Agriculture Reference
In-Depth Information
Assumptions 1-9 are
ad hoc
simplifications introduced in order to define a
standard system with characteristics that can be explored. Some of the assump-
tions (1-3, 5) are relatively uncontroversial; others (7-9) depend on an under-
lying supposition that root-mediated processes dominate.
Results
. Figure 8.2 gives steady-state profiles of O
2
and CH
4
and the corre-
sponding reaction rates calculated with the model for the fixed root system defined
in Assumption 9. Net O
2
consumption is 460
mol m
−
2
h
−
1
,netCH
4
emission is
µ
mol m
−
2
h
−
1
, the fractions of the O
2
and CH
4
fluxes through the plant are
0.84 and 0.97, respectively, and the fraction of CH
4
oxidized prior to emission
is 0.13. These are all credible numbers.
Figure 8.3 shows the consequences of varying the root transmissivity factor
k
T
and the substrate supply factor
k
V
. It shows that, other things being equal, the
CH
4
flux increases with
k
V
but decreases with
k
T
. The latter, perhaps counter-
intuitive, result reflects the fact that transport through roots allows O
2
into the
system as well as CH
4
out. Enhanced O
2
concentrations in the rhizosphere inhibit
methanogenesis and promote oxidation, and the combined effect of these two
processes more than compensates for the greater ease with which CH
4
can escape.
The model also shows, unsurprisingly, that the fraction of the CH
4
transmitted
through the plants increases as root transmissivity increases and decreases as
substrate supply increases.
Figure 8.3(b) shows that the fraction of CH
4
that is oxidized before reach-
ing the atmosphere is a sensitive function of
k
V
and
k
T
.Increasing
k
V
reduces
the fraction oxidized, presumably because the oxidation potential
V
O
is held
constant in these simulations and increased production simply overwhelms the
oxidation capacity; increasing
k
T
increases the fraction oxidized where transmis-
sivity is low and decreases it where transmissivity is high, presumably reflecting
480
µ
(a)
0
(b)
10
20
30
oxygen
methane
40
50
0.0
0.1
0.2
0.3
0
5
10
15
20
25
30
Reaction rate (mmol m
−
3
h
−
1
)
Concentration (mM)
Figure 8.2
Calculated profiles of O
2
and CH
4
concentrations (a) and reaction rates
(b) (Arah and Kirk, 2000). Reproduced by permission of Kluwer Academic Publishers
