Seasonal trends and environmental controls of methane emissions in a rice paddy field in Northern Italy.

Rice paddy fields are one of the greatest anthropogenicsources of methane (CH4), the third most importantgreenhouse gas after water vapour and carbon dioxide. Inagricultural fields, CH4 is usually measured with the closedchamber technique, resulting in discontinuous series of measurementsperformed over a limited area, that generally donot provide sufficient information on the short-term variationof the fluxes. On the contrary, aerodynamic techniques havebeen rarely applied for the measurement of CH4 fluxes inrice paddy fields. The eddy covariance (EC) technique providesintegrated continuous measurements over a large areaand may increase our understanding of the underlying processesand diurnal and seasonal pattern of CH4 emissions inthis ecosystem.For this purpose a Fast Methane Analyzer (Los Gatos ResearchLtd.) was installed in a rice paddy field in the Po Valley(Northern Italy). Methane fluxes were measured duringthe rice growing season with both EC and manually operatedclosed chambers. Methane fluxes were strongly influencedby the height of the water table, with emissions peakingwhen it was above 10-12 cm. Soil temperature and thedevelopmental stage of rice plants were also responsible ofthe seasonal variation on the fluxes. The measured EC fluxesshowed a diurnal cycle in the emissions, which was morerelevant during the vegetative period, and with CH4 emissionsbeing higher in the late evening, possibly associatedwith higher water temperature. The comparison between thetwo measurement techniques shows that greater fluxes aremeasured with the chambers, especially when higher fluxesare being produced, resulting in 30% higher seasonal estimations with the chambers than with the EC (41.1 and31.7 gCH4 m-2 measured with chambers and EC respectively)and even greater differences are found if shorter periodswith high chamber sampling frequency are compared.The differences may be a result of the combined effect ofoverestimation with the chambers and of the possible underestimationby the EC technique.