Agriculture Reference
In-Depth Information
4.3.1
TRANSFORMATIONS OF CARBON
In broad terms the decomposition of organic matter under anaerobic conditions
is expected to be slower than under aerobic conditions because the free energy
changes for the reactions involved are much smaller (Table 4.1 and Figure 4.3).
For example, for the aerobic decomposition of 'CH
2
O',
1
4
'CH
2
O'
+
1
4
O
2
=
1
4
CO
2
(
g
)
+
1
4
H
2
O
=−
119 kJ mol
−
1
G
o
at pH 7, whereas for its anaerobic decomposition in
methanogenesis,
1
1
1
4
'CH
2
O'
=
4
CO
2
(
g
)
+
4
CH
4
(
g
)
G
o
17
.
7kJmol
−
1
at pH 7. Consequently the microbes mediating the de-
composition derive less energy and produce fewer cells per unit of carbon
metabolized. The accumulation of organic matter in marshes and peat bogs illus-
trates this point. (But note the rarity of tropical wetland soils with large organic
matter contents, discussed in Section 3.7.)
The most striking difference between anaerobic and aerobic decomposition
is in the nature of the end products. In aerobic decomposition the main prod-
ucts are CO
2
,NO
3
−
,SO
4
2
−
and resistant residues; in anaerobic decomposition
they are CO
2
,H
2
,CH
4
,N
2
,NH
4
+
,H
2
S and various partially decomposed and
humified residues.
The decomposition proceeds in two stages. The first involves formation of
organic acids, particularly acetic, propionic and butyric, plus various aliphatics
and phenolics, some of which are toxic to plants. The second involves conversion
of organic acids to gaseous products and follows a characteristic pattern. In the
first few days, H
2
formed in fermentation reactions may be evolved together with
CO
2
. Nitrogen gas is also evolved, formed in denitrification of NO
3
−
. As inor-
ganic redox couples then begin to buffer the redox potential, H
2
evolution ceases
and CO
2
is the main end product of carbohydrate metabolism. This continues
until the pe and pH reach values at which methanogenesis is possible, typically
1 or 2 weeks after submergence. The concentration of CH
4
in the soil solution
and in gas bubbles then exceeds the concentration of CO
2
several-fold as a result
of solubility and precipitation effects. Although there is wide variation in the
composition of gases formed between soils, this general pattern is always seen.
At higher temperatures, CO
2
and CH
4
are formed sooner and at greater rates.
Also, at higher temperature and pH, the ratio of CH
4
to CO
2
in the soil gases
changes in favour of CH
4
because of solubility and precipitation effects and the
higher optimal temperatures for methanogens.
=−
4.3.2
TRANSFORMATIONS OF NITROGEN
The main transformations of N are summarized in Figure 4.10. In the absence of
oxygen, mineralization of organic N proceeds only as far as NH
4
+
,andNH
4
+
