Agriculture Reference
In-Depth Information
25
20
15
10
5
0
−
20
0
20
40
60
80
100
120
Time (days after planting)
Figure 8.1
Seasonal variation in CH
4
emission. Rice straw
(
tha
−
1
)
was incorporated in
the soil 14 days before planting the crop (data from Wassmann
et al
., 2000a). Reproduced
by permission of Kluwer Academic Publishers
field following a crop and there are no additions to the succeeding crop. The
early-season peak may then be absent.
(2) The availability of inorganic electron acceptors, both in the soil constituents
and added in mineral fertilizers. This affects the time course of soil reduction
and hence the rate at which sufficiently reducing conditions for methanogen-
esis develop.
(3) Temperature and radiation. The temperature regime affects rates of CH
4
pro-
duction, transport and oxidation, and generally high temperature favours high
rates of emission. The main effect of radiation is through its influence on
crop growth. Hence dry season emissions at a particular site are often much
smaller than wet season emissions, and a well-managed crop under optimal
conditions of temperature and radiation emits less CH
4
(see Section 8.1.5).
(4) Water regime, particularly where there is mid-season drainage allowing
escape of entrapped CH
4
but also oxidation of the soil. The water regime is
also affected by soil texture as this affects percolation rates; high percolation
rates tend to decrease emissions because less reducing conditions are
maintained in the soil. Texture may also affect gas entrapment and ebullition.
So the picture is complicated.
8.1.3 MODELLING METHANE EMISSION
Given the complexity, some form of mechanism-based modelling is required to
understand or predict emissions in given circumstances. Approaches to modelling
emissions from rice are reviewed by van der Gon
et al
. (2000). A complete
