Methane emissions from rice microcosms: The balance of production, accumulation and oxidation

In rice microcosms (Oryza sativa, var. Roma, type japonica), CH4 emission, CH4 production, CH4 oxidation and CH4 accumulation were measured over an entire vegetation period. Diffusive CH4 emission was measured in closed chambers, CH4 production was measured in soil samples, CH4 oxidation was determined from the difference between oxic and anoxic emissions, and CH4 accumulation was measured by analysis of porewater and gas bubbles. The sum of diffusive CH4 emission, CH4 oxidation, and CH4 accumulation was only 60% of the cumulative CH4 production. The two values diverged during the first 50 days (vegetative phase) and then again during the last 50 days (late reproductive phase and senescence) of the 150 day vegetation period. During the period of day 50-100 (early reproductive phase/flowering), the processes were balanced. Most likely, gas bubbles and diffusion limitation are responsible for the divergence in the early and late phases. The effect of rice on CH4 production rates and CH4 concentrations was studied by measuring these processes also in unplanted microcosms. Presence of rice plants lowered the CH4 concentrations, but had no net effect on the CH4 production rates.