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Fact Sheet 4.5. Management of Rice Cultivation
Management strategies for rice include irrigation, fertilization, and crop
residue management. Rice agriculture is an important source of methane globally;
hence, changes in methane emissions likely dominate the overall GHG effect of
riceland agriculture on short time horizons (<100 years). Little information
is available on carbon stock changes associated with rice paddy management.
Rice agriculture tends to increase soil carbon stocks in comparison with adjacent
areas without rice. Most practices that reduce methane emissions will likely
also reduce the rate of carbon storage in rice paddy soils, however.
Use and Potential
Changes in Soil Carbon Stocks under Rice Management
Increases in organic matter have been observed in paddy field soils that have
been in cultivation for 30-100 years in Japan and China. Organic matter may
increase in the plow layer by 40-50 percent through rice cultivation and triple
in other paddy soils compared with adjacent unflooded arable soils. In deeper
soil horizons, increases have been shown to be small (Mitsuchi, 1974). Measurements
of annual carbon balance made between May 1991 and April 1997 showed uptake
of 0.27-0.32 t C ha-1 for an upland single-cropping field, 0.16-0.27 C ha-1
for an upland double-cropping field, and only 0.02 t C ha-1 for a paddy rice
single-cropping field in Japan (Koizumi et al., 1998). These rates agree
with long-term rates observed over decades to a century of rice cultivation.
Addition of fertilizers, including manure (Wada et al., 1981; Wada,
1984), plant residues, and chemical amendments (Kimura et al., 1980)
increase carbon storage. Storage of 7-26 t C ha-1 was observed for 28 to 53
years of manure application in central and southern Japan (average of 0.25-0.5
t C ha-1 yr-1). Many researchers have demonstrated that incorporation of rice
straw and green manure into rice paddy soils dramatically increases methane
emissions. Yagi et al. (1997) showed that incorporation of rice straw in soil
at rates of 600-900 g m-2 after previous harvest increased methane emission
rates up to 3.5-fold in Japanese rice paddy fields; application of rice straw
compost slightly increased methane emissions (Yagi and Minami, 1990).
Strategies to Reduce Methane Emissions
Strategies to reduce methane emissions from rice cultivation include changes
in water management, fertilizer application, and chemical additions. Water management
strategies include midseason drainage (Yagi and Minami, 1990; Yagi et al.,
1996) and intermittent irrigation (Sass et al., 1992; Chen et al.,
1993; Cai et al., 1994). Chemical additions (e.g., sulfate or iron) decrease
the activity of methanogens by providing alternative electron acceptors and
restricting the availability of substrates in submerged soils (e.g., Hori et
al., 1990). Treatments with sulfate have reduced overall methane emissions
by 20-77 percent in different experiments (Schutz et al., 1989; Lindau
et al., 1993; Denier van der Gon and Neue, 1994). The effect may depend
on the amount applied, however: Wassmann et al. (1993) reported no effect
from sulfate addition to fields in China. Other chemical amendments have included
nitrate (Kitada et al., 1993-reduced emissions 23 percent), thiourea
(Cai et al., 1994-no effect), and calcium carbide (Bronson and Mosier,
1991-large reduction). Other options for reducing methane emissions include
changes in tillage and selection of rice cultivars that are associated with
lower methane emissions (Yagi et al., 1997).
Mosier et al. (1998) estimate the potential to reduce methane emissions
by 8-35 Mt CH4 yr-1 [total emissions estimated at 10-113 Mt CH4 yr-1 (Minami,
1994)] if practices were applied in all amenable areas. This reduction is equivalent
to reducing carbon emissions by 0.04-0.2 Gt C. By comparison, the global potential
for GHG reduction through carbon storage in rice paddy soils is small: Storage
at a rate of 0.25 t C ha-1 yr-1 over 30 percent of the total rice area (140
Mha) would remove 0.01 Gt C yr-1-equivalent to a reduction of about 1 Mt CH4
yr-1 globally. Carbon storage likely would actually decrease under most of the
management practices for methane reduction, decreasing the net GHG effect from
that determined from methane emissions reduction alone.
Scientific Uncertainties
There are large uncertainties regarding the area amenable to various rice cultivation
practices, and very few data exist on the rates of carbon accumulation or loss
and methane emission changes under these practices. The large range in estimates
of global methane sources associated with rice cultivation illustrate the large
uncertainties associated with extrapolation of methane emissions data over larger
land areas.
Time Scales
Data from areas where rice agriculture has been practiced continuously for a
century or more show that gains in soil carbon are long term (>100 years). Increased
drainage to decrease methane emissions may increase decomposition rates dramatically,
however, if the soil changes from largely anaerobic to aerobic conditions.
Monitoring, Verifiability, and Transparency
Measurement of methane fluxes is technically challenging and expensive, although
several models now attempt to predict methane emissions from rice. Because methane
fluxes are highly variable in space and time, monitoring of methane emissions
involves significant effort and cost. Changes in carbon storage may be monitored
as changes in bulk density and percent carbon, as discussed in Chapter
3.
Permanence
Carbon storage depends on the degree to which the soil remains anaerobic as
opposed to aerobic. Permanence of carbon storage therefore depends on the duration
of the cultivation practice. On long time horizons (>100 years), carbon storage
changes will dominate changes in methane because of the short atmospheric lifetime
of methane.
Associated Impacts
Rice is a major world food crop. The impact of management strategies on costs
to farmers and on rice yield and sustainability has yet to be assessed. Very
few data are available on nitrous oxide emissions and how they will be affected
by various management strategies.
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