Effect of methanogenic precursors (acetate, hydrogen, propionate) on the suppression of methane production by nitrate in anoxic rice field soil

Nitrate is known to suppress CH4 production in anoxic soil. Three hypothese s have been proposed to explain the mechanism: redox shift; competition for methanogenic substrates by denitrifying bacteria; or inhibition by toxic d enitrification intermediates (nitrite, NO, N2O). Since recent studies have shown that methanogens and methanogenesis are not necessarily prevented in being active under positive redox potential, only the latter two hypotheses remain valid. In order to test which of the remaining two hypotheses bette r explains this suppression, we studied the effect of added electron donors (acetate, H-2,H- propionate) on the suppression of CH4 production in rice soil. Addition of nitrate to a methanogenic rice soil slurry completely sup pressed CH4 production. The addition of acetate, H-2 or propionate in the a bsence of nitrate stimulated CH4 production. However, when they were added together with nitrate, CH4 production was also suppressed, although the com plete suppression of methanogenesis was shortened. Thus, addition of electr on donors could not prevent the inhibition of methanogenesis by nitrate. We conclude that the main mechanism involved in the suppression of CH4 produc tion by nitrate is the inhibition of methanogenesis by denitrification inte rmediates rather than the competition between denitrifiers and methanogens for substrates. The duration of the suppression by nitrate was closely rela ted to the ratio of electron donor to electron acceptor. The greater this r atio, the shorter the suppression, indicating that nitrate and its potentia lly toxic denitrification intermediates nitrite, NO and N2O were then faste r reduced to non-toxic N-2. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.