A simplified sampling procedure for the estimation of methane emission in rice fields.

Manual closed chamber methods are widely used for CH measurement from rice paddies. Despite diurnal and seasonal variations in CH emissions, fixed sampling times, usually during the day, are used. Here, we monitored CH emission from rice paddies for one complete rice-growing season. Daytime CH emission increased from 0800 h, and maximal emission was observed at 1200 h. Daily averaged CH flux increased during plant growth or fertilizer application and decreased upon drainage of plants. CH measurement results were linearly interpolated and matched with the daily averaged CH emission calculated from the measured results. The time when daily averaged emission and the interpolated CH curve coincided during the daytime was largely invariant within each of the five distinctive periods. One-hourly sampling during each of these five periods was utilized to estimate the emission during each period, and we found that five one-hourly samples during the season accurately reflected the CH emission calculated based on all 136 hourly samples. This new sampling scheme is simple and more efficient than current sampling practices. Previously reported sampling schemes yielded estimates 9 to 32% higher than the measured CH emission, while our suggested scheme yielded an estimate that was only 5% different from that based on all 136-h samples. The sampling scheme proposed in this study can be used in rice paddy fields in Korea and extended worldwide to countries that use similar farming practices. This sampling scheme will help in producing more accurate global methane budget from rice paddy fields.