OXIDATION OF METHANE IN THE RHIZOSPHERE OF RICE PLANTS

Oxidation of CH4 in the rhizosphere of rice plants was quantified usin g (1) methyl fluoride, a specific inhibitor of CH4 oxidation, and (2) measuring changes in plant-mediated CH4 emission after incubation unde r air, N-2, or 40% O-2. No significant rhizospheric CH4 oxidation was observed from rice plants in the ripening stage. CH4 emission from ric e plants 1 week before panicle initiation increased by 40% if CH4 oxid ation in the rhizosphere was blocked. The growth stage of the rice pla nt is an important factor determining the rhizospheric CH4 oxidation. Fluctuation of rhizospheric CH4 oxidation during the growing season ma y help to explain the observed seasonal CH4 emission patterns in field studies. Measurements from four rice varieties showed that one variet y, Pokkali, had higher rhizospheric CH4 oxidation. This was probably b ecause Pokkali was in an earlier growth stage than the other three var ieties. Both in the early and in the late growth stages, incubation un der N-2 caused a much stronger CH4 flux than inhibition of CH4 oxidati on alone. Apparently, N-2 incubation not only blocked CH4 oxidation bu t also stimulated methanogenesis in the rhizosphere. Incubation under a higher O-2 atmosphere (40% O-2) than ambient air decreased the CH4 f lux, suggesting that increasing the oxidation of the rice rhizosphere may help in reducing CH4 fluxes from rice agriculture. The O-2 pressur e in the rhizosphere is an important factor that reduces the plant-med iated CH4 flux. However, inhibition of methanogenesis in the rhizosphe re may contribute more to CH4 flux reduction than rhizospheric CH4 oxi dation.