Increased night temperature reduces the stimulatory effect of elevated carbon dioxide concentration on methane emission from rice paddy soil

To determine how elevated night temperature interacts with carbon dioxide concentration ([CO2]) to affect methane (CH4) emission from rice paddy soil, we conducted a pot experiment using four controlled-environment chambers and imposed a combination of two [CO2] levels (ambient: 380 ppm; elevated: 680 ppm) and two night temperatures (22 and 32 degrees C). The day temperature was maintained at 32 degrees C. Rice (cv. IR72) plants were grown outside until the early-reproductive growth stage and then transferred to the chambers. After onset of the treatment, day and night CH4 fluxes were measured every week. The CH4 fluxes changed significantly with the growth stage, with the largest fluxes occurring around the heading stage in all treatments. The total CH4 emission during the treatment period was significantly increased by both elevated [CO2] (P=0.03) and elevated night temperature (P < 0.01). Elevated [CO2] increased CH4 emission by 3.5% and 32.2% under high and low night temperature conditions, respectively. Elevated [CO2] increased the net dry weight of rice plants by 12.7% and 38.4% under high and low night temperature conditions, respectively. These results imply that increasing night temperature reduces the stimulatory effect of elevated [CO2] on both CH4 emission and rice growth. The CH4 emission during the day was larger than at night even under the high-night-temperature treatment (i.e. a constant temperature all day). This difference became larger after the heading stage. We observed significant correlations between the night respiration and daily CH4 flux (P < 0.01). These results suggest that net plant photosynthesis contributes greatly to CH4 emission and that increasing night temperature reduces the stimulatory effect of elevated [CO2] on CH4 emission from rice paddy soil.