Chapter 4 of the Environmental Protection Agency report Policy Options for Stabilizing Global Climate (Lashof and Tirpak 1990) provides data on current emissions of methane by source; regional contribution to greenhouse warming; wetland area and associated methane emissions (Figure 4-13); rough rice production (Figure 4-15); and rice area harvested (Figure 4-16).
According to an experiment detailed in "Factors Affecting Methane Production Under Rice," exogenous organic matter appears to be the largest contributor to methane production from flooded rice soils (Delwiche and Cicerone 1993). Figure 2 illustrates the influence of added organic matter on methane emissions from rice soils. In "Methane Production and Emission in Coastal Ricefields of Texas," the authors found that as the rice growing season progressed, methane production at lower depths and farther away from the plants increased in proportion with root density (Fisher et al. 1990).
Experimental studies indicate that reliable data on the importance of rice fields as a source of atmospheric methane can only be obtained by continuous measurement. Schutz et al. (1990) report on a continuous methane sampling and analyzing system in "Methane Emission from Italian Irrigated Ricefields."