Agriculture Reference
In-Depth Information
F
E
=
2.9
F
D
=
2.1
F
E
=
0.3
F
D
=
4.7
F
E
=
0.8
F
D
=
−
0.8
0
CO
2
N
2
CH
4
10
20
30
40
50
0.0
0.3
0.6
0.9
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4 0.0
0.3
0.6
Concentration of dissolved gas in water (mM)
Figure 2.10
Concentrations and fluxes of CH
4
,
CO
2
and N
2
in anoxic acidic marsh
(after Morel and Herring, 1993).
F
E
and
F
D
are the fluxes by ebullition and diffusion,
respectively Reproduced by permission of Wiley, New York
where
Z
is the depth of overlying water,
D
is the diffusion coefficient of the
solute in water,
C
Z
and
C
0
are the concentration of dissolved solute at sedi-
ment surface and water surface, respectively,
ε
is the rate of ebullition of all
gases together,
K
H
is Henry's law constant and
R
is the rate of generation of the
solute in the sediment.
An equation of this type can be written for N
2
,
CH
4
and CO
2
and combined
with Equation (2.35) and the resulting equation solved to obtain the rates of
ebullition and the concentrations of each gas at the sediment surface given the
ambient atmospheric concentrations, the rate of methanogensis and the depth of
the water.
Figure 2.10 compares the relative contributions of ebullition and diffusion to
fluxes of CH
4
,
CO
2
and N
2
in an anoxic marsh so calculated. The figure shows
that CO
2
escapes mainly by diffusion whereas more than half the CH
4
escapes by
ebullition. The bubbles contain 69% CH
4
,19%N
2
,5%H
2
O and only 7% CO
2
.
In practice gas bubbles may become entrapped under irregularly shaped soil
particles, and so the simple steady state described by Equation (2.36) does not
hold. The rate of ebullition is then sensitive to mechanical disturbances, induced
for example by wading animals or by the action of wind on plants in the sediment.
This is discussed further in Chapter 8.
2.4 MIXING BY SOIL ANIMALS
The upper few centimetres of the soil are subject to mixing by invertebrates bur-
rowing through the soil and ingesting soil particles. If populations are sufficiently
