Methane and methanogen community dynamics across a boreal peatland nutrient gradient

Field and lab-based methane (CH4) fluxes and methanogen community structure were characterized across three peatlands in central Ontario (Canada) representing a successional and nutrient gradient from rich to poor fens. Air temperature was a strong and significant predictor of both CH4 and carbon dioxide (CO2) fluxes among the three sites. Net CH4 efflux and in vitro CH4 production potential were significantly greater in the rich and intermediate than the poor fen site. Although the poor fen site had the lowest water table position, this was not a significant predictor of CH4 emissions and in general the 3 sites were relatively wet compared to many northern peatlands. Consistently, during spring and fall, ethanol stimulated in vitro CH4 production potential from the poor fen, but not the rich and intermediate sites, indicating substrate limitation for CH4 production in the poor fen. Lower rates of CH4 production and emissions in the poor fen site were consistent with our hypotheses based on poorer substrate quality and a lack of sedges in that peatland type. However phylogeny of dominant methanogens inferred from terminal restriction fragment length polymorphism (T-RFLP) analyses of 16S rDNA illustrated inconsistencies with previous reports of methanogens in northern peatlands. For example members likely of the family Methanosaetaceae (obligate high-affinity acetate fermenters) comprised a substantial portion of total methanogen population in the poor fen. In contrast, members of the order Methanomicrobiales (obligate CO2 reducers) were important methanogens in the rich and intermediate fens and not detected in the poor fen. Methanogen community structure based on T-RFLP across the 3 sites was distinct during spring, while during fall methanogen communities in the poor fen samples were still somewhat distinct from those in the rich and intermediate fens. Methanogen diversity (community richness and evenness) was not correlated with rates of CH4 production in the spring when soil respiration, and presumably rhizosphere activity, was slow. However, diversity was a significant predictor of CH4 production in the early fall (when both production and emissions rates were higher), indicating that methanogen diversity can potentially play a role in biogeochemical cycling and greenhouse gas emissions in northern peatlands.

► We characterized CH4 dynamics and methanogen communities across rich, poor, and intermediate fens.
► CH4 production and emissions were slowest in the poor fen in part due to substrate limitation.
► Purported obligate acetoclastic methanogens were important in the poor fen in contrast to other studies.
► Across sites, methanogen diversity correlated positively with CH4 production in fall, but not spring.