The activity and distribution of CH 4 -oxidizing bacteria (MOB) in flooded rice (Oryza sativa) soil microcosms was investigated. CH 4 oxidation was shown to occur in undisturbed microcosms by using 14 CH 4 , 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 CH 4 of 4 M and a V max of 0.1 mol g (dry weight) ؊1 h ؊1 . At a depth of 1 to 2

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... cm, there was no significant difference (P > 0.05) in numbers of MOB between soil from planted and nonplanted microcosms (mean, 7.7 ؋ 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 ؋ 10 6 g [fresh weight] ؊1 ), corresponding to a layer of 0.1 to 0.2 mm. Plant-associated CH 4 oxidation was shown in a double chamber with carefully washed intact rice plants. Up to 90% of the CH 4 supplied to the root compartment was oxidized in the plants. CH 4 oxidation on isolated roots was higher and had a larger variability than that in soil slurries. Roots had an apparent K m for CH 4 of 6 M and a V max of 5 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.