Photosynthates as dominant source of CH₄ and CO₂ in soil water and CH₄ emitted to the atmosphere from paddy fields

Emission rates of CH 4 from paddy soil with and without rice straw applications were measured with pot experiments to estimate the contribution of rice straw to the total CH 4 emission during the growth period of rice plants. The CH 4 derived from rice straw was calculated to be 44% of the total emission. 13 CO 2 uptake experiments were also carried out four times from June 30 to September 13, 1994, to estimate the contribution of photosynthesized carbon to CH 4 emission. The contribution percentages of photosynthesized carbon to the total CH 4 emitted to the atmosphere were 3.8% around June 30, 31% around July 25, 30% around August 19, and 14% around September 13 in the treatment with rice straw applications, and 52% around July 25, 28% around August 19, and 15% around September 13 in the treatment without rice straw applications. They were calculated to be 22% and 29% for the entire growth period in the treatments with and without rice straw applications, respectively. The contribution percentages of photosynthesized carbon to the total CH 4 and inorganic carbon (Σ CO 2 ) dissolved in soil water were 1.3%, 30%, 29%, and 34% for dissolved CH 4 and 3.0%, 36%, 30% and 28% for dissolved inorganic carbon around June 30, July 25, August l9, and September 13, respectively, in the treatment with rice straw applications. They were 70%, 23%, and 32% for dissolved CH 4 and 31%, 16%, and 19% for dissolved inorganic carbon around July 25, August 19, and September 13, respectively, in the treatment without rice straw applications.