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Rates and pathways of methane production in rice fields were found to change with season. However, field conditions are complex and thus, it is still unclear which of the many environmental factors cause these changes. One possible factor is the availability of degradable organic carbon. Therefore, we investigated the change in rate and pathway of CH4 production under controlled laboratory conditions by progressing incubation of Italian paddy soil under anaerobic conditions. We studied the functional (pathway, rate) and structural (abundance, taxonomic composition) responses of methanogenic archaeal communities without and with addition of rice straw. Initially, rice straw significantly enhanced CH4 production rates and acetate accumulation. Later on, the values strongly decreased with the progress of degradation. A high contribution of acetoclastic methanogenesis (62–75%) was initially observed, while hydrogentrophic methanogenesis became dominant in the late degradation phase, both in soil without and with amendment of rice straw. The percentage contribution of the hydrogenotrophic pathway scaled with the percentage of the organic carbon mineralized to CH4 plus CO2 during the incubation time, irrespectively whether or not the soil was amended with rice straw. It was low (<33%), when >60% of the carbon was still available, but increased to >50% when available carbon had decreased to <30%. The methanogenic archaeal communities also exhibited changes over the subsequent degradation phases, but especially when the soil was amended with rice straw. Thus, rice straw amendment initially favored acetoclastic Methanosarcina and hydrogentrophic Methanocella, which subsequently decreased, while acetoclastic Methanothrix (‘Methanosaeta’) was favored and increased in the late degradation phases. The results demonstrate strong functional and structural responses of methanogenic archaeal communities to progressing degradation of rice straw and/or soil organic matter, but indicate that the hydrogenotrophic methanogenic pathway was in particular controlled by the availability of degradable organic carbon.
