The addition of complex substrates (yeast extract, proteins, and broth) to frozen soil had mostly negative effect on DF, while volatile compounds and gases (ethanol, methanol, CH4 but not H2) stimulated DF as compared with unamended controls with added deionized water. The reason for inhibition by yeast extract and proteins is obscure. The regulatory repression of anaplerotic enzymes by complex substrates seems unlikely, as indicated by experiments on 14CO2 fixation by pure bacterial cultures grown on various substrates (Hesselsoe et al. 2005). On the other hand, stimulation of DF by gases and volatile compounds is in full agreement with our finding that frozen soils allow diffusion of these compounds into internal space. Probably, microbial species able to utilize the mobile C-compounds have a selective advantage in permafrost and seasonally frozen soils.
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