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

Abstract

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 14 CH4, and model calculations indicated that almost 90% of the oxidation measured had taken place at a depth where only roots could provide the O2 necessary. Slurry from soil planted with rice had an apparent K(m) for CH4 of 4 μM and a V(max) of 0.1 μmol 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 105 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 106 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 μM and a V(max) of 5 μmol 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.