Abstract: Permanently flooded rice fields are the rice fields which emit the largest amount of CH₄ in China. A 6-years (1995～2000) measurement carried out in a permanently flooded rice field in Chongqing, China showed that draining floodwater in winter and planting upland crops,either winter wheat or rape,instead of fallow under flooded conditions not only stopped CH₄ emission during the winter season, but also mitigated CH₄ emission during following rice growing period. CH₄ emission could also be mitigated by ridge cultivation. By using the results obtained from 1998～2000, statistical analysis indicated a significant relationship between the mean CH₄ emission over the rice growing period and averaged soil moisture in winter season, which explained 56% of the variation of the CH₄ emissions among the years and treatments. The averaged soil moisture (0～20 cm) in winter season and soil temperature (5 cm) over the rice growing period explained 78% of the variation. The significance of soil moisture in winter season in CH₄ emission during the following rice growing period was further demonstrated by a lysimeter experiment. The relationships implied that the precipitation during non rice growing period, which dominates soil moisture at a large spatial scale, and the soil temperature during the rice growing period would be the main factors controlling the annual variation of CH₄ emissions from rice fields.

Key words: Permanently flooded rice field, Methane emission, Factor analysis, Ridged cultivation, Nantong City, principal component analysis, quality evaluation., ecological landscape, arterial highway