METHANE EMISSION AND ENTRAPMENT IN FLOODED RICE SOILS AS AFFECTED BY SOIL PROPERTIES

Laboratory incubation experiments were conducted to study the effects of soil chemical and physical properties on CH4 emission and entrapment in 16 selected soils with a pH range of 4.7-8.1, organic matter content of 0.72-2.38%, and soil texture from silt to clay. There was no significant correlation with CH4 emission for most of the important soil properties, including soil aerobic pH (measured before anaerobic incubation), total Kjeldahl N, cation exchange capacity, especially soil organic matter, and soil water-soluble C, which were considered to be critical controlling factors of CH4 emission. A lower CH4 emission was observed in some soils with a higher organic matter content. Differences in soil Fe and Mn contents and their chemical forms contributed to the this observation. A significant correlation between the CH4 emission and the soil organic C content was observed only after stratifying soils into subgroups according to the level of CH4 emission in soils not amended with organic matter. The results also showed that the soil redox potential (Eh), anaerobic pH, anerobic pH, and biologically reducible Fe and Mn affected CH4 emission significantly. Urea fertilization promoted CH4 emission in some soils and inhibited it in others. This result appeared to be related to the original soil pH. CH4 entrapment was-positively correlated with soil clay content, indicating the importance of soil physical characteristics in reducing CH4 emissions to the atmosphere.