Agriculture Reference
In-Depth Information
and vice versa within a matter of days. Redox conditions are correspondingly
dynamic.
•
Transport of solutes and gases through the soil is much slower than through soil-
free water because of the restricted cross-sectional area for transport through
the soil pore network and because of adsorption and reaction on soil sur-
faces (Chapter 2). Redox conditions are therefore closely linked to transport
processes.
•
Mineral surfaces have a much greater influence through sorption and precipi-
tation of solutes and direct mediation of redox reactions.
•
The soil contains organic matter which is humified to a varying extent and
inputs of fresh organic matter are often much larger than in aquatic sys-
tems because of greater net primary productivity. The organic matter both
provides substrates for microbial processes and participates in sorption and
other reactions.
•
The micro- and macrobiological ecologies are different.
In this section the redox conditions developing in soils following submergence
are described and the processes governing these conditions are analysed in terms
of the soil chemistry and microbiology discussed so far.
4.2.1
CHANGES WITH DEPTH IN THE SOIL
The floodwater standing on the soil surface is usually sufficiently shallow, well
mixed by wind and thermal gradients, and oxygenated by photosynthetic organ-
isms that it is essentially aerobic. However transport of O
2
into the underlying
soil is too slow for more than a thin layer to be aerobic. In this layer the con-
centrations of Mn
2
+
,Fe
2
+
and other reduced species are negligible, and CO
2
is
the main end product of microbial respiration. In the underlying anaerobic soil,
only a few millimetres away, the concentrations of Mn
2
+
,Fe
2
+
and the vari-
ous organic products of anaerobic respiration can be very large. Thus conditions
change dramatically over a very short distance.
The distribution of reduced species with depth follows a characteristic pattern
reflecting the succession of terminal electron acceptors used by microbes — O
2
,
NO
3
−
, Mn(III, IV), Fe(III), SO
4
2
−
and organic C in fermentation reactions.
Sorption, precipitation and dissolution reactions also influence the distribution.
Figure 4.4 shows profiles of
E
H
and extractable Mn
2
+
,
Fe
2
+
and S
2
−
in soil
columns following flooding, illustrating some of these effects. A steady state
develops over time and the profiles of the reduced species in the soil then reflect
theprofileof
E
H
being progressively deeper for the less-easily reduced species.
Thus the profile of Mn
2
+
in the figure extends closest to the surface, Mn reduc-
tion taking place at the highest
E
H
,andtheS
2
−
profile extends least close to the
surface. The depth to which O
2
penetrates the soil, as indicated by the depth at
which
E
H
begins to decrease, is about 10 mm under the conditions of the figure.
This depth depends on such factors as the oxygenation of the floodwater, the
