Methane transport and oxidation in the unsaturated zone of a Sphagnum peatland

Efflux rates and oxidation rates of methane (CH 4 ) were measured in a northern Sphagnum bog, Thoreau's Bog in Concord, Massachusetts, by using a gradient methodology which does not change in situ conditions. A remote‐sampling technique was devised to obtain undisturbed CH 4 profiles as a function of depth in the peat; effective diffusion coefficients in peat were estimated both physically and by using propane as a tracer. By combining these techniques we estimated the average late summer CH 4 4 flux from deep, anaerobic methanogenic sediments to the unsaturated zone to be 3.5 × 10 −11 mol cm −2 s −1 (± 1.0 × 10 −10 mol cm −2 s −1 ), while the CH 4 flux from this unsaturated zone to the atmosphere was 3.7 × 10 −12 mol cm −2 s −1 (± 5.0 × 10 −12 mol cm −2 s −1 ). Therefore a large fraction of the CH 4 flux was consumed before it reached the atmosphere. Most CH 4 consumption, presumably by oxidation, occurred between the water table, located 12 to 15 cm below the bog surface, and about 6 cm below the bog surface. In this region, CH 4 concentrations and oxidation rates were unevenly distributed, probably following patterns of upward transport of CH 4 by bubbles via fissures and tubes in the saturated zone. Between the surface of the bog and 6‐cm depth, CH 4 concentrations were more uniformly distributed, most likely because of greater horizontal mixing in this depth range. Analysis of CH 4 distributions in unsaturated peat is a straightforward and practical technique to measure both net CH 4 efflux and CH 4 oxidation with minimal disturbance to the peat structure and gas exchange conditions.