Methane oxidation: isotopic enrichment factors in freshwater boreal reservoirs

Methane oxidation plays a vital role in controlling the flux of CH4 from many ecosystems. Release of the green house gas CH4 to the atmosphere during creation and operation of hydroelectric reservoirs is of concern because of the dramatic changes in C and nutrient cycling that result from flooding. Experimentally flooded reservoirs in the boreal forest at the Experimental Lakes Area, northwestern Ontario, Canada, have been under study for a decade. In these large-scale ecosystem experiments, stable C isotopic ratios are used to determine the importance of CH4 oxidation but quantification requires knowledge of the C isotope enrichment factor associated with CH4 oxidation under the appropriate environmental conditions. Laboratory incubations were used to assess the CH4 oxidation enrichment factors in 3 experimental boreal reservoirs with different soil and vegetation, and flood histories. As a result of flooding, new flooded surfaces were created with different temperature and hydrologic regimes and the importance of CH4 oxidation in controlling the flux of CH4 to the atmosphere changed significantly. However, isotopic ratio data from different systems could not be compared directly because the enrichment factor changed between systems. The enrichment factor in a flooded boreal wetland ecosystem (ELARP) decreased with temperature and the rate of CH4 oxidation increased with temperature. This was in contrast with two flooded upland boreal forest reservoirs (Flooded Upland Dynamics Experiment) where the enrichment factor was smaller than in ELARP and there was little or no temperature effect on the enrichment factors or rates of CH4 oxidation.