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