Spatial distribution of microbial methane production pathways in temperate zone wetland soils: Stable carbon and hydrogen isotope evidence

• Published in: Geochimica et cosmochimica acta Vol. 61; no. 4; pp. 745 - 753
• Main Authors: Hornibrook, Edward R.C., Longstaffe, Frederick J., Fyfe, William S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.1997
• Subjects: Freshwater
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(96)00368-7

The identity and distribution of substrates that support CH 4 production in wetlands is poorly known at present. Organic compounds are the primary methanogenic precursor at all depths within anoxic wetland soils; however, the distribution of microbial processes by which these compounds are ultimately converted to CH 4 is uncertain. Based on stable isotope measurements of CH 4 and ΣCO 2 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. CH 4 production by the acetate fermentation pathway is favored in the shallow subsurface, while methanogenesis from the reduction of CO 2 with H 2 becomes more predominant in older, less reactive peat at depth. This distribution can account for many of the reported CH 4 emission characteristics of wetlands, in particular, their sensitivity to changes in primary productivity, temperature, and hydrology. These factors play an important role in controlling the short-term supply of labile substrates to fermentive methanogens in the shallow subsurface where the most intense CH 4 production occurs. Predominance of the CO 2-reduction pathway at depth may help to explain reports of CH 4 with a semifossil age in lower peat layers.