Agitation of anoxic paddy soil slurries affects the performance of the methanogenic microbial community

Rates of CH4 production in slurries of anoxic Italian paddy soils were higher when incubated without agitation than with shaking or stirring. Stirring resulted in a drastically reduced transformation of [2-14C]acetate to 14CH4 and increased the relative contribution of CH4 production from H14CO3− to total methanogenesis. Numbers of acetotrophic methanogens were low (103 g−1 dry soil) in stirred slurries. An anoxic suspension of sterile sand which was amended with Methanosarcina barkeri and acetate produced only CH4 if it was not stirred. In stirred anoxic paddy soil, acetate accumulated to very high concentrations (<10 mM). Propionate, butyrate and/or isopropanol also increased in stirred slurries. Hydrogen partial pressures, on the other hand, reached in all treatments a similar value of about 3–5 Pa. However, H2 production was apparently inhibited by stirring, since H2 accumulated only if slurries in which methanogenesis was inhibited by chloroform were not stirred. Our results indicate that measurements of metabolic rates in anoxic paddy soil are better conducted in non-agitated incubations to avoid the potential destruction of acetotrophic methanogens, syntrophic microbial associations and other microorganisms that are sensitive to mechanical forces.