Sequential reduction processes and initiation of CH 4 production upon flooding of oxic upland soils

Abstract

Sequential reduction processes were studied in four oxic upland soils (cultivated, forest, savanna and desert soil) which were slurried and incubated under anoxic conditions. NO 3 −1 reduction began almost immediately and was followed by reduction of manganese(IV), sulfate and iron(III). The phases of reduction of Mn 4+, SO 4 2− and Fe 3+ overlapped, with SO 4 2− being depleted long before accumulation of Mn 2+ and Fe 2+ was finished. CH 4 production and growth of methanogenic bacteria began when all the other reduction processes were finished. Radiotracer experiments showed that CH 4 was produced from H 2 (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 H 2 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 CH 4 at Gibbs free energies of ΔG < −30 kJ mol −1 CH 4. However, exergonic production of acetate from H 2 CO 2 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.