Evaluation of methane oxidation in rice plant-soil system

Mechanisms of methane oxidation in the plant-soil system of rice were studied in a pot experiment using two cultivars (PSBRc-30 and IR72) at two growth stages (flowering and heading). Methane emission was measured by chambers, while methane oxidation was determined through propylene amendment as an alternative substrate to be propylene oxide (PPO) and acetylene as an inhibitor for methane oxidizing (methanotrophic) bacteria. Cell numbers (methanotrophic and methanogenic bacteria) were determined by the most probable number method. The cultivar PSBRc-30 consistently showed higher methane emission rates than IR72. Methane flux clearly decreased from flowering to heading stages in both cultivars. This observation was largely reflected by trends in the mechanisms involved: either methanogenic cell numbers or activities decreased with plant age while methanotrophic cell numbers or activities generally showed an increasing trend. The methanogenic population was in the order of 105g−1dry soil, while the population of methanotrophs ranged from 104to nearly 106g−1dry soil. Methanotrophic activity followed the order; root (1.7–2.8 nL PPO g−1DM h−1) > shoot (0.7–2.0) > soil (0–0.4) when the consumption of alternative substrate was related to dry matter. Derived from the estimated amounts of soil and plant biomass in the pot experiment, however, the soil generally accounted for more than 90% of the total methane oxidation. Within the plant segments, methane oxidation activities in the root exceeded those of the shoot by factor of approximately 10.