Sequential reduction processes and initiation of CH4 production upon flooding of oxic upland soils

Sequential reduction processes were studied in four oxic upland soils (cultivated, forest, savanna and desert soil) which were slurried and incubated under anoxic conditions. NO3−1 reduction began almost immediately and was followed by reduction of manganese(IV), sulfate and iron(III). The phases of reduction of Mn4+, SO42− and Fe3+ overlapped, with SO42− being depleted long before accumulation of Mn2+ and Fe2+ was finished. CH4 production and growth of methanogenic bacteria began when all the other reduction processes were finished. Radiotracer experiments showed that CH4 was produced from H2 (29–42%) and acetate. The respiratory index indicated that the acetate was predominantly degraded by methanogenic bacteria. The late onset of methanogenesis was not a consequence of limitation by the methanogenogenic precursors, since H2 and acetate were present long before the initiation of methanogenesis. Thermodynamic calculations showed that the concentrations of these substrates were always sufficient to allow exergonic production of CH4 at Gibbs free energies of ΔG < −30 kJ mol−1 CH4. However, exergonic production of acetate from H2CO2 was not possible. Propionate was also detected in the soil slurries. The redox potential in the soils decreased from > + 400 mV to final values of < − 150 mV, except in the forest soil where the redox potential stayed at + 50 mV. The onset of methanogenesis and of growth of methanogenic bacteria coincided with redox potentials between +70 and 0 mV, which is much higher than claimed in literature. We speculate that the redox-active substances in soil were the signal for methanogenic bacteria to initiate activity.