Factors affecting seasonal and vertical variations in the methanogenic activity in paddy soil determined by the addition of methanogenic substrates

Abstract

When slurries of paddy field soil were incubated anaerobically with the addition of acetate or H2, 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 H2 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.