Methane flux from drained northern peatlands: Effect of a persistent water table lowering on flux

Measurements of CH 4 flux from drained and undrained sites in three northern Ontario peatlands (a treed fen, a forested bog, and a treed bog) were made from the beginning of May to the end of October 1991. In the drained portions, the water table had been lowered between 0.1 and 0.5 m, compared to the water table of the undrained portion of the peatlands. The mean seasonal CH 4 flux from the undrained portions of three peatlands was small, ranging from 0 to 8 mg m −2 d −1 , but similar to the CH 4 flux from other treed and forested northern peatlands. The mean seasonal CH 4 flux from the drained portion of the peatlands was either near zero or slightly negative (i.e.,uptake): fluxes ranged from 0.1 to −0.4 mg m −2 d −1 . Profiles of CH 4 in the air‐filled pores in the unsaturated zone, and the water‐filled pores of the saturated zone of the peat at the undrained sites, showed that all the CH 4 produced at depth was consumed within 0.2 m of the water table and that atmospheric CH 4 was consumed in the upper 0.15 m of the peatland. On the basis of laboratory incubations of peat slurries to determine CH 4 production and consumption potentials, the lowering of the water table eliminated the near‐surface zone of CH 4 production that existed in the undrained peatland. However, drainage did not alter significantly the potential for CH 4 oxidation between the water table and peatland surface but increased the thickness of the layer over which CH 4 oxidation could take place. These changes occurred with a drop in the mean summer water table of only 0.1 m (from −0.2 to −0.3 m) suggesting that only a small negative change in soil moisture would be required to significantly reduce CH 4 flux from northern peatlands.