Agriculture Reference
In-Depth Information
10
10
mol m
−
2
h
−
1
)
(a) CH
4
flux (
µ
(b) Fraction of CH
4
oxidized
1000
0.1
1000
0.1
1000
0.2
1000
1
1
100
0.1
0.2
0.3
0.3
100
0.1
0.2
0.4
0.4
0.3
10
0.2
100
0.4
0.5
0.5
0.3
100
10
0.5
0.6
0.4
0.6
1
0.6
10
0.7
0.7
0.1
0.1
1
0.5
0.1
1
10
0.1
1
10
Multiple of standard transport factor k
T
Figure 8.3
Calculated effects of the CH
4
supply factor
k
V
(Assumptions 7 and 8 of
the simplified model) and the root transmissivity factor
k
T
(Assumption 4) on (a) the
CH
4
flux and (b) the fraction of CH
4
oxidized. Lines are contours of CH
4
flux or frac-
tion oxidized (Arah and Kirk, 2000). Reproduced by permission of Kluwer Academic
Publishers
the intricate balance between the twin effects of O
2
, namely inhibiting CH
4
production and promoting CH
4
oxidation (and thereby anaerobiosis, and thereby
CH
4
production).
Conclusions
. The model shows that the bulk of mid to late season emission
occurs via the plant; where organic matter has been added, large emissions early
in the season must occur by ebullition. For any given root-length density profile:
(1) rice cultivars with high specific substrate supply rates will lead to increased
CH
4
emissions;
(2) cultivars with high specific transmissivities will decrease CH
4
emissions;
(3) drainage leading to an air-filled porosity of just 0.01 decreases CH
4
emissions
practically to zero.
These findings broadly agree with experimental observations. Measured rates
of CH
4
oxidation in the rice rhizosphere range widely from 5 to 90% of the
CH
4
transported (Holzapfel-Pschorn
et al
., 1985; Epp and Chanton, 1993; van
der Gon and Neue, 1996). This agrees with the model. Rates of O
2
flow through
rice roots to the rhizosphere are of the order of a few mmol O
2
m
−
2
(soil surface)
h
−
1
(Section 6.4), which is sufficient to account for the rates of oxidation calcu-
lated with the model. Measured differences in emissions between rice cultivars
are largely due to differences in root biomass (Lu
et al
., 1999); the effects of
differences in root porosity are smaller (Aulakh
et al
., 2001a,b).
What little is known about the microbiology of CH
4
oxidation in
the rice rhizosphere indicates complicated kinetics and competition effects.
