MICROBIOLOGICAL ASPECTS OF METHANE EMISSION BY A RICEFIELD SOIL FROM THE CAMARGUE (FRANCE) .1. METHANOGENESIS AND RELATED MICROFLORA

The major trophic groups of methanogens and the methanogenic potential of a rice soil from Camargue (France) were studied under laboratory c onditions, with regard to the effects of soil desiccation and the addi tion of straw or algal material. The dynamics of cellulose and xylan f ermentation by the soil microflora were also established. Methanogens remained viable for several months in dry oxic soil, indicating that t heir density at the end of the crop cycle might not be markedly reduce d by the dry fallow that usually follows rice harvest. Methanogenic ac tivities were maximum two weeks after soil submersion. The native soil organic matter mineralized after submersion favoured acetotrophic met hanogens, while adding straw or algae favoured hydrogenotrophic methan ogens. Straw or algae addition increased populations of hydrogenotroph s and formatotrophs similarly, while four times more CH4 was produced with straw than with algae. Methanogenesis appeared to be more limited by the availability and nature of the substrate than by the density o f methanogens. Cellulose fermentation by the soil microflora exhibited a first stage, where hydrogenotrophs were mostly responsible for meth anogenesis through interspecies Hz transfer, and a second stage, where acetotrophs consumed accumulated acetate. Counts, microscopic observa tions, and strain isolations showed a diversified methanogenic microfl ora including rods, sarcinae, cocci and spirilla. Four mesophilic, neu trophilic strains were isolated and phenotypically characterized.