Fluxes and sup13/supC isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought

摘要

Peatlands contain a carbon stock of global concern and significantlycontribute to the global methane burden. The impact of drought and rewettingon carbon cycling in peatland ecosystems is thus currently debated. Westudied the impact of experimental drought and rewetting on intact monolithsfrom a temperate fen over a period of ~300 days, using a permanentlywet treatment and two treatments undergoing drought for 50 days. In one ofthe mesocosms, vegetation had been removed. Net production of CH₄ wascalculated from mass balances in the peat and emission using static chambermeasurements. Results were compared to 13C isotope budgets of CO₂and CH₄ and energy yields of acetoclastic and hydrogenotrophicmethanogenesis. Drought retarded methane production after rewetting for daysto weeks and promoted methanotrophic activity. Based on isotope and fluxbudgets, aerobic soil respiration contributed 32–96% in the wettreatment and 86–99% in the other treatments. Drying and rewettingdid not shift methanogenic pathways according to δ13C ratios ofCH₄ and CO₂. Although δ13C ratios indicated aprevalence of hydrogenotrophic methanogenesis, free energies of this processwere small and often positive on the horizon scale. This suggests thatmethane was produced very locally. Fresh plant-derived carbon inputapparently supported respiration in the rhizosphere and sustainedmethanogenesis in the unsaturated zone, according to a 13C-CO₂labelling experiment. The study documents that drying and rewetting in arich fen soil may have little effect on methanogenic pathways, but result inrapid shifts between methanogenesis and methanotrophy. Such shifts may bepromoted by roots and soil heterogeneity, as hydrogenotrophic methanogenesisoccurred locally even when conditions were not conducive for this process inthe bulk peat.