Communities of methanogenic bacteria in paddy field soils with long-term application of organic matter

Methanogenic communities were investigated in paddy field soil that had received long-term (over 30 y) applications of organic matter. Methanogenic bacteria were enumerated by the most probable number (MPN) method and by the analysis of ether-linked lipids. Three fields, one with chemical fertilizer (CF), one with application of rice straw plus chemical fertilizer (RS), and one with application of rice straw compost plus chemical fertilizer (RSC), were examined. The numbers of H2-CO2, methanol, and acetate utilizers estimated by the MPN method was 103–104, 104–105 and 104–105 g−1 dry soil in the CF and RS fields, respectively, and 105–106, 105–106 and 105–106 g−1 dry soil in the RSC field, respectively. Long-term application of rice straw compost significantly increased methanogenic numbers. Methanogenic cell numbers estimated by lipid analysis was 107–108 g−1 dry soil in the CF, RS and RSC fields. Rod-, coccus-, and sarcina-shaped cells were observed in the MPN tubes. Of the core lipids, hydroxyarchaeol was more abundant than caldarchaeol, which suggests that methanogenic bacteria of the family Methanosarcinaceae are predominant in these paddy field soils.