Methane transport and oxidation in the unsaturated zone of aSphagnumpeatland

Abstract

Efflux rates and oxidation rates of methane (CH4) were measured in a northernSphagnumbog, 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 CH4profiles 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 CH44 flux from deep, anaerobic methanogenic sediments to the unsaturated zone to be 3.5 × 10−11mol cm−2s−1(± 1.0 × 10−10mol cm−2s−1), while the CH4flux from this unsaturated zone to the atmosphere was 3.7 × 10−12mol cm−2s−1(± 5.0 × 10−12mol cm−2s−1). Therefore a large fraction of the CH4flux was consumed before it reached the atmosphere. Most CH4consumption, 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, CH4concentrations and oxidation rates were unevenly distributed, probably following patterns of upward transport of CH4by bubbles via fissures and tubes in the saturated zone. Between the surface of the bog and 6‐cm depth, CH4concentrations were more uniformly distributed, most likely because of greater horizontal mixing in this depth range. Analysis of CH4distributions in unsaturated peat is a straightforward and practical technique to measure both net CH4efflux and CH4oxidation with minimal disturbance to the peat structure and gas exchange conditions.