Agriculture Reference
In-Depth Information
CH
4
Oxidation
Again depending on the population and growth rates of methane oxidizing bacte-
ria, methane diffusing in from the anaerobic soil may be oxidized in the floodwa-
ter-soil interface or oxygenated rhizosphere. Figure 5.7 shows profiles of O
2
and
CH
4
measured with microelectrodes in the same way as for Figure 5.6. The rate
of oxidation is again sensitive to diurnal changes in oxygenation by algae in the
floodwater. But nonetheless, averaged over the day, of the order of 80% of CH
4
diffusing towards the floodwater will be oxidized (Conrad and Frenzel, 2002).
Oxidation of CH
4
in the rhizosphere is rather less efficient (10-30%) because
of the greater competition with alternative O
2
sinks (Section 8.1.3). Because dif-
fusion of CH
4
through the soil to the floodwater is slow, and oxidation in the
(a) illuminated
4
2
0
−
2
−
4
oxygen
−
6
methane
1
−
8
−
(b) not illuminated
4
2
0
2
−
−
4
−
6
1
−
8
−
0
200
400
600
800
1000
1200
1400
Concentration of O
2
(
µ
M)
0
50
100
150
200
250
300
Concentration of CH
4
(
µ
M)
Figure 5.7
Concentration profiles of O
2
and CH
4
in soil cores from a ricefield (a)
illuminated and (b) not illuminated (Damgaard
et al
., 1998). Reproduced by permission
of American Society for Microbiology
