Biological Processes in the Soil and Floodwater - The Biogeochemistry of Submerged Soils - page 142

comparison with the amount of reducible Fe. Hence SO 4 2 − reduction generally
does not exert a dominant influence on the soil ecology. Artificial amendment
with sulfate has been proposed as a means of ameliorating methane emissions
(Chapter 8).

5.1.5 METHANOGENESIS

While sufficient inorganic oxidants are present, CO 2 is the main end product
of organic matter decomposition. But after the inorganic oxidants are used up,
methanogenesis is obligatory and the proportion of CH 4 in the respiratory gases
increases. Methane is produced mainly by disproportionation of acetate to CO 2
and CH 4 or by reduction of CO 2 with H 2 (see review articles by Conrad, 1989;
Kiene, 1991; Zinder, 1993). The relative proportions of the two pathways and
the resulting ratio of CH 4 to CO 2 produced depend on the balance of elec-
trons among the reactants and products. Hence for organic matter whose average
oxidation state is zero—for example carbohydrates, which have an average com-
position CH 2 O—complete oxidation produces equal quantities of CH 4 and CO 2 .
But if the organic products are more oxidized or more reduced than the origi-
nal compounds, the ratio of CH 4 to CO 2 produced will be less or greater than
one. Yao and Conrad (2000) have used this principle to analyse organic matter
turnover in methanogenic soils. Because the rate of turnover is slow, and the cor-
responding changes in the organic matter small and therefore difficult to measure
accurately, this approach is potentially very useful. Yao and Conrad's method is
now outlined.
Once the inorganic oxidants have been used up and methanogenesis estab-
lished, the soil enters a pseudo steady state in which the gross composition of
the organic matter is little altered by decomposition and the rates of CO 2 and
CH 4 production are roughly constant (cf. Figure 5.2). A simple model of the
electron balance during the pseudo steady state is as follows. Decomposition
of soil organic matter (SOM) from SOM 0 to SOM 1 plus CO 2 and CH 4 occurs
through the following reactions:

SOM 0 + a H 2 O −−−→ SOM 1 + b CH 3 COOH + c H 2 + d CO 2 ( 5 . 1 )

CH 3 COOH −−−→ CH 4 + CO 2 ( 5.1a )

and

4H 2 + CO 2 −−−→ CH 4 + 2H 2 O ( 5.1b )

where a , b , c and d are coefficients, normalized for the flux of C (i.e. C =
2 b + d = 1). In the pseudo steady state, Reaction (5.1) is rate limiting and CO 2
abundant. It follows that the net rates of CH 4 and CO 2 production are

V CH 4 = (b + c/ 4 )V and V CO 2 = (b − c/ 4 + d)V