Further understanding CH4 emissions from a flooded rice field exposed to experimental warming with elevated [CO2]

Elevated atmospheric CO2 concentration ([CO2]) has the potential to increase CH4 emissions from rice fields. However, there is still inconclusive evidence due to limited data on whether elevated temperature (Ta) and [CO2] combined can modify CH4 emission. To study this issue further, we conducted a temperature gradient field chamber (TGC) experiment in Gwangju, Korea (126°53′E, 35°10′N, alt. 33 m). Rice (Oryza sativa L.) was grown at two [CO2] (396 vs 673 ppmV) and two Ta [24.8 (≈ambient) vs 26.5 °C] regimes in six independent field TGCs (three each for ambient and elevated [CO2]). CH4 fluxes were measured hourly using an automated gas sampling and analyzing system during the entire season. Elevated [CO2] significantly increased total CH4 emission by 17.4% (14.37 g/12.24 g CH4 m−2) at maturity, whereas elevated Ta only had a minor or insignificant effect (ca. +8.0%; 13.22 g/12.24 g CH4 m−2). However, the elevated Ta effect was significant when combined with elevated [CO2], resulting in an additive effect on CH4 emission (+29.3%; 15.83 g/12.24 g CH4 m−2). This suggests that ongoing rising atmospheric [CO2] and Ta may have a positive feedback on projected global warming. Nevertheless, the positive effects of elevated [CO2] on CH4 emission were greatly reduced with plant development, displaying an increase of 37.5% (or 53.2% in combination with elevated Ta), 23.4% (40.1%) and 17.4% (29.3%) at panicle initiation, full heading and grain maturity, respectively. We conclude that such seasonal dynamics of CH4 emission were attributed to the dwindling response of plant growth, including tiller number, above- and below-ground biomass, to elevated [CO2]. These are assumed to result in the reducing potential of C substrate availability for methanogens, as well as CH4 transport capacity.