The trend of the methane flux for direct-seeded paddy fields was different from that of transplanted paddy fields during the cultivation period. Thus we concluded that the soil physical and chemical properties of the plow layer were different for the two types of paddy fields. The relationship between methane production and soil physical properties with regard to soil macropores were analyzed using the soft X-ray radiographic method. The results were as follows. 1) The methane flux from the direct-seeded paddy fields increased during the early growth stage of rice plants. However, it decreased during the middle to late growth stage. This was not so for the transplanted paddy fields. 2) Eh of the transplanted paddy fields was from -lO0mV to -200mV, and that of the direct-seeded paddy fields was from +200 mV to -50 mV. 3) The distribution of soil macropores was determined by soft X-ray radiography. It was found in the direct-seeded paddy fields that these existed as the macropores of rice straw mixed with soil and that of root traces. On the other hand, in the transplanted paddy fields, the number of macropores was low and the fractuation of their diameter was high. 4) In the direct-seeded paddy fields, the rate of distribution of macropores under ψm=0.98 Pa was high, and saturated permeability was also high. In the trasplanted paddy fields, these were low. It is therefore concluded, based on these findings, that soil macropores of the direct-seeded paddy fields were more developed than those of the transplanted paddy fields. This shows that methane production at the early stage of submergence was increased due the presence of soil macropores.
