Geoscience Reference
In-Depth Information
After the microorganisms die, the nitrogen is mineralized to NH
4
+
.
It can then be absorbed in this form or as nitrate after nitrification.
However, nitrification by aerobic bacteria takes place only in the top
centimetres of soil that still contain oxygen. Of the NH
4
+
ions, those
that are manufactured in anoxic environment can thus be nitrified
only if they move to the surface by diffusion. To avoid having to
represent this movement in Figure 12.15, we have taken care to draw
the NH
4
+
compartment straddling the water, the aerated soil and the
anoxic soil.
Bubbling (NH
3
Bubbling (N
2
N
2
N
2
N
2
Atmosphere
Nonsymbiotic fixation
by blue-green algae
Nonsymbiotic
fixation in the
oxygenated
rhizosphere
Free water
(2-20 cm)
Organic nitrogen
Nitrification
NO
-
NH
+
Organic nitrogen
Denitrification
Organic nitrogen
Absorption into
roots
Fixation by anaerobic
heterotrophic nonsymbiotic
bacteria
N
2
The rhizosphere, drawn as a triangle pointing down, is an extension of the
aerated layer because the rice plant has the ability to transport air to the roots
Fig. 12.15
Fixation of atmospheric nitrogen, nitrifi cation and denitrifi cation in a fl ooded rice-
paddy soil (Savant and De Datta 1982, completed).
Nitrogen is returned to the atmosphere in two ways, through
bubbling of gas:
One part, generally small, will be lost as ammonia by direct
volatilization starting from NH
4
+
:
NH
4
+
(
ion in solution
) Æ
NH
3
(gas) +
H
+
(
ion in solution
)
Another part is linked to denitrification that will give
gaseous nitrogen N
(§ 12.3.3). But this mostly takes place in
