METHANE TRANSPORT AND OXIDATION IN THE UNSATURATED ZONE OF A SPHAGNUM PEATLAND

Efflux rates and oxidation rates of methane (CH4) 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 CH4 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 CH4 flux from deep, anaerobic methanogenic sediments to the unsaturated zone to be 3.5 x 10(-11) mol cm(-2) s(-1) (+/- 1.0 x 10(-10) mol cm(-2) s(-1)), while the CH4 flux from this unsaturated zone to the atmosphere was 3.7 x 10(-12) mol cm(-2) s(-1) ( 5.0 x 10-12 mol cm (-2) s(-1)). Therefore a large fraction of the CH4 flux was consumed before it reached the atmosphere. Most CH4 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, CH4 concentrations and oxidation rates were unevenly distributed, probably following patterns of upward transport of CH4 by bubbles via fissures and tubes in the saturated zone. Between the surface of the bog and 6-cm depth, CH4 concentrations were more uniformly distributed, most likely because of greater horizontal mixing in this depth range. Analysis of CH4 distributions in unsaturated peat is a straightforward and practical technique to measure both net CH4 efflux and CH4 oxidation with minimal disturbance to the peat structure and gas exchange conditions.