| Research Abstract |
Methane is produced by methane-producing archaea from acetate or H_2 + CO_2 as major substrates. In flooded paddy soil, methanogenesis proceeds extensively and it has been considered to be a major source of atmospheric methane. Iron-reduction is a key reaction in flooded soil, which reduces Fe(III) to Fe(II) by using H_2 and various organic matter as electron donors. Since in flooded paddy soil, soil temperature, especially in upper soil layers, shows extensive diurnal changes in sunny days, effects of the diurnal changes in the temperature on methanogenesis and iron-reduction were investigated. The results obtained are as follows;
(1) Soil temperature in the upper soil layers reached its maximum in the day from around noon to 14:00 and it usually reached its minimum in the day at dawn (around 4:00 a.m.).
(2) When the methanogenic activity was determined in the presence or absence of exogenous substrates (H_2 or acetate) by incubating soil slurries obtained from the field twice in a day
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(at noon and at dawn), the two activities determined without the substrates were significantly different. In contrast, the activities determined with H_2 were not significantly different in most cases.
(3) When the slurries of paddy soil were incubated at various temperatures, iron-reduction proceeded actively even at rather low temperatures around 10℃, however, methanogenesis was extremely low at lower than 20℃.
(4) When the incubation temperature was diurnally changed, iron-reduction proceeded based on the averages of temperature in a day and it proceeded actively even at lower than 15℃. While, methanogenesis did not proceed at the average temperature lower than 20℃.
(5) It is suggested from the results that immediately after flooding of soil, when the soil temperature is around 15℃, iron-reduction proceeds dominantly and accumulates Fe(II) in soil, which serves as a reducing agent in soil and lowers the oxidation-reduction condition of soil. When the soil temperature increases after the initial iron-reducing period, methanogenesis is activated in the reduced condition. Less
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