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.
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.
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.
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.
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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