Temporal change of C-13-isotope signatures and methanogenic pathways in rice field soil incubated anoxically at different temperatures

Production of CH4 and CO2 was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37degreesC. The delta(13)C of the accumulated CO2, CH4 and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH4 production. The calculations were further constrained by the determination of (CH4)-C-14 production from (CO2)-C-14 at steady state. At 50degreesC, CH4 was exclusively produced from CO2, indicating a fractionation factor of alpha(CO2/CH4) = 1.073. Between 10 and 37degreesC, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO2-derived fraction of CH4 production in the beginning was followed by a phase in which contribution of CO2-derived CH4 decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of C-13 during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation. Copyright (C) 2004 Elsevier Ltd.