Methane emission from rice paddies
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Methane release rates from rice paddies have been measured in Andalusia, Spain, during almost a complete vegetation period in 1982 using the static box system. The release rates ranged between 2 and 14 mg/m2/h and exhibited a strong seasonal variation with low values during the tillering stage and shortly before harvest, while maximum values were observed at the end of the flowering stage. The CH4 release rate, averaged over the complete vegetation period, accounted for 4 mg/m2/h which results in a worldwide CH4 emission from rice paddies of 35–59×1012 g/yr if we assume that the observed CH4 release rates are representative of global conditions. The CH4 release rates showed diurnal variations with higher values late in the afternoon which were most likely caused by temperature variations within the upper layers of the paddy soils. Approximately 95% of the CH4 emitted into the atmosphere by rice paddies was due to transport through the rice plants. Transport by bubbles or diffusion through the paddy water was of minor importance. Incubation experiments showed that CH4 was neither produced nor consumed in the paddy water. The relase of CH4 from rice paddies caused a diurnal variation of CH4 in ambient air within the rice-growing area with maximum values of up to 2.3 ppmv during the early morning, compared to average daytime values of 1.75 ppmv.
Key wordsMethane emission field studies rice paddies atmospheric CH4 budget CH4 transport seasonal variation
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