Publications

Rice agriculture is a large anthropogenic source of atmospheric methane (CH 4 ). The reliable estimation of CH 4 emissions requires frequent measurements to trace diurnal and seasonal variations. To evaluate the appropriate intervals and optimal time of day to measure CH 4 emissions using the manual closed chamber method, this study analysed four datasets of CH 4 fluxes in a Japanese irrigated rice paddy measured at 2-hour intervals using the automated closed chamber method. The typical diurnal variation in the CH 4 flux was observed after the rice's heading stage, during which the daily time-weighted mean CH 4 flux was observed twice, in the 08:00 11:59 and 18:00 21:59 time windows. During the flooded rice-growing period, the CH 4 emissions, which were estimated by weekly measurements once per day during the 10:00 11:59 time window, corresponded to 93 106% of the emissions calculated using the automated measurement method. In contrast, no regular measurement strategies produced a satisfactory estimate of the CH 4 emissions during the non-flooded rice-growing period because of a sharp increase in the CH 4 flux just after the drainage. Consequently, the combination of weekly measurements once per day at approximately 10:00 as local mean time for the flooded rice-growing period and daily measurements once per day during the daytime for 1 week after each drainage event is recommended as a strategy to obtain the estimation with a ±10% error.