Rice roots and CH4 oxidation: the activity of bacteria, their distribution and the microenvironment

Irrigated rice fields account for 10–30% of global methane emissions. Rice plants ventilate the soil and enlarge the oxic–anoxic interface by their root system, thus supplying the necessary O2 to aerobic CH4 oxidizing bacteria (MOB). Rice plants (Oryza sativa type japonica var. Roma) were grown in microcosms in a greenhouse. The roots were sandwiched between two blocks of flooded rice field soil separated by a nylon gauze bag. A root mat developed which mimicked the dense root texture in the upper layer of a natural rice field. Flux measurements under oxic and anoxic conditions showed that CH4 was oxidized with a constant rate of 19% of the anoxically emitted CH4, suggesting that CH4 oxidation in the rhizosphere was at least sometimes limited by CH4 availability. Washed rice roots could both produce and oxidize CH4, depending upon incubation conditions. CH4 production by washed rice roots accounted for at most 10% of the CH4 emitted under anoxic conditions. Initial CH4 oxidation rates of washed roots equaled oxidation rates calculated from the difference between oxic and anoxic fluxes in situ. Oxidation rates became twice as high after an induction period of 20 h, indicating a limitation by O2 or CH4 in situ. The micro-environmental conditions near to the root mat were measured using microelectrodes for O2, redox potential and NH4+ and diffusion probes for CH4. Up to 42 μM O2 was detected in the root mat and concentrations were >2.5 μM in 45% of all measurements. In the bulk soil, no O2 was detected below 2 mm depth, but the root mat significantly increased the redox potential. Plant roots and associated bacteria decreased porewater CH4 and NH4+ concentrations. In the root mat, concentrations of dissolved CH4 were below the detection limit of our probes (<5 μM). Cell numbers of MOB increased with time in the rhizosphere and in the rhizoplane. MOB and aerobic heterotrophic bacteria (AHB) each numbered from 106 to 108 cells g−1 dry weight of soil or root biomass). Active MOB occurred near to a root mat similar to the dense root texture in the upper layer of rice fields. We speculate about O2 or CH4 limitation of MOB.