Activity and distribution of methane-oxidizing bacteria in flooded rice soil microcosms and in rice plants (Oryza sativa)

The activity and distribution of CH4-oxidizing bacteria (MOB) in flooded rice (Oryza sativa) soil microcosms was investigated. CH4 oxidation was shown to occur in undisturbed microcosms by using (CH4)-C-14, and model calculations indicated that almost 90% of the oxidation measured had taken place at a depth where only roots could provide the O-2 necessary, Slurry from soil planted with rice had an apparent K-m for CH4 of 4 mu M and a V-max of 0.1 mu mol g (dry weight)(-1) h(-1), At a depth of 1 to 2 cm, there was no significant difference (P > 0.05) in numbers of MOB between soil from planted and nonplanted microcosms (mean, 7.7 x 10(5) g [fresh weight](-1)), Thus, the densely rooted soil at 1 to 2 cm deep did not represent rhizospheric soil with respect to the number of MOB, A significantly increased number of MOB was found only in soil immediately around the roots (1.2 x 10(6) g [fresh weight](-1)), corresponding to a layer of 0.1 to 0.2 mm. Plant-associated CH4 oxidation was shown in a double chamber with carefully washed intact rice plants, Up to 90% of the CH4 supplied to the root compartment was oxidized in the plants, CH4 oxidation on isolated roots was higher and had a larger variability than that in soil slurries, Roots had an apparent K-m for CH4 of 6 mu M and a V-max of 5 mu mol g (dry weight)(-1) h(-1), The average number of MOB in homogenized roots was larger than on the rhizoplane and increased with plant age, MOB also were found in surface-sterilized roots and basal culms, indicating the ability of these bacteria to colonize the interior of roots and culms.