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Rice paddies are an agricultural form of wetland that may replace natural wetlands or be drylands that have been converted through irrigation ( see Fact Sheet 4.5). Rice paddies are flooded for long periods. Anaerobic conditions associated with inundation slow decomposition rates and allow accumulation of large stores of carbon over long time scales. Organic matter storage in rice paddy soils may be considerably greater than in adjacent, unflooded arable soils, especially when organic amendments (rice straw and manures) are part of the agricultural inputs. Carbon may be stored or lost to the atmosphere on conversion of wetland or dryland to rice paddy fields, depending on the amount of carbon stored in soils prior to conversion.
Most practices in rice paddies affect CO2 and CH4 emissions in opposite ways. Therefore, management of rice agriculture for positive climate impact must consider the combined effects of carbon storage, CH4 emissions, and N2O emissions. Table 4-5 provides qualitative rather than quantitative estimates for carbon sequestration and net GHG effects of different management practices. As with managed wetlands (rice paddies) in general, the carbon impacts would constitute the rationale to include this activity under Article 3.4, whereas CH4 reductions from improved practices are already included under the Protocol. There is a lack of monitoring and measuring of carbon storage of rice paddies in the world. China's 200 years of data show that carbon storage of rice paddies from original dryland can reach levels of 0.2-0.45 t C ha-1 yr-1.
Intermittent rice field drainage, which is practiced widely in Asia to improve production, typically decreases CH4 production and emission. If nitrogen management is not linked to the practice, however, N2O emission can become quite large (Bronson et al., 1994). Where rice growth is now limited by nitrogen deficiencies, increased deposition of nitrogen associated with intensified production of available nitrogen (Vitousek et al., 1997) may accelerate rice growth-eventually resulting in more soil carbon storage (Wedin and Tilman, 1996; Nadelhoffer et al., 1999). The global impact of nitrogen deposition, however, may be comparatively small. On the other hand, increased soil temperatures associated with atmospheric CO2 are expected to result in increased soil respiration losses (Woodwell et al., 1998). Higher CO2 concentrations may also suppress decomposition of stored carbon because C:N ratios in residues may increase and because more carbon may be allocated below ground (Torbert et al., 1997).
Strategies for mitigating CH4 emission from rice cultivation are based on controlling production, oxidation, or transport processes (Yagi et al., 1997; Fact Sheet 4.5).
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