Agriculture Reference
In-Depth Information
capable or reducing Fe(III) without direct contact with the oxide. Ecological
interactions between these and other species of Fe reducers in natural anoxic
sediments reflect these and other mechanisms (Snoeyenbos-West
et al
., 2000;
Stein
et al
., 2001).
Demonstrating that Mn(III,IV) is reduced microbially is complicated by the
rapid abiotic reduction of Mn(III,IV) by Fe
2
+
and other reductants. Lovley and
Goodwin (1987) obtained indirect evidence for microbial mediation of Mn(III,IV)
reduction in experiments in which they follows the consumption of H
2
by anoxic
sediment. Addition of MnO
2
caused H
2
to decrease to smaller concentrations
than possible under Fe reducing conditions, suggesting that Mn reduction was
out competing Fe reduction for H
2
in the same way that Fe reduction out competes
SO
4
2
−
reduction and methanogenesis.
5.1.4 SULFATE REDUCTION
Widdel (1988) gives a comprehensive review of the microbiology and ecology
of sulfate reduction in natural environments. Dissimilatory reduction of SO
4
2
−
is carried out by certain heterotrophic bacteria, which use SO
4
2
−
as the terminal
electron acceptor in their respiration. The main genera are
Desulfovibrio, Desul-
fomaculum
and
Desulfobacter
. The bacteria are obligate anaerobes, and being
heterotrophs their activity is sensitive to the supply of carbon. Various organic
substrates are used with some preferences among species: lactate is the preferred
substrate for many species but there are also acetate oxidizing sulfate reducers.
Lactate oxidizers in particular will also grow well on H
2
. Competition for H
2
and acetate results in inhibition of methanogens by sulfate reducers.
Figure 5.2 shows that SO
4
2
−
reduction commences well before Fe(III) reduc-
tion is complete, in spite of the lower redox potential required. The onset of SO
4
2
−
reduction coincides with a rapid decline in concentrations of H
2
and acetate, for
which the sulfate reducers compete with Fe(III) reducers. The overlap between Fe
and SO
4
2
−
reduction is explained by clustering of sulfate reducers in microsites
within which they generate more strongly reducing conditions than in the sur-
rounding soil. They are able to do this because the SO
4
2
−
and organic substrates
on which they subsist are mobile in the soil solution and can therefore diffuse
to the microsites where the colonies of sulfate reducers develop. Iron reducers
cannot do this because they depend on access to immobile Fe(III) in the soil
solid. Hence the horizontal distribution of sulfate reducers in submerged soils
is generally found to be contagious (Watanabe and Furusaka, 1980). The degree
of clustering increases as the mean number of cells present increases, confirm-
ing that the clustering is self-induced. The distribution with depth follows the
profile of redox potential with a peak at an intermediate depth, below the zone
dominated by Fe reduction and above the zone of methanogenesis.
Except in some coastal soils, histosols, acid sulfate soils, and soils artificially
amended with sulfate, the total amount of sulfate present is usually small in
