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

In anoxic paddy soil, rice straw is decomposed to CH(4) and CO(2) by a complex microbial community consisting of hydrolytic, fermenting, syntrophic and methanogenic microorganisms. Here, we investigated which of these microbial groups 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 studied separately. Although the potential activities of polysaccharolytic enzymes were higher in the soil slurry than in the straw incubations, the actual release of reducing sugars was higher in the straw incubations. The concentrations of fermentation products, mainly acetate and propionate, increased steadily in the straw incubations, whereas only a little CH(4) was formed. In the soil slurries, on the other hand, fermentation products were low, whereas CH(4) production was more pronounced. The production of CH(4) or of fermentation products in the separated straw and soil incubations accounted in sum for 54-82% of the CH(4) formed when straw was not removed from the soil. Syntrophic propionate degradation to acetate, CO(2) and H(2) was thermodynamically more favourable in the soil than in the straw fraction. These results show that hydrolysis and primary fermentation reactions were mainly localized 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 that rice straw is mainly colonized by hydrolytic and fermenting bacteria that release their fermentation products into the soil pore water where they are further degraded to CH(4). Hence, complete methanogenic degradation of straw in rice soil seems to involve compartmentalization.