Does dissolved organic matter or solid peat fuel anaerobic respiration in peatlands?

Solid-phase soil organic matter is the largest carbon (C) pool in peatlands and, as such, has long been assumed to be the primary substrate driving anaerobic respiration in these systems. However, radiocarbon data from previous field samples suggest that dissolved organic matter (DOM) plays a key, and often dominant, role in fueling heterotrophic respiration across a variety of peatlands. In this study, we manipulated available C sources under laboratory conditions to empirically determine the primary C source - solid-phase peat or DOM - fueling anaerobic respiration at surface and deep depth increments within two bogs and a poor fen in northern Minnesota. We found that increasing DOM concentration from 0 to 50% during anaerobic incubations significantly increased rates of surface (25-50 cm) methane (CH4) production, but not carbon dioxide (CO2) production, indicating that DOM acts as a primary driver of surface methanogenesis in peatlands. Contrary to our expectations, this response was consistent across all three sites despite differences in plant communities and biogeochemical characteristics. However, we observed no effect of DOM availability on CH4 or CO2 production at deeper depths (75-100 or 150-200 cm). The lack of response of CO2 production to DOM manipulation highlights the sensitivity of surface CH4 production to changes in the DOM pool quality and quantity, which are likely to occur under future climate change scenarios. However, CH4 production in deeper peat appears to be limited by additional factors beyond labile C availability, at least on the timescale of the laboratory incubations.