HR: 0800h
AN: B51C-0957    [Abstracts]
TI: Isotopic Disequilibrium Between Carbon Fixed and Released in a Rice Paddy Ecosystem as Influenced by Methanogenesis From CO$_{2}$ Under Anaerobic Conditions
AU: * Han, G H
EM: hangh@niaes.affrc.go.jp
AF: National Institute for Agro-environmental Sciences, Kannondai 3-1-3, Tsukuba, 305-8604 Japan
AU: Yoshikoshi, H
AF: Kyushu University, Hokazaki 6-10-1, Fukuoka, 812-8581 Japan
AU: Nagai, H
AF: National Institute for Agro-environmental Sciences, Kannondai 3-1-3, Tsukuba, 305-8604 Japan
AU: Yamada, T
AF: National Institute for Agro-environmental Sciences, Kannondai 3-1-3, Tsukuba, 305-8604 Japan
AU: Ono, K
AF: National Institute for Agro-environmental Sciences, Kannondai 3-1-3, Tsukuba, 305-8604 Japan
AU: Miyata, A
AF: National Institute for Agro-environmental Sciences, Kannondai 3-1-3, Tsukuba, 305-8604 Japan
AU: Harazono, Y
AF: International Arctic Research Center, 930 Koyukuk Dr., Fairbanks, AK 99775 United States
AB: Stable carbon isotope ratios of various ecosystem components and ecosystem respiration (\delta$_{R}$) were measured in a Japanese rice paddy. An automated air sampling system was used to collect nighttime air samples to estimate \delta$_{R}$ by means of Keeling plot. Throughout the growing season in 2003, significantly (3\permil to 4\permil) higher \delta$^{13}$C values were observed in \delta$_{R}$ than those observed in plant tissue samples, indicating a strong decoupling process for carbon assimilated and respired in the ecosystem. It is well known that production of methane from CO$_{2}$ exhibits a larger isotope fractionation than that can be found in equilibration of CO$_{2}$ with soil water. CO$_{2}$ entrapped in soil showed 5.5\permil to 7.5\permil higher \delta$^{13}$C values than \delta$_{R}$. Given these isotopic differences, we partitioned total ecosystem respiration into plant respiration and soil (including root) respiration components with an assumption that there is no isotope fractionation associated with respiratory processes of rice plant. The estimated proportion of soil respiration to total ecosystem respiration was about 30% under flooded conditions, but increased to about 40% by floodwater drainage. The partitioned respiratory fluxes from soil contributed to reducing the discrepancy between measured plant biomass increase and accumulated net ecosystem exchange (NEE) for the entire growing season. Partitioning NEE into photosynthetic assimilation and ecosystem respiration based on the isoflux approach revealed that floodwater drainage increased daytime respiratory fluxes greater than the estimated respiratory fluxes from an exponential relationship between nocturnal NEE and air temperature.
DE: 4806 Carbon cycling
DE: 4815 Ecosystems, structure and dynamics
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting