Methane (CH4) has a global warming potential 23 to 32 times higher than carbon dioxide and its emission rate in rice fields should vary daily with water and soil temperature, and plant metabolism. This study aimed to identify the appropriate time interval for air sampling in order to quantify the mean daily CH4 emission rate, key information to future studies aiming the derivation of regional indexes of CH4 emission. Three campaigns were performed to evaluate diurnal variation in CH4 emissions (3h interval) from continuously flooded rice (Oryza sativa L. cv. 'IRGA-422 CL') fields at different crop stages (panicle differentiation and ripening) and management systems (conventional tillage-CT and no-tillage-NT) during 2002/2003 crop season in long-term experiment at the Instituto Rio Grandense do Arroz (IRGA), in Cachoeirinha, RS, Brazil. Static chamber method was used and the air samples collected with polystyrene syringes were analyzed by gas chromatography. Methane emission showed a same diurnal pattern in the three campaigns in the both tillage systems. The maximum range of emission (8-33mg CH4m-2 h-1) was observed in the early afternoon (12.00 to 15.00p.m.) followed by a decline to a minimum around midnight to next morning (4-19mg CH4 m-2 h-1), in which fluxes were related to flood water temperature (campaign 1 and 3) or soil temperature (campaign 2). Taking into account the daily emission curves, plus operational aspects like chromatographic analysis of samples into the 24h period after air sampling, the time interval from 9.00 to 12.00a.m. is recommended to studies aiming to evaluate mean daily CH4 emission in rice fields. The close relationship between temperature and methane emission allows us infer about the suitability of the gas sampling procedure for the different rice sub-regions of South of Brazil (littoral region and south of RS, and south of SC) that present the same daily behavior of solar radiation and air temperature. Even this, the appropriateness of this procedure should be tested for these different sub-regions of the South of Brazil but, mainly, for the world regions with diverse climatic conditions.
Management systems; activation energy; methanogenesis
