METHANOTROPHIC BACTERIA IN THE RHIZOSPHERE OF RICE MICROCOSMS AND THEIR EFFECT ON POREWATER METHANE CONCENTRATION AND METHANE EMISSION

CH4 emission from irrigated rice field is one of the major sources in the global budget of atmoshperic CH4. Rates of CH4 emission depend on both CH4 production in anoxic parts of the soil and on CH4 oxidation a t oxic-anoxic interfaces. In the present study we used planted and unp lanted rice microcosms and characterized them by numbers of CH4-oxidiz ing bacteria (MOB), porewater CH4 and O-2 concentrations, and CH4 flux es. Plant roots had a stimulating effect on both the number of total s oil bacteria and CH4-oxidizing bacteria as determined by fluorescein i sothiocyanate fluorescent staining and the most probable number techni que, respectively. In the rhizosphere and on the root surface CH4-oxid izing bacteria were enriched during the growth period of rice, while t heir numbers remained constant in unplanted soils. In the presence of rice plants, the porewater CH4 concentration was significantly lower, with 0.1-0.4 mM CH4, than in unplanted microcosms, with 0.5-0.7 mM CH4 . O-2 was detected at depths of up to 16 mm in planted microcosms, whe reas it had disappeared at a depth of 2 mm in the unplanted experiment s. CH4 oxidation was determined as the difference between the CH4 emis sion rates under oxic (air) and anoxic (N-2) headspace, and by inhibit ion experiments with C2H2. Flux measurements showed varying oxic emiss ion rates of between 2.5 and 29.0 mmol CH4 m(-2) day(-1). An average o f 34% of the anoxically emitted CH4 was oxidized in the planted microc osms, which was surprisingly constant. The rice rhizosphere appeared t o be an important oxic-anoxic interface, significantly reducing CH4 em ission.