(1999) Effects of organic matter application, temperature, and sunshine duration on seasonal and annual variations of methanogenic activity in wetland rice field soil, Soil Science and Plant Nutrition, 45:4, 811-823,

Changes in methanogenic activity were determined in slurries of soil collected from a paddy field. Acetate or H 2 added as a methanogenic substrate stimulated methane production in most samples. In soil collected when the field was continuously flooded, the two substrates affected production similarly, while in the samples collected when the field had been drained, H 2 stimulated production more strongly than acetate. The effect of temperature on methane production in paddy soil was also investigated. In continuously flooded soil, the temperature optimum for production was 40°C, however, this shifted to 45°C during a period of intermittent irrigation accompanied by a marked decrease in activity. The temperature optimum during the non-cropping season was also 45°C. It was suggested that the dominant methanogens in the drained paddy field are H 2 -utilizers, different from the dominant groups in flooded paddy soil.

When slurries of paddy field soil were incubated anaerobically with the addition of acetate or H 2 , methane production from the slurries was stimulated for most of the samples, compared with those without the addition. Methanogenic activity with or without the addition of exogenous acetate or ~ was determined based on the increasing rate of methane concentration in the headspace during the incubation, and vertical distribution and seasonal changes in the activity in the paddy field soil were monitored for 2 years. In the upper plow layers, the activity markedly increased during the continuous flooding period and decreased during the intermittent irrigation period. In the deeper plow layers, however, the activity was much lower than that in the upper layers throughout the year. When the relationship between the methanogenic activity and Fe(II) content in soil was investigated by calculating the correlation coefficients between them, the activity determined with the addition of exogenous H2 showed the highest positive correlation for almost all the cases. Changes in the soil temperature or water content in soil appeared to affect the methanogenic activity differently depending on the agricultural practices during the cropping season and the depth of soil. The effect of the soil temperature on the activity was more pronounced in the deeper soil layers than in the surface soil layers in the continuously flooded soil.Key Words: flooded soil, greenhouse effect, iron reduction, methane production, paddy field. 315 Soil environment of paddy fields is reduced after flooding and flooded paddy soil is considered to be a major source of atmospheric methane (Seiler et al. 1984;Holzapfel-Pschorn et al. 1985;Cicerone and Orernland 1988;Schutz et al. 1989;Yagi and Minami 1990; IPCC 1996).We have been monitoring seasonal and annual changes in the methanogenic activity in paddy soil to investigate the variations in the functions of methanogenic microbial ecosystems in paddy soil Kaku et al. 2000). In these investigations. we determined the methanogenic activity of soil by incubating slurries of soil obtained from the field periodically and measuring the methanogenic rate of the slurries. Methane is produced by methanogenic archaea in anaerobic microbial ecosystems from acetate or H2 + CO 2 as major substrates (Sowers 1995). These substrates are supplied by hydrolytic and fermentative microbes in the ecosystem through the decomposition of organic matter (Conrad and Babbel 1989;Mayer and Conrad 1990;Thebrath et al. 1992;Zinder 1993;Conrad 1999). Since we incubated the soil slurries without the addition of methanogenic substrates to determine the activity. the methanogenic rate of the soil slurry should depend on the pool sizes or production rates of the substrates by the microbes present in the slurry. If the supply of substrates in the slurry is limited. the methanogenic rate during the incubation should also be limited (Hattori et al. 2001).I To whom correspondence should be addressed.Therefore. the effects of the addition o...

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