Methanotrophic bacteria in the rhizosphere of rice microcosms and their effect on porewater methane concentration and methane emission

Abstract

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 at oxic-anoxic interfaces. In the present study we used planted and unplanted rice microcosms and characterized them by numbers of CH4-oxidizing bacteria (MOB), porewater CH4 and O2 concentrations and CH4 fluxes. Plant roots had a stimulating effect on both the number of total soil bacteria and CH4-oxidizing bacteria as determined by fluorescein isothiocyanate fluorescent staining and the most probable number technique, respectively. In the rhizosphere and on the root surface CH4-oxidizing bacteria were enriched during the growth period of tice, while their numbers remained constant in unplanted soils. In the presence of rice plants, the porewater CH4 concentration was significantly lower, with 0.1–0.4mM CH4, than in unplanted microcosms, with 0.5–0.7mM CH4. O2 was detected at depths of up to 16 mm in planted microcosms, whereas it had disappeared at a depth of 2 mm in the unplanted experiments. CH4 oxidation was determined as the difference between the CH4 emission rates under oxic (air) and anoxic (N2) headspace, and by inhibition experiments with C2H2. Flux measurements showed varying oxic emission rates of between 2.5 and 29.0 mmol CH4m-2 day-1. An average of 34% of the anoxically emitted CH4 was oxidized in the planted microcosms, which was surprisingly constant. The rice rhizosphere appeared to be an important oxic-anoxic interface, significantly reducing CH4 emission.