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

The impact of drought and rewetting on carbon cycling in peatland ecosystems is 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 CH₄ was calculated from mass balances in the peat and emission using static chamber measurements and results compared to ¹³C isotope budgets of CO₂ and CH₄ 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 32?96% in the wet and 86?99% in the other treatments. Drying and rewetting did not shift methanogenic pathways according to ? ¹³C ratios of CH₄ and CO₂. Although ?¹³C ratios indicated a prevalence of hydrogenotrophic methanogenesis, free energies of this process were small and often positive on the horizon scale, suggesting 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 ¹³C-CO₂ 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.