Abstract
Distributions of various bacteria and methanogenic activity on rice roots and plant residue in paddy soil were investigated. Methanogens were usually detected on the rice roots at the population density higher than that in the soil fraction . The density seasonally changed, and in one case it was more than 100 times higher than that in the soil fraction. The densities of sulfate-reducers and anaerobes on the rice roots were usually 10-20 times higher than those in the soil fraction, and that of aerobes was about 3 times higher. The density of methanogens on the plant residue also seasonally changed, and was usually higher than that in the soil fraction. In one case, the density on the plant residue was more than 500 times higher than that in the soil fraction. On the plant residue, the densities of sulfate-reducers and anaerobes were also usually 100-300 times higher than those in the soil fraction, and that of aerobes was usually 30-70 times higher. In all samples, the density of methanogens enumerated on acetate as a substrate was almost always higher than that on H2-CO2. In the rice roots and plant residue, as well as in the soil fraction, methanogenic activity was usually detected. The activities per dry weight of these samples showed distinct seasonal changes, respectively. The change in the total bacterial number accompanied with the decomposition of plant residue seemed to be a major factor bringing about the seasonal change in bacterial composition of the microbial ecosystem in paddy field soil .
