Measuring methane flux from irrigated rice fields by eddy covariance method using open-path gas analyzer

Abstract

The newly developed LI-7700 open-path methane analyzer was used to measure methane (CH4) fluxes from irrigated rice fields using the eddy covariance technique. The diurnal and seasonal variations of CH4 emissions over the whole cropping period of 2013 dry season were characterized. Clear diurnal cycles of CH4 fluxes were observed during the different growth stages of the rice plant (vegetative, reproductive, and ripening). Methane flux started to increase at around 0800H, reached a peak at around 1300–1500H, and then decreased to low values after 1900H. Peak CH4 flux (mean±standard deviation) was 0.082±0.048μmolCH4m−2s−1 during the vegetative stage (0–37 days after transplanting, DAT); 0.063±0.021μmolCH4m−2s−1 during the reproductive stage (38–72DAT); and 0.060±0.033μmolCH4m−2s−1 during the ripening stage (73–103DAT). The diurnal cycles were influenced by temperature (air, floodwater, and soil), surface energy flux (net radiation, soil heat flux, sensible heat flux, and latent heat flux), and ecosystem CO2 exchange (photosynthesis and respiration). The seasonal variations in daily CH4 emissions were primarily controlled by water management and the growth of the rice plants. This study has shown that intermittent irrigation during the vegetative stage was an effective water management strategy to lower the seasonal CH4 emissions to about 3.26gCm−2. The irrigated rice field sequestered 306.45gCm−2 of CO2 from the atmosphere, released 3.03gCm−2 of CH4 to the atmosphere during the growing period, and produced a grain yield of 5.44Mgha−1. Considering a global warming potential (GWP) of 25 over a 100-year horizon, we accounted for the C footprint during the growing period: the irrigated rice field has taken up 1.88kg CO2 equiv. per kg of grain produced. Additionally, the irrigated rice field had a net biome productivity of 17.22gCm−2 and it is a C sink.