Methane production in aerated marshland and model soils: effects of microflora and soil texture

Under oxic conditions the importance of the indigenous microflora and the soil texture on methane production was investigated using marshland soils (clayey silt) and different textured model soils (clay, sand, gravel). Under an oxic partial pressure >2.5% O2, soil slurries had a low methane production rate over a period of 50–70 h with acetate (0.35±0.1 nmol g−1 h−1) as well as with hydrogen (2.43±0.51 nmol g−1 h−1) as substrate. The rates amounted to 1 and 5%, respectively, compared to methane production rates under anoxic conditions. As soon as the oxygen concentration decreased to 2.5% (microaerophilic conditions) the methane production increased significantly (9.5±3.3 nmol g−1 h−1 with acetate; 16.3±3.3 nmol g−1 h−1 with hydrogen). In the absence of the indigenous microflora, the inoculated culture of Methanosarcina mazei (DSM 2053) did not produce any methane in soil slurries under oxic conditions. To inhibit methane oxidation, all samples (oxic and anoxic) were supplied with 60 nl acetylene ml−1 headspace. Furthermore, methane production in different textured model soils demonstrated that a high amount of negative surface charges increased methane production under oxic as well as under anoxic conditions. Consequently the methane production rates increased in the following sequence: sand<gravel<clayey silt (marshland soil)≤clay. Our results show that the indigenous microflora in combination with the sorptive quality of soil particles (clay, silt, organic matter) enables methanogenic activity in the presence of oxygen, promoting microscale anoxia within the slurries.