Methane production in unamended and rice-straw-amended soil at different moisture levels

CH4 production in an alluvial soil, unamended or amended with rice straw (1% w/w), was examined under nonflooded [–1.5 MPa, –0.01 MPa and 0 MPa (saturated) and flooded (1 : 1.25 soil to water ratio)] conditions during a 40-day incubation in closed Vacutainer tubes. CH4 production was negligible at –1.5 MPa, but increased with an increase in the moisture level. Addition of rice straw distinctly increased CH4 production in the soil at all moisture levels including –1.5 MPa. Evidence, in terms of the drop in redox potential and Fe2+ accumulated, suggested that the addition of rice straw hastened the reduction of the soil, even under nonflooded conditions; thus its addition stimulated even the nonflooded soil to produce CH4 in substantial amounts. Our results indicate that many currently unidentified sources of CH4, possibly including organic-amended nonflooded soils, may make a significant contribution to the global CH4 budget.