Can Sphagnum leachate chemistry explain differences in anaerobic decomposition in peatlands?

• Sphagnum-derived dissolved organic matter increased anaerobic CO2 production.
• The response of CH4 to Sphagnum-derived dissolved organic matter was complex.
• Changes in CH4 production were driven by shifts in acetoclastic methanogenesis.
• Sphagnum-derived dissolved organic matter can regulate CO2 and CH4 in peatlands.

Peatlands are important ecosystems in the global carbon cycle, serving as both the largest terrestrial soil carbon pool and a significant source of the greenhouse gas methane (CH4). In Sphagnum moss-dominated wetlands, anaerobic decomposition, and in particular the production of CH4, is highly variable and controlling factors are poorly understood. The main objective of this study was to determine if leachates of Sphagnum can explain differences in anaerobic decomposition and CH4 production from three Sphagnum-dominated peatlands.Soils from each peatland were incubated anaerobically for 40 days with Sphagnum-derived organic matter (S-DOM) extracted using distilled water at 25 or 60 °C in a fully-crossed experimental design. S-DOM extracted at 25 °C had a minimal effect on decomposition, but S-DOM extracted at 60 °C increased CO2 production in all soils. The magnitude of the increased CO2 production in response to S-DOM depended on the source site of the S-DOM. The response of CH4 production to additions of S-DOM extracted at 60 °C was more complex. Soils from one peatland produced no CH4 during the incubation, regardless of S-DOM source. The same S-DOM additions led to an increase in CH4 production in a second soil, but a decrease in CH4 production in the third soil. Stable isotopic evidence suggests that these patterns were driven by the selective inhibition or stimulation of acetoclastic methanogenesis. Taken together, these data suggest S-DOM alone does not explain differences in anaerobic decomposition in peatlands, but may play a role in regulating CO2 and CH4 production.