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Free keywords:
CH4 flux; Diel pattern; Nocturnal peak; Eddy covariance; Rice paddy
Abstract:
The diel pattern of CH4 emissions used in field sampling strategies and time-scale extrapolation is generally regarded as peaking during daytime rather than at night. However, under specific conditions such as water limitation and high temperatures that rice widely undergo, the diel patterns of CH4 emissions remain unclear. We identified diel patterns in CH4 flux among rice stages that experienced different water and temperature conditions, using continuous high-frequency measurements over three consecutive years of CH4 flux in rice paddies. The results showed that a pronounced single peak in CH4 flux occurred during the daytime (13:30–14:30) in the early rice stage. During the reproductive stage, however, the daytime CH4 flux decreased significantly, resulting in a distinct reverse diel pattern frequently (80–86%) observed. The daytime CH4 emissions showed no peaks and were averaged only 41.67% of the nighttime levels under water-limited conditions with high temperatures. Missing the nighttime CH4 flux would underestimate 28.49–32.98% of the daily CH4 emissions during the reproductive stage if the nighttime CH4 flux was calculated using the nighttime/daytime ratio at the vegetative stage. The discovery of a distinct nocturnal peak diel pattern of CH4 flux, contrary to the well-known daytime peak pattern, indicates that measurements and extrapolations based only on daytime data could underestimate CH4 emissions from rice paddies.
