Geology Reference
In-Depth Information
diffuses through water 10
4
times more slowly than through air, and so
oxygen concentration falls. The microbial population changes in re-
sponse to the decreased oxygen, from aerobic organisms, requiring
oxygen for respiration, to facultative anaerobes, which can use sources
of electrons other than oxygen, to obligate anaerobes, which cannot
survive if oxygen is present. As the microbial population changes, their
use of alternative electron sources to oxygen causes a series of important
redox changes in soil (Table 8).
As each of these reduction processes occurs, the E
H
is buffered, or
poised, at a particular value. So, for example, while there is nitrate
present, the system will be poised at around
þ
0.22 V; once the nitrate
has been used up, reduction of Mn
IV
will poise the system at around
þ
0.2 V, and so on. E
H
in oxidized systems is dicult to measure because
there is not a specific reaction controlling it. Generally, when oxygen is
present, the E
H
is greater than
þ
0.3 V.
The phase changes that occur are important: the gaseous products can
be lost from the soil to the atmosphere; Mn and Fe become soluble and
can be moved within or out of the system.
Reduction of NO
3
,Mn
IV
, and Fe
III
can occur under moderately
reduced conditions, such as intermittently waterlogged soil or sediment.
Reduction of nitrate causes the loss of a major nutrient. Often the
reduction does not proceed through to the formation of N
2
gas, and
nitrous oxide (N
2
O) is formed, which is a potent greenhouse gas and can
Table 8 Redox reactions in soil
Redox reaction
Range of soil E
H
values (V)
O
2
þ
4H
þ
þ
4e
Ð
2H
2
O
Disappearance of oxygen
0.6-0.4
2NO
3
þ
12H
þ
þ
10e
Ð
N
2
þ
6H
2
O
solution
Disappearance of nitrate
0.5-0.22
gas
MnO
2
þ
4H
þ
þ
2e
Ð
Mn
2
þ
þ
2H
2
O
solid
Appearance of Mn
II
0.4-0.2
solution
Fe
ð
OH
Þ
3
þ
3H
þ
þ
e
Ð
Fe
2
þ
þ
3H
2
O
solid
Appearance of Fe
II
0.3-0.1
solution
SO
2
4
þ
10H
þ
þ
8e
Ð
H
2
S
þ
4H
2
O
solution
Disappearance of sulfate
0to
0.15
gas
CO
2
þ
8H
þ
þ
8e
Ð
CH
4
þ
2H
2
O
gas
Appearance of methane
0.15 to
0.22
gas
Source: adapted from Bohn, McNeal and O'Connor, Soil Chemistry, Wiley Interscience, 1985.
