CH₄ and CO₂ production below two contrasting peatland micro-relief forms: An inhibitor and δ¹³C study

Two peatland micro-relief forms (microforms) - hummocks and hollows - differ by their hydrological characteristics (water table level, i.e. oxic-anoxic conditions) and vegetation communities. We studied the CH₄ and CO₂ production potential and the localization of methanogenic pathways in both hummocks and hollows at depths of 15, 50, 100, 150 and 200cm in a laboratory incubation experiment. For this purpose, we measured CH₄ and CO₂ production rates, peat elemental composition, as well as δ¹³C values of gases and solids; the specific inhibitor of methanogenesis BES (2-bromo-ethane sulfonate, 1mM) was aimed to preferentially block the acetoclastic pathway. The cumulative CH₄ production of all depths was almost one fold higher in hollows than in hummocks, with no differences in CO₂. With depth, CO₂ and CH₄ production decreased, and the relative contribution of the hydrogenotrophic pathway of methanogenesis increased. The highest methanogenic activity among all depths and both microforms was measured at 15cm of hollows (91%) at which the highest relative contribution of acetoclastic vs. hydrogenotrophic pathway (92 and 8%, respectively) was detected. For hummocks, the CH₄ production was the highest at 50cm (82%), where relative contribution of acetoclastic methanogenesis comprised 89%. The addition of 1mM BES was not selective and inhibited both methanogenic pathways in the soil. Thus, BES was less efficient in partitioning the pathways compared with the δ¹³C signature. We conclude that the peat microforms - dry hummocks and wet hollows - play an important role for CH₄ but not for CO₂ production when the effects of living vegetation are excluded.