Sources of methane emitted from paddy fields

Soil organic matter, roots (photosynthates) and applied organic materials (rice straw etc.) are the main sources of methane (CH₄) emitted from paddy fields. The potential CH₄ production in Japanese paddy fields were estimated from chemical properties of paddy soils of respective soil series, their acreage and thermal regimes during the rice growing period. The estimated amounts of potential CH₄ production were from 24 to 54 kg-C ha^-1 among 7 Districts in Japan, which are around one fifth of the amounts of CH₄ emission observed from paddy fields in the world. ¹³CO₂ uptake pot experiments were carried out three times from Aug. 8 to Sept. 25 to the treatment without rice straw applications in 1993 and four times from June 30 to Sept. 13 to the treatments with and without rice straw applications in 1994 to estimate the contribution of photosynthesized carbon to CH₄ emission. The contribution percentages of photosynthesized carbon to the total CH₄ emitted to the atmosphere were calculated to be 22% and 29-39% for the entire growth period in the treatments with and without rice straw applications, respectively. The relationship between the amount of CH₄ emission to the atmosphere from submerged paddy soils with rice plants and the application level (0-8 g kg^-1) of rice straw in soil was investigated in a pot experiment. The increase (Y) in cumulative amounts of CH₄ with the increase in the application level of rice straw was formulated with a logistic curve: Y=k[a/(1 +be^-cx)]; x, application level of rice straw; k, a coefficient for relative CH₄ emission. Since the seasonal variations in coefficients a, b and c in the equation were also formulated as the function of the sum of effective temperature (E, Σ (T-15); T, daily average temperature), Y from any paddy soil by any level of rice straw application was known to be estimated by the equation: Y=k[a(E)/(1 +b(E)e^-c(E)x)]. This revised version was published online in August 2006 with corrections to the Cover Date.