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, l iving rice roots and plant residue) in wetland rice field soil during the c ropping season of rice were determined by incubating them anaerobically in diluent in tubes. The soil fraction was prepared by removing various materi als 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 soi l) collected were washed in anoxic diluent to remove adhering soil and cut to pieces before the incubation. The methanogenic activity of the soil frac tion was rather low from mid-May, when rice seedlings were transplanted, un til late-June. Thereafter, the activity rapidly increased and at early-July it reached the maximum level of the activity in the year. The high activit y 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 th e soil fraction, however, the activity on a dry weight basis was much highe r than that of the soil fraction on almost all the days of determination. F or the plant residue fraction, the pattern of seasonal changes was much dif ferent from those of the other two fractions and the activity increased soo n after the start of flooding of the held. At the early-cropping season, th e activity on a dry weight basis was much higher (up to 150 times) than tha t 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 whi ch was applied with rice straw and transplanted with rice seedlings. From t he changes in the methanogenic activities and the weights of the three frac tions in the field, the seasonal variations in the contribution of methanog enic activity of each fraction to the total methanogenic activity in the so il ecosystem were estimated. It was shown that plant residue should signifi cantly enhance the methanogenic activity in the first half of the cropping season, and in contrast, rice roots should enhance it in the latter half. ( C) 2000 Elsevier Science Ltd. All rights reserved.