The fine scale variability of dissolved methane in surface peat cores

Abstract

Peat forming wetlands are globally important sources of the greenhouse gas CH 4. The variability of flux recordings from peatlands is however considerable and the distribution of CH 4 below the water table poorly described. Surface peat (0–500 mm below the water table) is responsible for the bulk of emissions and a localised region of intense CH 4 concentration may exist within this region but the structure of peat and presence of gas bubbles make the determination of in situ gas distributions problematic. We report on the in situ distribution and concentrations of CH 4, CO 2 and O 2 in surface bog peat cores using Quadrupole Mass Spectrometry and relate this to peat physical structure. Replicate cores collected in spring and autumn from both hollows and hummocks are used ( n = 10). CH 4 recorded in almost every profile was localised in intense peaks reaching concentrations up to 350 μM at depths where O 2 was absent. Each CH 4 peak had a coincident CO 2 peak with a minimum mean ratio of ∼20:1 (CO 2:CH 4) and we found more CH 4 beneath hollows than hummocks. In statistical comparisons CH 4 concentration and distribution differed significantly between profiles for each depth. We demonstrate that variability found within a single core is at least as great as that between cores collected across the bog. The distribution of CH 4 was negatively correlated with bulk density and in some cases the location of roots matched those of intense CH 4 concentration where bubbles had formed and been trapped. Our comparisons suggest variability in gas distribution is caused by a heterogenous peat structure that controls the movement of gas bubbles and contains localised hotspots of gas production. The small and fine root systems of vascular plants on the peatland surface may cause high levels of methanogenic activity in their vicinity and also represent a physical barrier capable of trapping CH 4 bubbles.