Fluxes and 13 C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought

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Peatlands contain a carbon stock of global concern and significantly contribute to the global methane burden. The impact of drought and rewetting on carbon cycling in peatland ecosystems is thus currently debated. We studied the impact of experimental drought and rewetting on intact monoliths from a temperate fen over a period of ∼300 days, using a permanently wet treatment and two treatments undergoing drought for 50 days. In one of the mesocosms, vegetation had been removed. Net production of CH 4 was calculated from mass balances in the peat and emission using static chamber measurements. Results were compared to 13 C isotope budgets of CO 2 and CH 4 and energy yields of acetoclastic and hydrogenotrophic methanogenesis. Drought retarded methane production after rewetting for days to weeks and promoted methanotrophic activity. Based on isotope and flux budgets, aerobic soil respiration contributed 32ĝ€"96% in the wet treatment and 86ĝ€"99% in the other treatments. Drying and rewetting did not shift methanogenic pathways according to δ 13 C ratios of CH 4 and CO 2 . Although δ 13 C ratios indicated a prevalence of hydrogenotrophic methanogenesis, free energies of this process were small and often positive on the horizon scale. This suggests that methane was produced very locally. Fresh plant-derived carbon input apparently supported respiration in the rhizosphere and sustained methanogenesis in the unsaturated zone, according to a 13 C-CO 2 labelling experiment. The study documents that drying and rewetting in a rich fen soil may have little effect on methanogenic pathways, but result in rapid shifts between methanogenesis and methanotrophy. Such shifts may be promoted by roots and soil heterogeneity, as hydrogenotrophic methanogenesis occurred locally even when conditions were not conducive for this process in the bulk peat.




Knorr, K. H., Glaser, B., & Blodau, C. (2008). Fluxes and 13 C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought. Biogeosciences, 5(5), 1457–1473. https://doi.org/10.5194/bg-5-1457-2008

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