Methanogenic activities on rice roots and plant residue and their contributions to methanogenesis in wetland rice field soil

Changes in potential of methanogenic activities of three fractions (soil, living rice roots and plant residue) in wetland rice field soil during the cropping season of rice were determined by incubating them anaerobically in diluent in tubes. The soil fraction was prepared by removing various materials such as rice roots and plant residue mixed in core soil samples of the plow layer. Rice roots and plant residue (mainly rice straw plowed into soil) collected were washed in anoxic diluent to remove adhering soil and cut to pieces before the incubation. The methanogenic activity of the soil fraction was rather low from mid-May, when rice seedlings were transplanted, until late-June. Thereafter, the activity rapidly increased and at early-July it reached the maximum level of the activity in the year. The high activity was kept for about one month and then, during the period of intermittent irrigation, it dropped rapidly to much lower levels. The overall pattern of changes in the activity of the rice roots fraction resembled to that of the soil fraction, however, the activity on a dry weight basis was much higher than that of the soil fraction on almost all the days of determination. For the plant residue fraction, the pattern of seasonal changes was much different from those of the other two fractions and the activity increased soon after the start of flooding of the field. At the early-cropping season, the activity on a dry weight basis was much higher (up to 150 times) than that of the soil fraction of the same day. Changes in the weights of the three fractions in the rice field were measured by using hill plots, each of which was applied with rice straw and transplanted with rice seedlings. From the changes in the methanogenic activities and the weights of the three fractions in the field, the seasonal variations in the contribution of methanogenic activity of each fraction to the total methanogenic activity in the soil ecosystem were estimated. It was shown that plant residue should significantly enhance the methanogenic activity in the first half of the cropping season, and in contrast, rice roots should enhance it in the latter half.