Agriculture Reference
In-Depth Information
where
C
G0
is the concentration at
z
=
0and
κ
is a transmission constant which
depends on such variables as the root length density, root porosity, the perme-
ability of root tips and laterals, and root architecture. Similarly for root-mediated
efflux at a particular depth:
R
=
κD
G
C
G
(
8
.
8
)
The rate of ebullition,
S
, of a particular substance depends on its gas-phase
concentration. Most simply this is expressed:
S
=
σC
G
(
8
.
9
)
where
σ
is a rate constant.
The boundary conditions for solving Equation (8.1) are (a) for volatile solutes
that the concentration at the surface is known and (b) for non-volatile solutes
that the flux is zero.
Parameter Values
With appropriate values for
H,D
G
,D
L
,v,C
0
and the depth-profiles of
θ
G
,θ
L
,κ
and
σ
, Equations (8.1) to (8.9) apply to any non-adsorbed substance. To simulate
CH
4
production, transport, oxidation and emission, we need to consider at least
two mobile substances-O
2
and CH
4
-and at least three reactions:
Oxic respiration
CH
2
O
+
O
2
−−−→
CO
2
+
H
2
O
Methanogenesis
CH
2
O
+
CH
2
O
−−−→
CO
2
+
CH
4
CH
4
oxidation
CH
4
+
2O
2
−−−→
CO
2
+
2H
2
O
Here CH
2
O represents oxidizable organic matter. In reality the reactions with
inorganic terminal electron acceptors, particularly Fe(III) and SO
4
2
−
, should also
be considered. But in the absence of a complete understanding of these processes
(see Chapter 5), and for the sake of simplicity, we exclude them.
Production, P
. Methanogenesis is inhibited by solution-phase O
2
:
P
CH
4
=
IV
M
(
8
.
10
)
where
V
M
(
z, t
)istheCH
4
production potential and
I
(
z, t
) is an inhibition
function which we take to be:
1
I
=
0
≤
I
≤
1
(
8
.
11
)
1
+
ηC
LO
2
where
η
is an inhibition efficiency constant. No reaction produces O
2
,i.e.
P
O
2
=
0.
