CH₄ emission from various rice fields in P.R. China

Summary

The CH₄ emission rates from Chinese rice fields have been measured in five typical areas representing all of the five major rice culture regions in People's Republic of China (P.R. China). Four types of diurnal variations (afternoon peak, night peak, afternoon-night double peaks and random pattern) of CH₄ emission rates have been found. The first pattern was normally found in clear weather, the second and the third types were only found occasionally in particular place, while the fourth were found in cloudy or rainy weather. Due to the irregular pattern of the methane production observed in the morning-afternoon comparison experiment, the transport pathway influenced by certain factors, may be the major factor governing the diurnal variation of CH₄ emission. Seasonal variation patterns of CH₄ emission differ slightly with different field locations, where climate system, crop** system and other factors are different. Two and three emission peaks were generally found during single and early rice vegetation periods, with the peak magnitude and time of appearance differing to small degree in individual sites. A decreasing trend of seasonal variation was always observed in late rice season. A combination of seasonal change of transport efficiency and that of CH₄ production rate in the paddy soil explains well the CH₄ emission. The role of rice plant in transporting CH₄ varied over a large range in different rice growing stages. The reasons for internnual changes of CH₄ flux are not yet clear.

Great spatial variation of the CH₄ emission has been found, which can be attributed to the differences in soil type and soil properties, local climate condition, rice species, fertilizer and water treatment. Experiments showed that while the application of some mineral fertilizers will reduce the CH₄ emission and CH₄ production in the soil, the application of organic manure will enhance CH₄ emission and CH₄ production in the soil. Any measures which can get off easily decomposed carbon from organic manure may reduce C supply for CH₄ production, and hence reduce CH₄ emission. Fermented sludges from biogas generators and farmyard-stored manure seem to be promising. In some parts of China, separate application of the organic and mineral manure instead of mixed application could be another option. Frequent Scientific drainage and ridge cultivation, which are often used water management techniques in Chinese rice agriculture, have been proved in the experiments to be a very efficient mitigation measures to reduce CH₄ emission from rice fields.

By summarizing the present available data, China's rice fields contribute about 13.3 Tg yr⁻¹ (11.4–15.2) CH₄ to the atmosphere. The total methane emission from global rice fields can be estimated 33–60 Tg yr⁻¹, much less than the estimates made before.

If we extrapolate the measured data in China with a consideration of measured data in other Asian country, the total global emission of CH₄ from rice fields are estimated to be about 35–60 Tg yr⁻¹