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- reduction began almost immedia tely 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 fi nished. CH4 production and growth of methanogenic bacteria began when all the other reduction processes were finished. Radiotracer experimen ts showed that CHI was produced from H-2 (29-42%) and acetate. The res piratory index indicated that the acetate was predominantly degraded b y methanogenic bacteria. The !ate onset of methanogenesis was not a co nsequence 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 su bstrates were always sufficient to allow exergonic production of CH4 a t Gibbs free energies of Delta G < - 30 kJ mol(-1) CH4. However, exerg onic production of acetate from H-2/CO2 was not possible. Propionate w as also detected in the soil slurries. The redox potential in the soil s decreased from > + 400 mV to final values of < - 150 mV, except in t he forest soil where the redox potential stayed at + 50 mV. The onset of methanogenesis and of growth of methanogenic bacteria coincided wit h redox potentials between + 70 and 0 mV, which is much higher than cl aimed in the literature. We speculate that the redox-active substances in soil were the signal for methanogenic bacteria to initiate activit y.