Effect of temperature on reduction of iron and production of carbon dioxide and methane in anoxic wetland rice soils

Wetland rice soils from Italy (Pavia) and the Philippines (Bugallon, Luisia na, Maligaya) were incubated under anoxic conditions at 31 different temper atures ranging from 4.7 degreesC to 49.5 degreesC. Production of CO2 was mo st intensive at the beginning of the incubation (0-4 days) and was predomin antly coupled to the reduction of free Fe(III). The optimum temperature for these processes was between 32 degreesC and 41 degreesC. After 9-16 days, CO2 production rates had decreased and the available Fe(III) had been compl etely reduced at the optimum temperatures. However, Fe(III) was still avail able at temperatures below and above the optimum. Maximum CH4 production ra tes were observed after 4-16 days (except in soil from Maligaya) with tempe rature optima between 32 degreesC and 41 degreesC, similar to those for CO2 production and Fe reduction. Since ongoing Fe reduction is known to suppre ss CH4 production, the temperature range of optimum CH4 production was rest ricted to those temperatures at which Fe(III) had already been depleted. Ne vertheless, the temperature characteristics of both CO2 and CH4 production often exhibited two temperature optima at some time during the incubation, suggesting a complex pattern of adaptation of the methanogenic microbial co mmunity to temperature. When available Fe(III) was completely depleted by a noxic pre-incubation at 30 degreesC, CH4 was produced at a constant rate (s teady state conditions) which increased with increasing temperature. Steady state CH4 production reached a first maximum at about 40 degreesC, but inc reased further up to at least 50 degreesC, suggesting the presence of therm ophilic microorganisms whose activity was apparently masked when Fe had not been completely reduced. The apparent activation energy of CH4 production at steady state ranged between 48 kJ mol(-1) and 65 kJ mol(-1).