presence of
oxygen while others nitrify). On the other hand, oxygenation conditions
vary with depth as could be see in the section devoted to soils of rice
fields. When all is said and done, in a wet environment, some bacteria
make a small quantity of nitrates to ensure their biosyntheses while, at
the same time, others destroy these nitrates in large amounts to be able
to obtain their energy from organic matter. Some bacteria even degrade
ammonium NH
4
+
to gaseous nitrogen N
2
via the formation of nitrates
NO
3
-
! The strangest are those bacteria that are capable of nitrification
or denitrification according to environmental conditions.
Faced with such complexity, it is necessary to retain the main point:
in soil, the bacteria responsible for transfer of electrons in absence of
oxygen are mostly facultative anaerobes yielding place gradually to
obligate anaerobes when reducing conditions become very severe. The
obligate anaerobes can be heterotrophic organisms that use the organic
matter present as source of energy and of carbon.
12.4 GLEYSOLS AND STAGNOSOLS
Gleysols and Stagnosols are soils that are waterlogged for more or less
prolonged periods of time, but in their case the mineral phase is greatly
predominant. The former soils develop a
gleyic colour pattern
(iron redox
mottles) and the latter have a temporary perched water table with
bleaching that accompanies reduction and loss of iron.
12.4.1 Location
Various situations that may lead to a temporary excess of water in
high-rainfall periods are brought together in Fig. 12.9.
