Small differences in ombrotrophy control regional-scale variation in methane cycling among Sphagnum-dominated peatlands

Although methane (CH4) dynamics are known to differ at broad scales among peatland types and with climate, there is limited understanding of the variability associated with anaerobic carbon (C) cycling, and, the mechanisms that control that variability, among low pH, Sphagnum moss-dominated peatlands within a geographical region with similar climate. This is important because upscaling of CH4 emissions to regional and global scales often considers peatlands as a single, or at most two, ecosystem type(s). Here, we report the results from two studies exploring the controls of CH4 cycling in peatlands from the Upper Midwest (USA). Potential CH4 production and resultant CO2:CH4 ratios varied by several orders-of-magnitude among these soils. These differences were only partially explained by pH and fiber content (a measure of degree of decomposition in peat), suggesting other, more complicated controls may drive CH4 cycling in ombrotrophic peat soils. Based in part on the results from this survey, we more intensively examined CH4 dynamics in three bog-like, acidic, Sphagnum-dominated peatlands in northern Minnesota that differed in their degree of ombrotrophy. Net CH4 flux was lowest in the peatland with well-developed hummocks, and the isotopic composition of the CH4 along with methanotroph gene expression indicated a strong role for CH4 oxidation in controlling net CH4 flux. There were limited differences in porewater chemistry (CH4 and dissolved inorganic C concentrations) or microbial community composition among sites, and potential CH4 production was also similar among the sites. Taken together, these experiments demonstrate that high variation in CH4 cycling in seemingly similar peatlands within a single geographical region is common. We suggest a one peatland represents all approach is inappropriate-even among Sphagnum-dominated peatlands-and caution must be used when extrapolating data from a single site to the landscape scale, even for outwardly very similar peatlands. Instead, the macroscale development of peatlands, and concomitantly their microtopography as expressed in the proportion of hummocks, hollows, lawns and pools, need to be considered as central controls over CH4 emissions.