In the present study, incubation experiments were done using different plant residues. Alfalfa (Medicago sativa L.), clover (Trifolium repens L.), duckweed (Lemna Paucicostata Hegelm), water hyacinth (Eichhornia crassipes Solms-Laub), rice (Oryza sativa L.) and wheat (Triticum aestivum L.) were added to the paddy soil on the basis of the same C content. Our research attempts to reveal the CH_4 and CO_2 formation process from paddy soil and the relationship between the amount of CH_4, CO_2 and carbohydrate composition of the plant residues. The results were as follows. The formation pattern of CH_4 differs according to different plant residue applications. The alfalfa and clover were faster, and the duckweed and water hyacinth were slower in the early incubation period. The amount of methane produced during the 12th week of incubation varied among various kinds of plant residues. It was in the order of : clover>alfalfa>water hyacinth>duckweed>rice>wheat. The amount of CH_4 was higher than that of CO_2 throughout the different plant residue applications. The CH_4 and CO_2 contents showed positive regression with reducing sugar, non-reducing sugar, starch and hemicellulose amount in plant residues. The CH_4 and CO_2 contents showed negative regression with the cellulose amount in plant residues.
