Agriculture Reference
In-Depth Information
where
ρ
is the soil bulk density. The rate of microbial O
2
consumption is
described by a Michaelis - Menten type equation:
S
2
=
ρv
max
[O
2
]
L
/(K
M
+
[O
2
]
L
)
(
4
.
44
)
Kirk and Solivas (1994) measured profiles of Fe(II) and Fe(III) concentrations
and pH in columns of reduced soil exposed to O
2
at one end and compared the
results with the predictions of the model using independently measured parameter
values. The agreement between the observed and calculated results, shown in
Figure 4.17, is good. The measured profiles of [Fe(II)] (Figure 4.17a) are scat-
tered, probably because of the spatial variability inherent in soil reduction and
the clustering of microbes around favourable microsites. There was much less
scatter in the Fe(OH)
3
and pH profiles which are the result of abiotic reactions.
The zone of Fe(II) depletion extends further than the zone of Fe(OH)
3
accu-
mulation, as expected because Fe
2
+
is mobile but Fe(OH)
3
is not. As a result,
Fe(OH)
3
accumulated in the oxidation zone close to the source of O
2
, as shown
in the inset in Figure 4.17(b).
The good agreement between observed and calculated results and the fact that
the model contains no arbitrary fitting parameters show that the important pro-
cesses are well understood and that the model provides a satisfactory description
of the system. It can therefore be used to explore other conditions through a sen-
sitivity analysis (Figure 4.18). The figure shows that over the range of parameter
values expected for submerged soils, substantial amounts of iron are transferred
towards the O
2
-exposed surface leading to a well-defined zone of Fe(OH)
3
accu-
mulation. For a given soil Fe(II) content, the accumulation is sensitive to the soil
Fe
2
+
buffer power, the oxidation rate constant and the soil bulk density. The fall
in pH in the oxidation zone is sensitive to the initial soil pH, the soil pH buffer
power, and the partial pressure of CO
2
.
By contrast if the soil dries to any extent resulting in partially air-filled pores,
the penetration of O
2
increases dramatically: for an air-space of just 1 % of total
6
5.5
0.14
[Fe]
b
Fe
b
Hs
k
s
5
0.12
5.0
P
CO
2
r
4
0.10
4.5
r
[Fe]
[H
+
]
3
0.08
[Fe]
r
4.0
2
0.06
3.5
k
s
1
0.04
k
s
0.01
3.0
0.0
0.01
0.1
1
10
100
0.01
0.1
1
10
100
0.1
1
10
100
Multiple of standard parameter value
Figure 4.18
Sensitivity of the model used for the calculations in Figure 4.17 to its
parameters: [Fe] is the initial concentration of mobile Fe
2
+
,
b
Fe
is the soil Fe
2
+
buffer
power,
b
HS
is the soil pH buffer power,
k
S
is the Fe
2
+
oxidation rate constant and
ρ
is
the soil bulk density. Standard values as for calculations in Figure 4.17
