LONG-TERM GROWTH AT ELEVATED CARBON-DIOXIDE STIMULATES METHANE EMISSION IN TROPICAL PADDY RICE

Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO 2 and CH4) which absorb infra-red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attentio n has focused on potential interactions between gases which could infl uence emission rates. In the current experiment, the effect of enhance d CO2 (300 mu L L-1 above ambient) and/or air temperature (4 degrees C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field sea sons (1995 wet and dry seasons 1996 dry season). For all three seasons , elevated CO2 concentration resulted in a significant increase in dis solved soil methane relative to the ambient control. Consistent with t he observed increases in soil methane, measurements of methane flux pe r unit surface area during the 1995 wet and 1996 dry seasons also show ed a significant increase at elevated carbon dioxide concentration rel ative to the ambient CO2 condition (+49 and 60% for each season, respe ctively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolv ed or emitted methane compared to growth at elevated CO2 alone. The ob served increase in methane emissions were associated with a large, con sistent, CO2-induced stimulation of root growth. Results from this exp eriment suggest that as atmospheric CO2 concentration increases, metha ne emissions from tropical paddy rice could increase above current pro jections.