Effect of Rice Straw Application Time in Previous Crop Season on CH4 Emission from Rice Paddy Field

A pot experiment was carried out to investigate the effect of rice straw application time in the previous crop season on CH4 emissions from rice field. Applying rice straw just before soil is flooded for rice transplanting (DFL) significantly enhanced CH4 flux by 3.04 times in 1996 and 7.12 times in 1997 compared with rice straw application just before the previous crop season (DFE). Seasonal variation of soil Eh and CH4 fluxes were also affected by rice straw application time in the previous crop season. Soil Eh of DFL treatment was in the active range of methanogenic bacteria during the whole rice-growing period, so CH4 emission occurred during the corresponding period (except being imposed by soil drying). While it took 42 days after rice transplanting when soil Eh of DFE treatment dropped to negative value and CH4 emission was very low during the period.