SPATIAL-DISTRIBUTION OF MICROBIAL METHANE PRODUCTION PATHWAYS IN TEMPERATE ZONE WETLAND SOILS - STABLE CARBON AND HYDROGEN ISOTOPE EVIDENCE

The identity and distribution of substrates that support CH4 productio n in wetlands is poorly known at present. Organic compounds are the pr imary methanogenic precursor at all depths within anoxic wetland soils ; however, the distribution of microbial processes by which these comp ounds are ultimately converted to CH4 is uncertain. Based on stable is otope measurements of CH4 and Sigma CO2 extracted from soil porewaters in two temperate zone wetlands, we present evidence that a systematic spatial distribution of microbial methanogenic pathways can exist in certain anoxic, organic-rich soils. CH4 production by the acetate ferm entation pathway is favored in the shallow subsurface, while methanoge nesis from the reduction of CO2 with H-2 becomes more predominant in o lder, less reactive peat at depth. This distribution can account for m any of the reported CH4 emission characteristics of wetlands, factors play an important role in controlling the short-term supply of labile substrates to fermentive methanogens in the shallow subsurface where t he most intense CH4 production occurs. Predominance of the CO2-reducti on pathway at depth may help to explain reports of CH4 with a semifoss il age in lower pear layers. Copyright (C) 1997 Elsevier Science Ltd.