Agriculture Reference
In-Depth Information
However, as yet monitoring networks on atmospheric mixing ratios are not
sufficiently extensive or close to continental CH
4
sources to make this approach
entirely reliable for rice.
8.1.5 POSSIBILITIES FOR DECREASING EMISSIONS
Methane emissions from rice are expected to increase over the next few decades
as rice production increases to meet projected increases in population and demand.
Intensified production to produce more rice on a smaller land area, with more
crops per year, greater use of fertilizers, greater quantities of crop residues to
be disposed of, legislation against burning residues, and mechanization allowing
incorporation of residues are all likely to exacerbate emissions.
However the better crop management necessary to increase yields will of itself
tend to lessen emissions. Hence van der Gon
et al
. (2002) found that emissions
over five years at an irrigated site in the Philippines were inversely related to
grain yield (Figure 8.5). They found that large emissions were associated with
small ratios of grain to biomass, particularly in the wet season, and hypothesized
that this caused greater CH
4
production from root carbon released into the soil.
In a greenhouse experiment, removing spikelets to reduce the plants' capacity to
store photosynthate in grains increased CH
4
emissions, possibly via more carbon
entering the soil. Unfavourable conditions for spikelet formation in the wet sea-
son may similarly explain high CH
4
emissions. For similar reasons, modern high
yielding rice varieties generally emit less CH
4
than traditional varieties (Neue
et al
., 1997; Aulakh
et al
., 2001a,b), and there are possibilities for breeding vari-
eties with low emission potentials, exploiting differences in biomass partitioning
and gas transport.
500
dry season
wet season
400
300
y
=
−
61x
+
558
r
2
=
0.94
200
100
0
3
4
5
6
7
8
9
Grain yield (t ha
−
1
)
Figure 8.5
Seasonal CH
4
emission as a function of grain yield in wet and dry seasons
at Maligaya, Luzon, Philippines (van der Gon
et al
., 2002). Reproduced by permission of
National Academy of Sciences, USA
