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Methane plays an important role in anoxic habitats as a final product in the anaerobic decomposition of organic material. An example of anoxic habitats are flooded habitats, such as rice fields. In these soils, methane is produced by the methanogenic microorganisms and is formed primarily from acetate or carbon dioxide in combination with hydrogen. The adaptations of diversity and activity of the methanogenic community have not been sufficiently investigated under different water levels in the soil. Therefore, this study focused on the influence of different water levels on two different rice fields (Philippine and Italian origin) using classical analyzes (GC, HPLC, GC-IRMS) as well as molecular biological methods (qPCR, T-RFLP and Sequencing).
Methanogenesis correlated positively with the water content. For a detectable methane production a water availability of at least 40% of the maximal water retention capacity of the soil was needed, while the carbon dioxide emission already increased by 17%. It is suggested that the release of carbon dioxide from different fermentations is possible at low soil moistures and the formation of methane requires higher soil moistures.
The stable carbon isotope fractionation data indicated that hydrogenotrophic methanogenesis dominated at the beginning of the incubation for all the incubations with different moisture levels. The initial methane production changed faster from hydrogenotrophic to aceticlastic in the incubations with low soil moisture conditions than in completely flooded incubation. The change in water content showed only a small influence on the population dynamics of methanogenic community. Adaptation to the different moisture levels was achieved by a change in the activity of certain methanogenic archaea. High-throughput sequencing reveals that there were mcrA transcripts of different methanogenic families present. Methanosarcinaceae dominated in the transcriptional active methanogenic community under low water levels. In general, the number of mcrA transcripts and thus the methanogenic activity increased by a slight increase of the moisture content. The mcrA transcripts in the Philippine rice field doubled, and the mcrA transcripts in the Italian rice field increased by 100 times in the soil with a low moisture increase. Completely flooded incubation samples reached mcrA transcript levels from 10^8 to 10^9 per gram of dry soil. The mcrA transcript levels in the Philippine soil were always somewhat lower than those in the Italian rice field soil, although the methane production rates in the Philippine soil were about twice as high as in the Italian rice field soil. However, the diversity in the soil at the DNA level was insensitive to dehydration and rewetting. The experiments have thus shown that the anaerobic decomposition of organic material in rice field soils did not require complete flooding. However, the activity of the methanogenic community changed under different moisture levels.
In a second series of experiments, the influence of a moving system (rolled, shaken or magnetically stirred) on methanogenesis was analyzed compared to a static control. In the three investigated environmental systems (digested sludge, river sediment, rice field soil) the difference of methane emission in the moving systems was not significant compared to the static control. In some cases, the methane production in the moving systems reduced or reached a similar order of magnitude as in the static control. The results show that the moving system had little influence on the methanogenic activity, and the chosen rate of motion was not sufficient to damage the syntrophic relationship of the methanogens. Nevertheless, the highest diversity was observed in the static control of the rice field soil incubations, suggesting the effect on the diversity composition of methanogenic community in the rice field soil. The results were deeply dependent on the chosen environmental systems. The effect of mechanical stress on the methanogenic community needed further investigation.