CH 4 production, oxidation and emission in a U.K. ombrotrophic peat bog: Influence of SO 42− from acid rain

Abstract

Factors influencing the rates of production and emission of CH 4, CH 4 oxidation and rates of SO 4 2− reduction, were measured in the peat of an ombrotrophic bog in New Galloway, Scotland. Vertical concentration profiles of CH 4 and O 2 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 O 2 into the peat increased under illumination when photosynthesis was active, but decreased in the dark. Emission of CH 4 from the peat surface was faster from peat-bog hollows than from hummocks, where most CH 4 was reoxidized before emission. CH 4 emission rates also varied seasonally, being greatest during summer. For most of the year the amount of organic C oxidized to CO 2 by SO 4 2− reduction by anaerobic bacteria exceeded that being transformed to CH 4 by methanogenic bacteria, except during summer when SO 4 2− reduction became SO 4 2− limited. Laboratory experiments showed that the addition of SO 4 2− to peat inhibited CH 4 formation, confirming that there was competitive inhibition of CH 4 formation by active SO 4 2− reduction, as demonstrated in other environments. The degree of acid rain deposition of SO 4 2− onto peat bogs may therefore be extremely important in regulating the production and emission of CH 4 from peat. CH 4 formation was most active in the strata of peat 5–15 cm below the water table, although actual rates of CH 4 formation were slower in the peat beneath hummocks than that below hollows. In contrast, CH 4 oxidation occurred nearer the peat surface (only 3–7 cm below the water table) where the methanotrophic bacteria could intercept vertically migrating CH 4. Surprisingly, the peak for CH 4 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 CH 4-oxidizing bacteria persisted, or the presence of a community of facultatively anaerobic CH 4-oxidizing bacteria which, in anoxic conditions, metabolized substrates other than CH 4. There was no evidence of anaerobic CH 4 oxidation.