Methane Emissions During the Flooded Rice Growing Season As Affected by Different Straw Managements in Southern Brazil.

Methane (CH₄) is a key anthropogenic greenhouse gas in the atmosphere. Flooded rice soils have been recognized as an important source of atmospheric CH₄. In Southern Brazil is cultivated annually approximately 1.2 million hectares of rice under flooded regime. The flooded rice fields in Southern Brazil were responsible for the emission of 350 Gg CH₄ in 2005, 82% of the Brazilian emissions from that source. In these fields, rice is cultivated from October to March, and on the fallow period different straw managements are performed by the farmers. Therefore is essential to select straw managements applied after the rice harvest, which promote the reduction of CH₄ emissions in rice fields. The aim of this study was to evaluate the CH₄ emissions during the rice growing season as affected by different straw managements applied during the fallow period. Eight treatments were tested: non-straw management, cage roller after harvest, disk harrow after harvest, rice straw removal, non-straw management + ryegrass cropping, late disk harrow, disk harrow after harvest + late disk harrow and cage roller after harvest + late disk harrow. During the fallow period all plots remained drained. Methane flux was determined weekly by the static chamber technique. Gas samples were analyzed for CH₄ concentration with a gas chromatograph (Shimadzu GC-2014 Greenhouse) equipped with a flame ionization detector. The treatments that leaded to a higher CH₄ evolution during the rice growing season were non-straw management (585 kg CH₄ ha^-1) and ryegrass cropping (530 kg CH₄ ha^-1), the ones that had more rice straw on the soil surface available for CH₄ production by methanogenic bacteria. The disk harrow after harvest treatment was the one with the least CH₄ emissions (334 kg CH₄ ha^-1), probably due to the lack of available straw-C for the methanogenic bacteria, since it had already been degraded by the moment of its incorporation to the soil prior to the rice cultivation. Managements that stimulate the decomposition of the rice straw before its cultivation lead to a decrease of CH₄ production after the soil flooding during rice growing season.