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[1] Temporal variability in the carbon isotope composition of ecosystem respiration (d R ) has been closely related to environmental variables that influence photosynthetic isotope discrimination (D). We show that belowground methanogenesis has a strong impact on d R and leads to a partial decoupling between d R and D. The d R , observed in a Japanese rice paddy, varied from À26.3% to À22.8% over the growing season in 2003 and was consistently more positive (by 2%À5%) than leaf bulk d 13 C (d P ). Interestingly, d R increased by 1.1%-3.1% upon drainage, consistent with increased methane and nighttime CO 2 fluxes. As the season progressed, d 13 C of soil CO 2 (d S ) was gradually enriched in 13 C, from À23.2% to À16.6%, reflecting a large isotopic fractionation associated with CO 2 -dependent methanogenesis. Correlation analyses between d R and driving factors showed that ecosystem factors related to methanogenesis had shorter lags and better correlation with d R than environmental variables that influence photosynthesis. Given the distinct isotopic differences between d R and its above-and belowground components (d P and d S , respectively), we partitioned ecosystem respiration into plant and soil respiration assuming negligible fractionation in respiration and CO 2 transport. The estimated proportion of soil respiration varied from 20% to 50% of the ecosystem respiration depending on vegetation stage, temperature, and flooding/drainage conditions. We found a good agreement between net primary production obtained by field sampling and that estimated from the inferred soil respiration and measured CO 2 fluxes. We also performed a sensitivity analysis to constrain the probable range of isotopic fractionations in respiration and CO 2 transport.Citation: Han, G. H., H. Yoshikoshi, H. Nagai, T. Yamada, K. Ono, M. Mano, and A. Miyata (2007), Isotopic disequilibrium between carbon assimilated and respired in a rice paddy as influenced by methanogenesis from CO 2 ,

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