Peatland types influence the inhibitory effects of a humic substance analog on methane production

- AQDS inhibited fermentative production in bog soil, but not in fen soil.
- CH4 production in bog and fen soils were inhibited by AQDS amendment.
- In bog soils AQDS acted as a toxic compound.
- AQDS served as a competitive terminal electron acceptor in fen soils.
- AQDS caused complicated temporal dynamics in temperature sensitivity of methanogenesis.

Methane (CH4) production is often impeded in many northern peatland soils, even though more thermodynamically favorable inorganic terminal electron acceptors (TEAs) used in anaerobic respiration are often present in low concentrations in these soils. Recent studies suggest that humic substances in wetland soils can be utilized as organic TEAs for anaerobic respiration and may directly inhibit CH4 production. Here we utilize the humic analog anthraquinone-2, 6-disulfonate (AQDS) to explore the importance of humic substances, and their effects on the temperature sensitivity of anaerobic decomposition, in two peatland soils. In a bog peat, AQDS was not initially utilized as a TEA, but greatly inhibited the fermentative production of acetate by > 98%, carbon dioxide by > 49%, hydrogen by > 90%, and CH4 production by 86%. When added together with glucose, < 47% of added AQDS was reduced after a lag period of 5 to 10 days. In contrast, no inhibitory effect of AQDS on fermentation was found in a fen peat, and AQDS was readily reduced. The addition of glucose and AQDS to both bog and fen peats caused complicated temporal dynamics in the temperature sensitivity of CH4 production, reflecting temporal changes in the temperature responses of other decomposition processes with effects on methanogenesis. Our results show that the inhibitory effects of a humic analog on CH4 production depend on peatland type, acting primarily as a toxic compound in bog soils and as a competitive TEA in fen soils. The results also suggest that the high concentrations of humic substances in northern peatlands may impact the temperature sensitivity of soil carbon cycling in these ecosystems.