A series of researches have been made on the mechanism of methane emission from rice fields in Nanjing.
Paddy fields are an important source of greenhouse gas methane, which has an important impact on global climate change. Methane emissions from rice fields are the result of the combined effects of the three processes of methane production, oxidation and transport in the soil. For a long time, due to the limitation of measurement technology and research methods, the research on the methane emission mechanism in rice fields, especially the methane production pathway and the percentage of oxidation, has been rather inadequate. The stable carbon isotope natural abundance method is a sensitive method developed in the past decade or more. It quantitatively evaluates the methane production pathway and percentage of oxidation based on the changes in the stable carbon isotope composition during the determination of methane emissions. It has played an important role in the research in this field.
In recent years, China's research in this area has made important progress. The Xu Hua team of the Nanjing Institute of Soil Science, Chinese Academy of Sciences, through years of field and cultivation experiments, adopted the method of natural abundance of stable carbon isotopes to systematically clarify the effects of water management in rice season and non-rice season (winter) on rice methane emissions. (Soil Biology & Biochemistry, 2012, 52:108-116; Plos One, 2013, 8, e73982. doi:73910.71371/journal.pone.0073982), seasonal variations of methane production pathways and percent oxidation in continuously flooded paddy fields. The effects of rice straw and straw returning on the methane emission process were studied in of Geophysical Research, 2011, 116, G04025, doi:10.1029/2011JG001696) and it was reported for the first time in the world that methane oxidation fractionation coefficient in paddy soil and the impact of straw returning on it. (Biogeosciences, 2013, 10: 3375-3389).
Recently, the team further researched and found that there are still some deficiencies in the observation methods of methane transmission fractionation in the past rice fields, and a new observation method has been designed for this purpose, that is, the separation box + cutting plant method. This method is more scientific in theory, easier to operate, and easier to promote. This achievement has been published on the research of SCIENCE CHINA: Earth Sciences (2014, 57: 1664-1670). Since there have been reports on the percentage of methane oxidation in paddy fields, which δ13CH4 is used as δ13CH4 (after oxidation) (the δ13CH4 that has been oxidized by methane oxidizing bacteria but has not yet been transported and released), there is a large uncertainty, so according to the choice of different δ13CH4 ( The results obtained after oxidation) are difficult to compare with each other. The team summarized the various options in the existing studies, discussed in depth their advantages and disadvantages, and the conditions for their use. The optimal δ13CH4 (post-oxidation) selection scheme was defined, ie, the δ13CH4 in the rhizosphere was recommended as the δ13CH4 in the rice season ( After oxidation), in winter, the δ13CH4 of the soil surface water is δ13CH4 (after oxidation). The results were published on Plant and Soil (2014, DOI 10.1007/s11104-014-2365-5).
After rice is harvested, the paddy fields will naturally drain and dry, and the effect of straw returning in winter on the methane emission process in the subsequent rice season may be relatively small. However, due to the large number of rural laborers working outside the rice harvest, a considerable portion of paddy fields are idle after winter harvesting, which leads to the growth of a large number of weeds, and their impact on the subsequent production of methane emissions during the rice season should not be underestimated. The study confirms that straw returning to winter has no significant effect on methane production potential, production pathways, oxidation potential, percentage of oxidation, and methane emission flux in the subsequent rice season, but a large amount of weeds before rice transplanting significantly promotes the production of methane and emission. According to statistics, the winter leisure rice fields in Jiangsu Province in 2008 and 2009 were as high as 155,000 hectares, which could lead to a 0.14~0.37Tg increase in methane emissions in the subsequent rice season. The results were published in Atmospheric Environment (2015, doi: 10.1016/j. atmosenv. 2015.01.018).
If paddy fields continue to maintain water layer after rice harvest, then returning straw to winter will significantly increase methane emissions in the current and subsequent rice seasons, but there is no report on the impact of winter methane production pathway and percentage of oxidation. The two-year experimental results show that straw returning to winter significantly increases the methane production potential, and its effect on methane production pathways is regulated by temperature: when the temperature is lower, acetic acid production by methane is promoted, but the increase in temperature lowers the production of methane from acetic acid, which is related to the rice season. The effect of returning straw to the methane production pathway is different. The return of straw in winter has little effect on the oxidation potential of methane, but it significantly reduces the percentage of methane oxidation. The reason for this may be that straw returning significantly increases methane production but has no significant effect on methane oxidation, thus reducing the percentage of methane oxidation. The results were published in Soil Biology & Biochemistry (2015, 84: 75-82).
The above study was funded by the National Fund Project (40971154 and 41071169).
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