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The input of organic substances (e.g., rice straw) in rice field soils usually stimulates the production and emission of the greenhouse gas methane (CH4). However, the amount of CH4 derived from the applied rice straw, as well as the response of bacterial and archaeal communities during the methanogenic phase, are poorly understood for different rice field soils. In this study, samples of five different rice soils were amended with 13C-labeled rice straw (RS) under methanogenic conditions. Immediately after RS addition, the RS-derived CH4 production rates were higher in soils (Uruguay, Fuyang) that possessed a stronger inherent CH4 production potential compared with other soils with lower inherent potentials (Changsha, the Philippines, Vercelli). However, soils with higher inherent potential did not necessarily produce higher amounts of CH4 from the RS applied, or vice versa. Quantitative PCR showed copy numbers of both bacteria and methanogens increased in straw-amended soils. High-throughput sequencing of 16S rRNA genes showed distinct bacterial communities among the unamended soil samples, which also changed differently in response to RS addition. Nevertheless, RS addition generally resulted in all the rice field soils in a relative increase of primary fermenters belonging to Anaerolineaceae and Ruminococcaceae. Meanwhile, RS addition also generally resulted in a relative increase of Methanosarcinaceae and/or Methanocellaceae. Our results suggest that after RS addition the total amounts of RS-derived CH4 are distinct in different rice field soils under methanogenic conditions. Meanwhile, there are potential core bacterial populations that are often involved in primary fermentation of RS under methanogenic conditions. [ABSTRACT FROM AUTHOR]