Oda Masato, Nguyen Huu Chiem
Japan International Research Center for Agricultural Sciences, Tsukuba, Japan.
Department of Environmental Science, Can Tho University, Can Tho, Vietnam.
F1000Res. 2019 Sep 23;8:1675. doi: 10.12688/f1000research.20046.6. eCollection 2019.
The Mekong Delta paddies are known as hotspots of methane emission, but these emissions are not well studied. We analyzed methane emission patterns based on monitoring data from typical triple rice cropping paddies collected over 5 years. We found that the total emissions in a crop season doubled in the second crop, tripled in the third crop, and reset after the annual natural flood of the Mekong River. The emission peaks occurred around 0 to 3 weeks after starting irrigation, then gradually decreased. In general, the main source of emitted methane is rice-derived carbon by current-season photosynthates and the emission peaks at the rice heading stage. However, the contribution of the rice-derived carbon is negligible in the hotspot paddies because total emission is high. The increase in emission levels from the first to the third crop can be explained by the accumulation of rice residue from the preceding crops, especially rice straw incorporated into the soil. The reset of emission levels after the annual flood means that the rice straw is decomposed without methanogenesis in water with dissolved oxygen. Thus, the annual emission pattern shows that avoiding rice straw incorporating into soil and decomposing rice straw in paddy surface-water reduces methane emissions.
湄公河三角洲的稻田是甲烷排放热点地区，但这些排放尚未得到充分研究。我们基于5年收集的典型三季稻种植稻田的监测数据分析了甲烷排放模式。我们发现，一个作物季的总排放量在第二季作物中增加了一倍，在第三季作物中增加了两倍，并在湄公河年度自然洪水后重置。排放峰值出现在开始灌溉后的0至3周左右，然后逐渐下降。一般来说，排放的甲烷的主要来源是当季光合产物产生的水稻衍生碳，且在水稻抽穗期排放达到峰值。然而，在热点稻田中，水稻衍生碳的贡献可以忽略不计，因为总排放量很高。第一季到第三季作物排放水平的增加可以用前茬作物的水稻残茬积累来解释，特别是混入土壤中的稻草。年度洪水后排放水平的重置意味着稻草在含有溶解氧的水中分解而不产生甲烷。因此，年度排放模式表明，避免稻草混入土壤并在稻田地表水分解稻草可减少甲烷排放。
