Agriculture Reference
In-Depth Information
bacteria, which oxidize reduced S compounds to SO
4
2
−
, and Mn(II) and Fe(II)
oxidizing bacteria, which produce insoluble Mn and Fe oxides, though it is not
certain that useful energy is derived from this. Examples of autotrophs using
different electron sources in fixing CO
2
are green plants which derive electrons
from the oxidation of water, sulfide oxidizers which oxidize H
2
S(g) to colloidal
S, and ammonium oxidizers which oxidize NH
4
+
to NO
2
−
.
Organisms that obtain carbon from ingested organic compounds are called
heterotrophs, and most also derive their energy and electrons from these organic
compounds. Examples are fungi, protozoa and most bacteria. A wide range of
organic and inorganic oxidants are used as end electron acceptors in oxidizing the
organic compounds, as in the reactions shown in Figure 4.3(b). Also a wide range
of organic compounds are oxidized. The resulting free energy changes may differ
substantially from those in Figure 4.3(b) for oxidation of the average compound
'CH
2
O'. For example the oxidation of glucose yields about 54 kJ more energy
per mole of C than oxidation of acetate. This makes an increasingly significant
difference the lower the pe
o
∗
of the oxidizing couple. Thus it makes only a small
difference for O
2
or NO
3
−
reduction (12 and 15 %, respectively), but a large
difference for SO
4
2
−
reduction (69 %).
An important component of the overall efficiency of energy production by
microbes is the location of the linked couples and the resulting need to trans-
port reactants and products across cell membranes. In denitrification and SO
4
2
−
reduction, because all of the NO
3
−
andtoalesserextenttheSO
4
2
−
are dissolved
in the soil solution, they are readily imported into the cell and their reduction
linked directly to the oxidation of organic compounds via electron transfer sys-
tems. But in Mn and Fe reduction, the oxides are only sparingly soluble, and so
the concentrations of Mn(III, IV) and Fe(III) in solution are small, even when
large concentrations of the solid oxides are present. This presented a problem in
establishing that Mn and Fe reduction was directly linked to microbial respiration
in natural systems, rather than being an indirect effect through abiotic reactions
involving side products of respiration. The evidence for the direct involvement
of microbes is discussed in Chapter 5.
4.2 REDOX CONDITIONS IN SOILS
This topic has a long history of research (Harrison and Aiyer, 1920; Sturgis,
1936; Pearsall and Mortimer, 1939; Shioiri, 1943; De Gee, 1950; Takai, 1952;
Ponnamperuma, 1955; Baas-Becking
et al
., 1960; Jeffrey, 1961; Patrick, 1966;
Ponnamperuma, 1972; Yu, 1985; Kyuma, 2003). The following factors result in
conditions differing from those in simple aquatic systems:
•
The soil has a structure and contains a network of pores filled to a varying
extent with water, and the soil is overlain by a layer of standing water of vary-
ing depth and degree of oxygenation. The filling and emptying of the pores
is often very dynamic changing from complete saturation to near emptiness
