Feedbacks of Methane and Nitrous Oxide Emissions from Rice Agriculture

Abstract

The effect of global warming on methane (CH 4 ) and nitrous oxide (N 2 O) emissions from agriculture was investigated and simulated from a soil warming experiment.Experiments were designed and installed in a temperature controlled greenhouse. The relationships between elevated temperatures and CH 4 and N 2 O emissions were determined and calculated as the Q 10 s of production, emission and oxidation. A study of the populations of methanogens and methanotrophs at a range of soil temperatures was performed based on soil molecular DNA analysis.This study showed that global warming would increase CH 4 emissions from rice agriculture and that the resultant emissions will be potentially large enough to cause changes in the present atmospheric concentrations. This research also showed that this increase was most evident for soil temperatures below 30 o C, above which emissions decreased with increasing temperature. The seasonal average Q 10 s of CH 4 emission, production, oxidation, methanogen and methanotroph populations were found to be 1.7, 2.6 and 2.2, 2.6 and 3.8, respectively, over a temperature of 20-32 o C. Considering that the processes of CH 4 production and emission are similar to those in natural wetlands, which is the largest source of atmospheric CH 4 , the contribution of this feedback is likely to cause a significant increase to the present CH 4 atmospheric budget if the current global warming trend persists over the next century.ii The Q 10 s of N 2 O emissions and production were 0.5-3.3 and 0.4-2.9, respectively. The low Q 10 values found for N 2 O suggest that although global warming will have a direct impact on the production and emission rates. Nevertheless, the magnitude of the impact of global on both CH 4 and N 2 O emissions from agriculture is likely to vary from one region to another due to the spatial variations in agricultural soil temperatures and the likely changes in the global regional distribution of water resources (water tables, rainfall patterns), water management practices and the responses of terrestrial CH 4 and N 2 O sources such as natural wetlands and plants.iii Acknowledgements