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-C-14 ]acetate to (CH4)-C-14 and increased the relative contribution of CH4 production from (HCO3-)-C-14 to total methanogenesis. Numbers of aceto trophic methanogens were low (10(3) g(-1) dry soil) in stirred slurrie s. An anoxic suspension of sterile sand which was amended with Methano sarcina barkeri and acetate produced only CH4 if it was not stirred. I n stirred anoxic paddy soil, acetate accumulated to very high concentr ations (<10 mM). Propionate, butyrate and/or isopropanol also increase d in stirred slurries. Hydrogen partial pressures, on the other hand, reached in all treatments a similar value of about 3-5 Pa. However, H- 2 production was apparently inhibited by stirring, since H-2 accumulat ed only if slurries in which methanogenesis was inhibited by chlorofor m were not stirred. Our results indicate that measurements of metaboli c rates in anoxic paddy soil are better conducted in non-agitated incu bations to avoid the potential destruction of acetotrophic methanogens , syntrophic microbial associations and other microorganisms that are sensitive to mechanical forces.