Methane formation and emission from flooded rice soil incorporated with 13C-labeled rice straw

Pot experiments with or without 13C-labeled rice straw and rice plants were made to investigate the contribution of organic sources to the formation and emission of CH4 from flooded rice soil. The 13C abundance in emitted CH4 and contained in soil bubbles peaked at 20–40 d after flooding (DAF). After 40 DAF, 13C abundance was higher in unplanted pots than in planted pots. A similar pattern was found in the abundance of 13CCO2 in the soil bubbles. Using the δ13C values in the soil bubbles of the pots without straw application as the background values, the percentages of the CH4 emitted from soil to the atmosphere or contained in the soil bubbles originating from straw were calculated. These percentages were multiplied by the CH4 flux to estimate the quantities of the emitted CH4 from rice straw. The most active CH4 emission from the straw was found during 40–80 DAF, when the first peak of CH4 emission was observed. The contribution of sources other than added straw to the formation and emission of CH4 increased sharply only in the planted pots after 80 DAF (heading stage), suggesting this increase was due to the release of organic materials from rice plants. The relative contribution of 13C derived from straw in CH4 and CO2 was not significantly different, indicating that methanogens did not have any preference for the straw-derived substrates. After the cropping season, ca. 5% of the rice straw carbon remained in the soil and 1% was incorporated into the rice plants.