Agriculture Reference
In-Depth Information
Abiotic catalysis is generally less important than biotic but may be important.
Examples are Mn(III,IV) and Fe(III) reduction by microbial metabolites, and
Fe(II) oxidation which is catalysed by sorption onto soil particles.
•
Abiontic, involving free extracellular enzymes or solubilizing agents, enzymes
bound to soil surfaces, enzymes within dead or non-proliferating cells, or
enzymes associated with dead cell fragments. Extracellular enzymes are impor-
tant in the initial stages of organic matter oxidation, in which polysaccharides
and proteins are hydrolysed to soluble compounds that can be absorbed by
microbial cells and further oxidized in biotic processes.
•
Above all, biotic catalysis by microbes is important.
Biotic catalysis is complicated. Different communities of microbes deal with dif-
ferent parts of the sequence of processes degrading organic matter. Anaerobic
decomposition involving organic electron acceptors (i.e. fermentation) generally
occurs concurrently with respiration involving inorganic electron acceptors, and
both produce intermediates that act as both oxidants and reductants. There are
often syntrophic relationships between microbes in which the metabolisms of two
or more organisms are linked and mutually beneficial. For example, in methano-
genesis, oxidation of fatty and amino acids to H
2
,
CO
2
and acetate is endergonic
under standard conditions (i.e.
P
H
2
=
1 atm), but a sufficiently small concentra-
tion of H
2
is maintained locally by methanogens that utilize H
2
(Conrad
et al
.,
1986; Zehnder and Stumm, 1988; Krylova and Conrad, 1998). Likewise there are
antagonisms between microbes, for example where one microbe maintains the
concentration of a substrate below the threshold of a competitor, such as in the
inhibition of methanogens by SO
4
2
−
reducers competing for H
2
(Achtnich
et al
.,
1995). There are also specific inhibitory effects through particular metabolites,
such as in the inhibition of methanogens by denitrifiers (Roy and Conrad, 1999).
Hence the initial microbial populations, growth rates and community structures
may all be important in the overall course of reduction.
The main pathways of organic matter oxidation in anaerobic soil are as fol-
lows. In the initial stages, fermenting bacteria excrete extracellular enzymes that
hydrolyse polysaccharides and proteins to soluble compounds. These may then be
absorbed by microbial cells and converted to alcohols, fatty acids and H
2
. If inor-
ganic electron acceptors are available, the alcohols and fatty acids are completely
oxidized to CO
2
in sequential reduction reactions. If inorganic electron acceptors
are not available—whether because they have been exhausted or because they
are otherwise inaccessible—communities of fermenting bacteria decompose the
alcohols and fatty acids to acetate, H
2
and CO
2
. These then serve as substrates
for methanogenic archaea. Sugar monomers may also be directly converted to
acetate by homacetogenic bacteria. Likewise proteins are hydrolysed to amino
acids by extracellular enzymes, and the amino acids then ultimately oxidized to
acetate, H
2
,NH
4
+
and CO
2
.
Figure 5.2 shows the sequential reduction of inorganic electron acceptors and
production of CO
2
,CH
4
and intermediaries in two representative soils from a
