Methane emissions from flooded rice soils and plants under controlled conditions

A split-chamber system was constructed to quantify CH4 emissions directly from flooded rice soils and through intact rice plants in two experiments with two soils. By varying the humidity surrounding the plant, the transpiration rate could be varied by a factor of 3. No effect on CH4 transport was found, indicating that CH4 emissions through the plant are not affected by water movement. Of the CH4 emissions measured in experiments conducted in the summer, approximately 79–87% occurred through the plants, whereas in the winter experiment 61–68% of the total emissions occurred directly from the soils, apparently through ebullition. As the soils were dried at grain maturity, both plant and soil emission rates increased about 10-fold for 1–1.5 days, accounting for 7–8.5% of the total seasonal emissions. Unexpectedly, nighttime emission from the soil was frequently much less than the daytime emission, possibly because of increased gas mobility due to soil drying during the day. Plant emissions varied during the seasons from 0.2 to 1.0 mg m−2 min−1; however, they did not vary dielly over the 2- or 3-day measurement periods, whereas emissions directly from the soil varied greatly. The total emissions of 100–400 g CH4 m−2 during the cropping seasons and the daily emission rates were similar to those reported for field studies.