Abstract. There is some evidence that rice cultivars respond differently to elevated CO 2 concentrations ([CO 2 ]), but [CO 2 ] Â cultivar interaction has never been tested under open-field conditions across different sites. Here, we report on trials conducted at free-air CO 2 enrichment (FACE) facilities at two sites in Japan, Shizukuishi (2007 and2008) and Tsukuba (2010). The average growing-season air temperature was more than 5 C warmer at Tsukuba than at Shizukuishi. For four cultivars tested at both sites, the [CO 2 ] Â cultivar interaction was significant for brown rice yield, but there was no significant interaction with site-year. Higher-yielding cultivars with a large sink size showed a greater [CO 2 ] response. The Tsukuba FACE experiment, which included eight cultivars, revealed a wider range of yield enhancement (3-36%) than the multi-site experiment. All of the tested yield components contributed to this enhancement, but there was a highly significant [CO 2 ] Â cultivar interaction for percentage of ripened spikelets. These results suggest that a large sink is a prerequisite for higher productivity under elevated [CO 2 ], but that improving carbon allocation by increasing grain setting may also be a practical way of increasing the yield response to elevated [CO 2 ].

Paddy fields are an important source of atmospheric CH₄, the second most important greenhouse gas. There is a strong concern that the increasing atmospheric CO₂ concentration ([CO₂]) and global warming are further stimulating CH₄ emissions, but the magnitude of this stimulation varies substantially by study, and few open-field evaluations have been conducted. Here we report results obtained at a Japanese rice free-air CO₂ enrichment (FACE) site under water and soil temperature elevation during two growing seasons. Our objectives were to evaluate the effects of high [CO₂] (ambient + 200 μmol mol⁻¹) and elevated soil temperature (+ 2 °C) on CH₄ emissions under completely open-field conditions. We found about 80% enhancement in total seasonal emissions by the additive effects of FACE and warming, indicating a strong positive feedback effect of global warming. The enhancement in CH₄ emission from the FACE-effect alone (+ 26%) was statistically non-significant (P = 0.19). Nevertheless, observed positive correlations between CH₄ emissions and rice biomass agreed well with previous studies, suggesting that higher photosynthesis led to greater rhizodeposition, which then acted as substrates for methanogenesis. Soil warming increased the emission by 44% (P < 0.001), which was equivalent to a Q₁₀ of 5.5. Increased rice biomass by warming could only partly explain the enhanced CH₄ emissions, but stoichiometric analysis of the electron budget indicated that even a moderate enhancement in organic matter decomposition due to soil warming can cause a large increase in CH₄ production under conditions where Fe(III) reduction, which was little affected by soil warming, dominates electron-accepting processes. At later rice growth stages, advanced root senescence due to elevated temperature probably provided more substrate for methanogenesis. Our stoichiometric evaluation showed that in situ Fe reduction characteristics and root turnover in response to elevated temperature should be understood to correctly predict future CH₄ emissions from paddy fields under a changing climate. Challenges remain for determination of in situ root-exudation rate and its response to FACE and warming

The DNDC (DeNitrification-DeComposition)-Rice model, one of the most advanced process-based models for the estimation of greenhouse gas emissions from paddy fields, has been discussed mostly in terms of the reproducibility of observed methane (CH 4 ) emissions from Japanese rice paddies, but the model has not yet been validated for tropical rice paddies under alternate wetting and drying (AWD) irrigation management, a water-saving technique. We validated the model by using CH 4 and nitrous oxide (N 2 O) flux data from rice in pots cultivated under AWD irrigation management in a screen-house at the International Rice Research Institute (Los Bañ os, the Philippines). After minor modification and adjustment of the model to the experimental irrigation conditions, we calculated grain yield and straw production. The observed mean daily CH 4 fluxes from the continuous flooding (CF) and AWD pots were 4.49 and 1.22 kg C ha À1 day À1 , respectively, and the observed mean daily N 2 O fluxes from the pots were 0.105 and 34.1 g N ha À1 day À1, respectively. The root-mean-square errors, indicators of simulation error, of daily CH 4 fluxes from CF and AWD pots were calculated as 1.76 and 1.86 kg C ha À1 day À1, respectively, and those of daily N 2 O fluxes were 2.23 and 124 g N ha À1 day À1, respectively. The simulated gross CH 4 emissions for CF and AWD from the puddling stage (2 days before transplanting) to harvest (97 days after transplanting) were 417 and 126 kg C ha À1 , respectively; these values were 9.8% lower and 0.76% higher, respectively, than the observed values. The simulated gross N 2 O emissions during the same period were 0.0279 and 1.45 kg N ha À1 for CF and AWD, respectively; these values were respectively 87% and 29% lower than the observed values. The observed total global warming potential (GWP) of AWD resulting from the CH 4 and N 2 O emissions was approximately one-third of that in the CF treatment. The simulated GWPs of both CF and AWD were close to the observed values despite the discrepancy in N 2 O emissions, because N 2 O emissions contributed much less than CH 4 emissions to the total GWP. These results suggest that the DNDC-Rice model can be used to estimate CH 4 emission and total GWP from tropical paddy fields under both CF and AWD conditions.