Isotopic disequilibrium between carbon assimilated and respired in a rice paddy as influenced by methanogenesis from CO2

Abstract

Temporal variability in the carbon isotope composition of ecosystem respiration (δR) has been closely related to environmental variables that influence photosynthetic isotope discrimination (Δ). We show that belowground methanogenesis has a strong impact on δR and leads to a partial decoupling between δR and Δ. The δR, observed in a Japanese rice paddy, varied from −26.3‰ to −22.8‰ over the growing season in 2003 and was consistently more positive (by 2‰−5‰) than leaf bulk δ13C (δP). Interestingly, δR increased by 1.1‰–3.1‰ upon drainage, consistent with increased methane and nighttime CO2 fluxes. As the season progressed, δ13C of soil CO2 (δS) was gradually enriched in 13C, from −23.2‰ to −16.6‰, reflecting a large isotopic fractionation associated with CO2‐dependent methanogenesis. Correlation analyses between δR and driving factors showed that ecosystem factors related to methanogenesis had shorter lags and better correlation with δR than environmental variables that influence photosynthesis. Given the distinct isotopic differences between δR and its above‐ and belowground components (δP and δS, respectively), we partitioned ecosystem respiration into plant and soil respiration assuming negligible fractionation in respiration and CO2 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 CO2 fluxes. We also performed a sensitivity analysis to constrain the probable range of isotopic fractionations in respiration and CO2 transport.