Effect of application of rice straw and cellulose on methane emission and biological nitrogen fixation in a subtropical paddy field .2. Enumeration of populations of methanogenic bacteria by most probable number method and roll tube method

Paddy soil samples were taken from three plots as follows: control (T1), application of rice straw at 10 t ha(-1) (T3), and application of cellulose at 10 t ha(-1) (T5) during the rice cultivation period. Populations of methanogenic bacteria were enumerated by the most probable number method (MPNM) and roll tube method (RTM). In both methods, methanogenic populations in T3 and T5 were higher than those in the control (T1). Values of the populations enumerated by MPNM tended to be higher than those by RTM but sometimes the values were close to each other in both methods. The population level of methanogens in T3 was nearly the same as that in T5. In RTM, the percentages of methanogenic colonies to total ones grown on the medium mere the highest in T5 among the three treatments. On the 14th day after transplanting, the percentages in T1 and T5 decreased. Then the percentage in T5 increased and reached a value of 15.4% after heading, and that in T3 also increased at around heading and reached a value of 8.5%, whereas the percentage in T1 was very low (0-2.1%). These results suggest that the application of rice straw and cellulose at high doses may enhance the level of methanogenic populations and that some changes in the microflora surrounding methanogenic bacteria might occur at rice heading and ripening stages.