Agriculture Reference
In-Depth Information
sample of 16 rice soils studied by Yao
et al
. (1999). Three distinct phases can
be distinguished:
(1) an initial reduction phase lasting 19-75 days in the 16 soils, during which
most of the inorganic electron acceptors are depleted and the rate of CO
2
production, given by the slope of the CO
2
accumulation line in the figure,
is maximal;
(2) a methanogenic phase starting after 2-87 days and lasting 38-68 days, during
which the rate of CH
4
production is maximal; and
(3) a pseudo steady-state phase during which rates of CH
4
and CO
2
production
and concentrations of H
2
and acetate are roughly constant.
The line of H
2
accumulation in the figure is informative because H
2
is turned
over rapidly as it is produced in fermentation and consumed in Fe(III) and SO
4
2
−
reduction and methanogenesis. Hence there are peaks in H
2
pressure in the early
stages of Fe(III) and SO
4
2
−
reduction and again at the transition from Fe(III)
and SO
4
2
−
reduction to methanogenesis. Because consumption tends to increase
with the concentration of H
2
but production is independent of it, there is a
point at which consumption equals production, characterized by H
2
concentra-
tions in the nM range. Acetate is also produced in fermentation and consumed
in methanogenesis, but its turnover is slower and larger concentrations build up.
Figure 5.3 compares the quantities of electrons consumed in reduction of inor-
ganic electron acceptors and methanogenesis in the 16 soils with those donated in
the oxidation of organic matter to CO
2
. At the end of the initial reduction phase,
the former exceeded the latter in nine of the soils, probably in part because CO
2
was precipitated in carbonates and in part because some of the organic carbon was
converted to forms more oxidized than that in CO
2
. However by the end of the
incubation the electron balance was zero in all but three of the soils. At the end
of the incubation, only 6-17% of the organic carbon in the soils was released as
gases: 61-100% as CO
2
,
<
0
.
1-35% as CH
4
and
<
5% as non-methane hydro-
carbons. Most of the CO
2
was produced in Fe(III) reduction during the initial
reduction phase.
Yao and Conrad (1999) calculated the free energy changes in methanogenesis
during the three phases above. During the initial reduction and while redox poten-
tials were still positive (360-510mV), acetate and H
2
concentrations allowed
exergonic methanogenesis with
G <
−
30 kJ mol
−
1
CH
4
. After about 4 days
CH
4
accumulation slowed and ceased in most soils. At this time CH
4
partial
pressures were still small (about 10-100 Pa), but H
2
depleted by Fe or S reduc-
tion and
G
increased to
−
10 kJ mol
−
1
CH
4
, indicating that methanogenesis was
not possible. At the end of Fe and S reduction,
G
decreased to
<
−
25 kJ mol
−
1
CH
4
and CH
4
production resumed. Vigorous CH
4
production continued until the
pseudo steady state was reached. In a few soils the initial CH
4
production was not
interrupted by an intermediate increase of
G
so CH
4
was released throughout
the experiment, resulting in the highest maximum CH
4
production rates.
