Survival of methanogens in air-dried paddy field soil and their heat tolerance

Population density of methanogens in moist paddy field soil during the cultivation period of rice was usually at the order of 105 MPN g−1 (dry soil). When the soil was air-dried and preserved under oxic condition, the population of methanogens decreased to about one fourth of that in the original moist soil after four months of storage. The population density of methanogens in the air-dried soil, however, did not decrease significantly during the following storage and the population in the soil after two years was almost identical to that in the soil preserved for four months. The population of anaerobic bacteria in the air-dried soil was 34% of that in the original moist soil after two years and that of aerobic bactertia was about 1 to 3%. When slurries of various air-dried paddy field soils which were preserved for different times were incubated anaerobically in the presence of added methanogenic substrates, production of methane usually proceeded actively. When these air-dried paddy field soils were heated at various temperatures for different times and the slurries of the soils were incubated anaerobically in the same way, it was shown that methanogens surviving in the air-dried paddy field soils could withstand temperatures up to 105°c for 10hrs or 140°c for 30 min. H2-producing bacteria in the air-dried soils also endured almost the same level of heat treatment.