Methane and nitrous oxide emissions from rice paddy fields

Recent field studies in rice paddy fields in Japan and China have shown that N2O is emitted to the atmosphere not only after the application of nitrogen fertilisers under flooded conditions but also in a period from the final drainage to submergence in the following rice-growing season through nitrification or denitrification. Those results clearly indicate that CH4 and N2O emissions in rice paddy fields are in a trade-off relationship. In some cases, the emission rate of N2O equivalent to CO2 emission was higher than that of CH4, by using global warming potential (CO2: CH4: N2O = 1: 21: 310). A long-term field study in Okayama, Japan, showed that no-tilled direct seeding cultivation (N-TD) could reduce CH4 emission for several years after the start of N-TD, compared with tilled transplanting cultivation (TT), while N2O was emitted from N-TD more than from TT. Two field experiments which were conduced in different years and sites in China showed that the annual N2O emission in double-rice cropping fields was much higher in the plot where Chinese milk vetch was planted in winter than in the plot with dry fallow in winter, while the CH4 emission rate was much lower in the Chinese milk vetch plot in the rice-growing season. From these results strongly indicate that possible techniques to reduce N2O and CH4 emissions simultaneously from rice fields should be developed, investigating nitrogen and carbon cycles in rice paddy fields and measuring the flux of trace gases, not only in rice growing season but through a year. The introduction of methodology of life cycle inventory to agro-ecosystems indicates that the annual emission was highest in CH4 among three greenhouse gases in rice paddy fields, and the total emission rate was much lower in rice paddy fields than in upland fields.