[Factors influencing CH4 emissions from a permanently flooded rice field during rice growing period]

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.