UC Irvine

Boreal peatlands, a major source of atmospheric CH4, are characterized by a rapidly fluctuating water table position and meter-scale variations in relief. Regional and ecosystem-based studies show that water table position generally controls CH4 emission from boreal peatlands by influencing the relative extent of the zones of CH4 oxidation and production within the peat profile. We used a combined field and laboratory study to assess the influence of local hydrology on the short-term dynamics of CH4 production, oxidation, and emission from sites in an Alaskan boreal peatland that were characterized by temporarily (site LB1A) and permanently (LB2) water-saturated subsurface peat during the thaw season. The two sites contrasted sharply with respect to the dynamics of CH4 cycling. Site LB1A, which showed low CH4 concentrations in pore water (<2 μM) and unsaturated peat (<2.6 nM), consumed both atmospheric CH4 and CH4 diffusing upward from the saturated zone for a net flux of -0.9 mg CH4 m-2 d-1. In contrast, LB2 had pore water CH4 concentrations, 300 μM and emitted CH4 at 69 mg m-2 d-1. Roughly 55% of the CH4 diffusing upward from the saturated zone at LB2 was oxidized in transit to the peat surface. Methane oxidation potentials (V(ox)) were maximum in the 10-cm zone immediately above the local water table at both sites but were greater on a dry mass (dw) basis at LB2 (498-650 ng CH4 g(dw)-1 h-1) than at LB1A (220-233 ng CH4 g(dw)-1 h-1). Methane production potentials (V(p)) were low (<2 ng CH4 g(dw)-1 h-1) at LB1A, but the maximum at LB2 (139 ng CH4 g(dw)-1 h-1) was spatially coupled with the maximum V(ox). Methanogens exposed to O2 produced no CH4 in a subsequent 48 h anoxic incubation, whereas methanotrophs incubated anoxically oxidized CH4 vigorously within 20 h of return to an oxic environment, indicating that the former are more sensitive than the latter to adverse O2 conditions. Experiments with 14CH4 showed that ˜71% of assimilated 14CH4 was respired as 14CO2. Respiration by methanotrophs contributes at most ˜1.1-1.7% (molar basis) of gross ecosystem respiration (15.6-17.9 mg CO2 m-2 d-1) at these sites.