Nocturnal peak methane flux diel patterns in rice paddy fields

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.