METHANE PRODUCTION AND ITS FATE IN PADDY FIELDS .8. SEASONAL-VARIATIONS IN THE AMOUNT OF METHANE RETAINED IN SOIL

Seasonal variations in the amount of CH4 retained in submerged paddy soil were investigated in a pot experiment. In the non-planted treatment without rice straw (RS) application, the amount of CH4 in soil was very small throughout the cropping season. It was also small in the planted treatment without RS application until mid-August, and then drastically increased, which corresponded to the increase in the CH4 emission rates. In the planted and nonplanted treatments with RS application, a large amount of CH4 was present in soil at the beginning of the tillering stage. The amount of CH4 in soil in the non-planted treatment did not vary appreciately during the cropping season. On the other hand, the amount of CH4 in the planted treatment abruptly decreased in contrast to the increase in the CH4 emission rates during the tillering stage. Both the amount of CH4 in soil and CH4 emission rates decreased around the maximum tillering stage and increased from the reproductive growth stage onwards. The CH4 produced in soil was estimated to be emitted within 2d during these stages. The amount of CH4 in soil in the rice-planted treatments continuously increased until the harvesting stage irrespective of RS application, which suggested that the decrease of the CH4 emission rates during the ripening stage was not due to the low production of CH4 under low temperature.