Localization of processes involved in methanogenic in degradation of rice straw in anoxic paddy soil

In anoxic paddy soil, rice straw is decomposed to CH4 and CO2 by a complex microbial community consisting of hydrolytic, fermenting, syntrophic and me thanogenic microorganisms. Here, we investigated which of these microbial g roups colonized the rice straw and which were localized in the soil. After incubation of rice straw in anoxic soil slurries for different periods, the straw pieces were removed from the soil, and both slurry and straw were st udied separately. Although the potential activities of polysaccharolytic en zymes were higher in the soil slurry than in the straw incubations, the act ual release of reducing sugars was higher in the straw incubations. The con centrations of fermentation products, mainly acetate and propionate, increa sed steadily in the straw incubations, whereas only a little CH4 was formed . In the soil slurries, on the other hand, fermentation products were low, whereas CH4 production was more pronounced. The production of CH4 or of fer mentation products in the separated straw and soil incubations accounted in sum for 54-82% of the CH4 formed when straw was not removed from the soil. Syntrophic propionate degradation to acetate, CO2 and H-2 was thermodynami cally more favourable in the soil than in the straw fraction. These results show that hydrolysis and primary fermentation reactions were mainly locali zed on the straw pieces, whereas the syntrophic and methanogenic reactions were mainly localized in the soil. The percentage of bacterial relative to total microbial 16S rRNA content was higher on the straw than in the soil, whereas it was the opposite for the archaeal 16S rRNA content. It appears t hat rice straw is mainly colonized by hydrolytic and fermenting bacteria th at release their fermentation products into the soil pore water where they are further degraded to CH4. Hence, complete methanogenic degradation of st raw in rice soil seems to involve compartmentalization.