Effects of nitrate, nitrite, NO and N2O on methanogenesis and other redox processes in anoxic rice field soil

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We studied the inhibitory mechanism of nitrate and its denitrification products (nitrite, NO, N2O) on the production of CH4 and the concentrations of reductants (H2, acetate, propionate, etc.) and oxidants (NO−3, NO−2, NO, N2O, Fe(III), SO2−4) in slurries of anoxic Italian rice soil. Addition of each of the N-compounds caused a complete but largely reversible inhibition of methanogenesis. Nitrate, nitrite and N2O significantly decreased the H2 partial pressure. With nitrate and N2O it decreased below the threshold of methanogens, thus not allowing exergonic production of methane (ΔG>0). Furthermore, significant production of the electron acceptors Fe(III) and/or sulfate was observed after addition of nitrate and N2O, probably due to the oxidation of reduced iron and sulfur species with nitrate and/or N2O as electron acceptors. Methanogenic activity did not resume until all electron acceptors were reduced and, as a consequence, H2 had reached the methanogenic threshold again. Thus competition for H2 with denitrifying bacteria, iron- and sulfate-reducing bacteria seemed to be one important factor for the inhibition of methanogenesis. Addition of rice straw to reduce competition for electron donors did not prevent inhibition of methanogenesis after addition of nitrate but decreased the inhibition period. Especially after addition of nitrite and NO, toxic effects may have been more important than competition. Although addition of nitrite or NO caused a decrease of the H2 concentration, exergonic methanogenesis from H2/CO2 was always possible (ΔG<0). Nevertheless, CH4 production was inhibited. Furthermore, acetate concentrations were generally sufficient for exergonic methanogenesis in all experiments, even so CH4 production was completely inhibited. Turnover times of [2-14C]acetate to 14CH4 were higher in soil slurry that had resumed methanogenesis after nitrate inhibition than in the untreated control indicating toxic effects on acetate-utilizing methanogens. However, since the contribution of 14CO2 reduction to CH4 was almost the same (26–29%) in soil slurry that had resumed methanogenesis after nitrate inhibition as in the untreated control, hydrogenotrophic methanogens must have been affected by toxic N-compounds to a similar extent as acetoclastic methanogens.