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 flood ed 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 measure d had taken place at a depth where only roots could provide the O-2 ne cessary, Slurry from soil planted with rice had an apparent K-m for CH 4 of 4 mu M and a V-max of 0.1 mu mol g (dry weight)(-1) h(-1), At a d epth of 1 to 2 cm, there was no significant difference (P > 0.05) in n umbers of MOB between soil from planted and nonplanted microcosms (mea n, 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 th e number of MOB, A significantly increased number of MOB was found onl y 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 o xidation was shown in a double chamber with carefully washed intact ri ce plants, Up to 90% of the CH4 supplied to the root compartment was o xidized in the plants, CH4 oxidation on isolated roots was higher and had a larger variability than that in soil slurries, Roots had an appa rent 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 t hese bacteria to colonize the interior of roots and culms.