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Abstract
Methanogenic activity of the soil fractions obtained after soil collection from the plow layer of a wetland rice field treated with rice straw was measured by incubating the soil slurries in vitro and seasonal and annual variations were monitored during consecutive 5 y. In this rice field, after a sharp increase in activity, the high activity was maintained for 1 or 2 months and then the activity rapidly decreased. Although almost the same pattern of variations was repeated every year, the period of increase and decrease in activity was different depending on the year and the variations did not correspond to the growth of the rice plants or water management of the field. Differences in the time of the first sharp increase in methanogenic activity were more related to the cumulative sunshine duration than to the cumulative air temperature. Fe2+ concentration in the soil treated with rice straw increased after flooding of soil and it decreased to lower levels until the middle of September. The period of high Fe2+ concentration in soil corresponded to the period of high methanogenic activity. Methane concentration in soil gas increased rapidly soon after flooding and reached a level of 35 to 50% until the beginning of June. In rice field soil without application of organic matter, the methanogenic activity never reached such high levels as those in soil treated with rice straw. Methanogenic activity was also detected in soil after harvest of rice as well as in winter, although, with some exceptions, the activity was much lower than that during the cropping season. The vertical profiles of methanogenic activity in the plow layer showed that the activity was distributed rather heterogeneously and the pattern of distribution significantly changed depending on the season.
