Carbon isotopic composition, methanogenic pathway, and fraction of CH4oxidized in a rice field flooded year-round

Abstract

[1] Values of δ13C were investigated of CH4 trapped in the soil pore water and floodwater of and emitted from a rice field under continuous flooding throughout the fallow and following rice seasons, and CH4 produced via different pathways and fraction of CH4 oxidized was calculated by using the isotopic data. Pore water CH4 was relatively 13C depleted, with δ13C values about −65‰ over the season except between July and August (around −55‰). Also, hydrogenotrophic methanogenesis was very important (around 50%) for most of the season, while acetoclastic methanogenesis dominated (about 70%) only between July and August. Floodwater CH4 was heavier in δ13C value (from −50‰ to −34‰) than pore water CH4 (from −68‰ to −54‰) over the season, demonstrating that it is highly influenced by methanotrophy. The δ13C value of emitted CH4was negatively correlated with flux in temporal variation (P <0.05), and it was more positive in the fallow season (between −56‰ and −44‰) than in the rice season (between −68‰ and −48‰). This indicates that plant-mediated CH4 transport is probably a more important pathway and causes less CH4 oxidation during the rice season than during the fallow season, which is further confirmed by the fraction of CH4 oxidized being generally greater in the fallow season (60%–90%) than in the rice season (10%–80%). These findings suggest a low contribution of acetoclastic methanogenesis and a high fraction of CH4 being oxidized in the field, especially in the fallow season.