FACTORS AFFECTING METHANE EMISSIONS FROM RICE FIELDS

Methane emissions from rice fields are affected by a number of environ mental and agricultural factors. We have analyzed our 7-year data set on methane emissions from rice fields in Tu Zu, China, to delineate th e relationships between emissions and a number of variables that were measured at the same time. Our work was done in fields that were manag ed under prevailing agricultural practices of the region. Consequently , only the effect of factors that vary from year to year or during the growing season can be calculated. In our study we measured the effect s of environmental variables (soil temperature, wind speed, sky cover) and agricultural factors (planting density, water level, rice cultiva rs, organic fertilizer amounts, yield). Of these variables, soil tempe rature had the most significant effect on methane emissions resulting in Q(10) values of about 2 (1.5-3). The effect of sky cover, and even water levels, was to change the soil temperature, which in turn affect ed the methane flux. Wind tended to increase emissions, possibly by ag itation of the soil. Of the agricultural variables, planting density h ad the most significant but complex effect on methane emissions. We st udied emissions from up to 4 times the normal planting density under o therwise similar agricultural conditions in the same fields. For a fou r fold increase in planting density the seasonal average emissions inc reased by about a factor of 2. Rice cultivars had a small but detectab le effect. The amount of organic fertilizer and the yields did not aff ect methane emissions in our fields. The lack of an effect from the fe rtilizers is attributed to a saturation phenomenon whereby methane emi ssions do not respond to continual increases in organic material after some sufficiently high level.