Dynamics of the methanogenic archaeal community in anoxic rice soil upon addition of straw

Addition of rice straw, which is a common practice in rice agriculture, generally results in enhanced production and emission of the greenhouse gas methane (CH4). However, it is unclear whether straw addition affects only the activity or also the composition of the methanogenic microbial community. It is also unclear to what extent methanogenic archaea would be able to proliferate in the soil. Anoxic slurries of Italian rice-field soil produced CH4 after a lag, during which ferric iron and sulfate were reduced. Addition of rice straw slightly decreased this lag and greatly enhanced the subsequent production of CH4. At the same time, addition of rice straw enhanced the intermediate production of H-2 and acetate that served as the methanogenic substrates. Compared with the unamended control, the addition of rice straw resulted in an increased concentration of phospholipid fatty acids in the soil. Quantitative 'real-time' PCR targeting the 16S rRNA gene also showed increased copy numbers of both Bacteria and Archaea in the straw-amended soil at the end of the experiment. The composition of the archaeal community was followed over time by terminal restriction length polymorphism (T-RFLP) analysis of the archaeal 16S rRNA genes extracted from straw-amended soil and the control. Rice Cluster-I (RC-I) methanogens and Methanosarcinaceae were the most abundant methanogenic populations, followed by Methanobacteriales, Methanomicrobiales and Methanosaetaceae. Addition of rice straw resulted in a relative increase of Methanosarcinaceae and Methanobacteriales and a relative decrease of RC-I methanogens and Methanomicrobiales. Our results revealed a dynamic methanogenic community in anoxic rice-field soil and showed that addition of organic matter selectively enhanced the growth of particular methanogenic populations, which were apparently better adapted to the presence of straw than the others. The extent of archaeal growth was consistent with that expected theoretically from the ambient Gibbs free energies of hydrogenotrophic and acetoclastic methanogenesis.