Methane Emission from Dam-Lake and Methane Uptake by Forest Soil Surrounding the Lake

Abstract

Methane is one of the greenhouse gasses, having a global warming effect approximately 10 times stronger than CO_2. It is important to identify sources of methane emission and sinks of methane consumption, and to understand their processes. We measured the annual methane emission from a dam-lake and methane consumption by forest soil surrounding the dam-lake, at Morai, Hokkaido, Japan. Methane was continuously emitted from the surface of dam-lake. The methane emission rate increased with the increase of air temperature, from 0.022 mg C m^<-2> h^<-1> in spring to 0.431 mg C m-2 h-1 in fall, followed by a flash of 0.922 mg C m^<-2> h^<-1> in the ice-melting season. The emission rate was significantly correlated with the dissolved methane concentration in the surface water of the dam-lake (r=0.98, n=11). The annual amount of methane emitted from the surface of the dam-lake was estimated as 0.4 Mg C. Methane emission associated with water-discharge from the dam was estimated as 0.43 Mg C by calculation from the data of amount of water discharged from the dam-lake and the difference in concentration between the lake-water and discharged water. Consequently, the total amount of methane emission was 0.83 Mg C, in which 75% occurred during the period of waterdischarge from the dam. Forest soil surrounding the dam-lake always took up methane, even in the snow-covered season. The methane uptake rate increased as soil temperature increased, and there was a decrease of methane concentration in the surface soil, ranging from 0 to 0.1 mg C m-2 h-L Methane uptake during the snow-covered season accounted for 18% of the annual uptake. Taking the watershed area (2050 ha) and dam area (63 ha) into consideration, methane uptake by forest soil surrounding the dam-lake was 10 times larger than the methane emission from the dam-lake. However, the methane uptake rate was about 4 times smaller than the methane emission rate. Therefore, the methane emitted from the dam-lake was not directly consumed by the forest soil surrounding the dam-lake.