CH4 production, oxidation and emission in a U.K. ombrotrophic peat bog: Influence of SO42− from acid rain

Factors influencing the rates of production and emission of CH4, CH4 oxidation and rates of SO42− reduction, were measured in the peat of an ombrotrophic bog in New Galloway, Scotland. Vertical concentration profiles of CH4 and O2 showed that the water table essentially represented the oxic-anoxic boundary in the peat. This boundary was usually at the surface in the case of peat-bog hollows, but up to 20 cm of oxic peat occurred above the water table in peat-bog hummocks. Penetration of O2 into the peat increased under illumination when photosynthesis was active, but decreased in the dark. Emission of CH4 from the peat surface was faster from peat-bog hollows than from hummocks, where most CH4 was reoxidized before emission. CH4 emission rates also varied seasonally, being greatest during summer. For most of the year the amount of organic C oxidized to CO2 by SO42− reduction by anaerobic bacteria exceeded that being transformed to CH4 by methanogenic bacteria, except during summer when SO42− reduction became SO42− limited. Laboratory experiments showed that the addition of SO42− to peat inhibited CH4 formation, confirming that there was competitive inhibition of CH4 formation by active SO42− reduction, as demonstrated in other environments. The degree of acid rain deposition of SO42− onto peat bogs may therefore be extremely important in regulating the production and emission of CH4 from peat. CH4 formation was most active in the strata of peat 5–15 cm below the water table, although actual rates of CH4 formation were slower in the peat beneath hummocks than that below hollows. In contrast, CH4 oxidation occurred nearer the peat surface (only 3–7 cm below the water table) where the methanotrophic bacteria could intercept vertically migrating CH4. Surprisingly, the peak for CH4 oxidation potential occurred at about 5 cm below the water table, in peat which was apparently anoxic. This may reflect either a transiently oxic peat environment, in which aerobic CH4-oxidizing bacteria persisted, or the presence of a community of facultatively anaerobic CH4-oxidizing bacteria which, in anoxic conditions, metabolized substrates other than CH4. There was no evidence of anaerobic CH4 oxidation.