SEASONAL-VARIATIONS IN METHANE FLUX AND DELTA(CH4)C-13 VALUES FOR RICE PADDIES IN JAPAN AND THEIR IMPLICATIONS

We have made measurements of the methane (CH4) flux and deltaC-13 value in CH4 from rice paddies in Ryugasaki, Japan. This study is the first we are aware of in which a significant change in the deltaC-13 signature of emitted CH4 has been documented over the rice growing season. Nutrient treatments studied were of two kinds: compound mineral fertilizer either with or without rice straw from the previous growing season incorporated into the inorganic fertilizer. The calculated annual emission rates during the 1990 growing season were 43.1 g/m2 (straw) and 40.6 g/m2 (no straw) for the two treatments. In both treatments, CH4 started out relatively enriched in C-13, became lighter in C-13, and then became more enriched again during the latter part of rice growth. The 1991 growing season showed a lower integrated flux in both nutrient treatments than for 1990 but plots of the fluxes versus time had the same general shape as the flux curves in 1990 and a similar although less pronounced trend in delta(CH4)-C-13 signal. Seasonal changes in deltaC-13 are probably related to changes in CH4 production and oxidation and plant-mediated transport. The likelihood of each process occurring and its effect on C-13 values is discussed. The range of delta(CH4)-C-13 values from seasonal effects was approximately 12 parts per thousand in 1991 for both nutrient treatments. The delta(CH4)-C-13 range for 1991 was approximately 10 parts per thousand (no straw) and approximately 5 parts per thousand (no straw). Our data indicate that when using flux-weighted isotopic signatures to put constraints on the tropospheric CH4 budget, attention should be paid to seasonal changes in isotopic signature from rice paddy CH4 in a manner similar to that previously suggested from measurements in natural wetlands.